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Climatic Aspects of the 1993 Upper Mississippi River Basin Flood
Abstract
The 1993 record-breaking summer flood in the Upper Mississippi River
Basin resulted from an unprecedentedly persistent heavy rain pattern.
Rainfall totals for the Upper Mississippi River Basin were, by a large
margin, the largest of this century for the 2-, 3-, 4-, and 12- month
periods encompassing the 1993 summer. The totals for these periods are
estimated to have a probability of occurrence of less than 0.005
yr1. In addition, the number of reporting stations receiving
weekly totals in excess of 100 mm (events identified in a previous study
as being closely correlated with floods) was the largest in at least the
last 45yr. Other conditions contributing to the flood include
above-normal soil moisture levels at the beginning of June 1993;
large-sized areas of moderate to heavy rains; occurrence of rain areas
oriented along the main stems of major rivers; a large number of
localized extreme daily rainfall totals (greater than 150 mm); and
below-normal evaporation. The large-scale atmospheric circulation
patterns during the summer of 1993 were similar to the patterns
associated with past heavy rain events, although much more persistent
than past events.
... The intensification of the North American ridge resulted in a poleward-displaced Jetstream and, hence, a northward displaced cyclone track. In contrast, the flood of 1993 was characterized by a western US trough and eastern US ridge during later spring and summer [6][7][8]. Specifically, Bell and Janowiak [8] noted that this pattern became established in June 1993, with a western ridge dominant in the previous months. The resultant pattern featured a southward displacement of the Jetstream implying a southward-displaced cyclone track [9]. ...
... For the 1993 flood, antecedent wet soil moisture anomalies likely played a role in exacerbating the event [15]. For example, Kunkel et al. 1994 discussed that conditions were wet from July 1992 to the summer of 1993 with seven of eleven months preceding the flood having above normal precipitation. In addition, Kunkel et al. [7] noted that AMJ 1993 was anomalously wet in the Midwest resulting in saturated soil; hence, additional rainfall went into run-off into the Advances in Meteorology 3 Mississippi as opposed to being absorbed locally. ...
... For example, Kunkel et al. 1994 discussed that conditions were wet from July 1992 to the summer of 1993 with seven of eleven months preceding the flood having above normal precipitation. In addition, Kunkel et al. [7] noted that AMJ 1993 was anomalously wet in the Midwest resulting in saturated soil; hence, additional rainfall went into run-off into the Advances in Meteorology 3 Mississippi as opposed to being absorbed locally. Although not completely responsible, Trenberth and Guillemot [9] described how a mature El Niño in the spring of 1993 and its associated anomalous convection resulted in an alteration of the large-scale circulation and hence a rearrangement of the planetary waves, jetstream, and cyclone track via an atmospheric response due to heating anomalies. ...
To assess the role of cyclone tracks in contributing to floods and droughts, we highlight the role of midlatitude cyclones played in the 1988 drought and the 1993 flood. Our results demonstrate that the 1988 drought featured a poleward-displaced cyclone track with a reduced role for cyclone-induced precipitation, especially in the spring of 1988. The 1993 flood featured a cyclone track from Mexico northeast to Missouri in the spring, while the summer featured two cyclone tracks: one in the southwestern US and the other across Canada linked to the right-entrance and left-exit regions of a strong 200 hPa Jetstream across the upper Midwest. Enhanced 850 hPa inflow from the Caribbean northeast to the Midwest with high precipitable water values occurred in conjunction with the right entrance portion of the 200 hPa Jetstream. Linking storm tracks and the 200 hPa Jetstream to a storm-rain index for the Midwest showed that these extreme events conformed to features of the general circulation normally associated with wet/dry episodes in the warm half of the year. Although El Niño did not play a role in the 1993 flood, the 1988 drought was associated with a poleward displacement of cyclone tracks in response to La Niña.
... The physical mechanisms that cause extreme floods have been extensively studied, particularly over the central Great Plains of the US. Large-scale atmospheric circulation anomalies are key to the development of extreme floods (Bell & Janowiak, 1995;Flanagan et al., 2018;Kunkel et al., 1994;Zhang & Villarini, 2019). Anomalous circulation can sustain strong southerly winds (Budikova et al., 2010;Lavers & Villarini, 2013;Nakamura et al., 2013) that transport warm and moist air from the subtropical oceanic moisture sources into the Great Plains (Dirmeyer & Brubaker, 1999;Dirmeyer & Kinter, 2009Erlingis et al., 2019). ...
... It has been suggested that local and remote evapotranspiration can enhance precipitation or sustain circulation anomalies through soil moisture feedbacks during extreme floods (DeAngelis et al., 2010;Pal & Eltahir, 2002;Trenberth & Guillemot, 1996). Antecedent wet soil moisture conditions can exacerbate flood conditions (Bell & Janowiak, 1995;Budikova et al., 2010;Kunkel et al., 1994), and supply moisture to enhance precipitation (Erlingis et al., 2019;Pal & Eltahir, 2002;Trenberth & Guillemot, 1996). However, the results of Dirmeyer and Kinter (2010), based on monthly moisture source anomalies, suggest that the nearby terrestrial sources of the Midwest cannot support floods due to their limited evaporation rate compared to oceanic sources. ...
A comprehensive picture of the development of warm season extreme floods in the Midwestern US is presented. We first identify the climatological moisture sources for precipitation in the Midwest using the two‐layer dynamic recycling model (2L‐DRM) with ECMWF Reanalysis v5 (ERA5) data. Terrestrial sources supply most of the moisture for Midwestern US precipitation during the warm season, while oceanic sources dominate during the cold season. The Empirical Orthogonal Function (EOF) analysis is used to select extreme flood events characterized by both positive soil moisture and precipitation anomalies. During the warm season flood events, moisture coming from oceanic sources increases by more than 45% compared to the climatology. In addition, our results show that moisture can come from remote regions due to sustained anomalous circulation. Low‐level circulation anomalies associated with wave trains that traverse the continent enhance moisture contributions from terrestrial sources along narrow paths. However, moisture budget analysis reveals that the primary flood‐producing mechanism is the convergence of moisture due to intense circulation anomalies. Moisture advection and thermodynamic terms are responsible for flood termination. Our results suggest that knowledge of antecedent wet soil moisture conditions is unlikely to improve the predictability of flood‐producing storms.
... It has been widely recognized that heavy precipitation and the associated flooding events have serious socioeconomical consequences. For example, the 1993 summer floods in the upper Mississippi River Basin of the United States devastated many low-lying communities in the basin and cost the economy nearly $20 billion (Kunkel et al., 1994). Most recently in winter 2019-2020, extremely heavy rainfall associated with three powerful extra tropical cyclones caused widespread flooding across the United Kingdom, resulting in loss of lives and at least €150 million property damages. ...
... In recent decades, extreme precipitation events have been on the rise in some parts of the world while decreasing in other parts, depending on various factors such as season, period, and geographical location (Easterling et al., 2000;Ghosh et al., 2012). For example, several studies have documented an upward trend of extreme precipitation events across much of the United States and Canada (e.g., Karl et al., 1996;Kunkel et al., 1994;Easterling et al., 2000), Japan (Iwashima and Yamamoto, 1993), Indian (Mukherjee et al., 2018), and Australia (Suppiah and Hennessy, 1998). However, in the United Kingdom (UK) heavy rainfall events have been increasing in winter, but decreasing in summer (Osborn et al., 1999) with substantial interannual variability (Jones et al., 2013;Simpson and Jones, 2014;Brown, 2018). ...
This study examines trends in the intensity and frequency of short-duration (5 min to 3 h) rainfall extremes in Hong Kong for the period of 1984 to 2010 and the drivers for the trends using gauge observations and gridded reanalysis. Both the intensity and frequency of rainfall extremes exhibit an upward trend, with the slope for the intensity (frequency) trend increasing (decreasing) as duration lengthens from 5 min to 3 h. The upward intensity (frequency) trends appear to be a manifestation of an abrupt change around 1991/1992 (1992/1993) that separates a period of lower and fewer rainfall extremes before from a period of higher and more extremes after. The increase in Hong Kong’s extreme rainfall after the early 1990s is likely caused by a combination of stronger rising motion along Southeast China Coasts and enhanced moisture transport into South China Sea resulting from the strengthening and westward shift of the western Pacific subtropical high associated with anomalous convective activities over the tropical western Indian Ocean and a positive phase circumglobal teleconnection wavetrain.
... Summer (JJA) years classified as SIC + , SIC − , and SIC neu during the 1979-2013 period from de-trended mean Arctic summer sea ice concentration (SIC) levels. SIC + 1983, 1992, 1994, 1996, 1999, 2000, 2004, 2009SIC − 1980, 1981, 1990, 1993, 1995, 2007, 2011, 2012SIC neu 1979, 1982, 1984, 1985, 1986, 1987, 1988, 1989, 1991, 1998, 2000, 2002, 2003, 2006 downloaded from the Hadley Office Meteorological Centre at http://www.metoffice.gov.uk/hadobs/hadisst/. Each SIC value characterizes the percentage of ice covered area relative to the total area of a 1 ∘ latitude-longitude grid. ...
... These conditions are indicative of a well-developed and active GPLLJ and southerly air flow from the Gulf of Mexico and westerly flow from the Pacific Ocean merge over the study area just to the southwest of the UMRV region. Together these coincidental atmospheric flow patterns are known to produce the majority of precipitation across the study area and UMRV region in the summer Weaver and Nigam, 2008) and in some cases have the potential to increase the incidence of flooding such as during spring and summers of 1993 and 2008 (Kunkel et al., 1994;Bell and Janowiak, 1995;Mo et al., 1997;Coleman and Budikova, 2009). The location of the trough along the eastern slopes of the Rocky Mountains promotes leeside cyclogenesis and significantly contributes to the generation of atmospheric pressure gradients needed for the development of the GPLLJ (Uccellini, 1980;Song et al., 2005). ...
Arctic sea ice has been shrinking at unprecedented rates over the past three decades. These cryospheric changes have coincided with greater incidence of global extreme weather conditions, including increased severity and frequency of summer heatwaves and extreme rainfall events. Recent studies identify potential physical mechanisms related to Rossby wave and resonance theories that may attribute the observed changes in extreme summer weather patterns to Arctic sea ice decline. This study explores the linkages between summer Arctic sea ice variability and hydroclimate of the north-central United States (US) during the 1979 to 2013 period. Since 1979, summers with low sea ice conditions have coincided with significant increases in mean, minimum, maximum, and dew point air temperatures. Also apparent are increases in seasonal precipitation, the number of wet days, heavy (>95th percentile) precipitation days, and accumulated precipitation over the region. These moisture changes coincide with atmospheric patterns typically observed during anomalously wet summers, known to prompt flooding across the Upper Mississippi River Valley (UMRV) region. Low sea ice summers have coincided with (1) enhanced southerly air flow and increased activity of the Great Plains Low Level Jet (GPLLJ) over the study area, (2) increased occurrence of moist tropical air masses over the UMRV region, and (3) amplified 500 hPa flow over the Pacific-North American region with a ridge situated over the central-eastern portions of the North American continent emanating from Greenland and the central Arctic basin. The results suggest summer Arctic sea ice variability has been associated with recent hydroclimate anomalies of the north-central United States and the UMRV region and add to our growing knowledge of the connections between a changing Arctic environment and concurrent mid-latitude climate variability.
... Very heavy precipitation events can cause costly and sometimes catastrophic floods in regions that may not be adequately prepared to combat them. Although details of these events may vary, such as the Midwest floods of 1993 (e.g., Kunkel et al. 1994) and 2008 (e.g., Coleman and Budikova 2010), there is no question that these events cause immense social and economic stress to those that are affected. Furthermore, very heavy precipitation events are often highly localized in time and space and can occur independently from changes in the seasonal mean, making these events difficult to predict (Gershunov 1998;Kunkel et al. 2002). ...
... This pattern provides substantial fetch for moistening air before it enters the southern United States. Similar results were found in Brubaker et al. (2001), Brubaker et al. (2001) focused on the warm season, Fig. 8 highlights the importance of the moisture fetch during the winter season when, climatologically, Gulf of Mexico moisture does not often penetrate our upper Mississippi region, and existing terrestrial moisture supply within the region is low (Kunkel and Liang 2005;Brubaker et al. 2001). Moreover, this flow pattern passes over the Loop Current, where SST tends to be warmer because of a consistent flow of warmer Caribbean waters into the southern Gulf (Vukovich 2007). ...
The authors analyze the ability of the North American Regional Climate Change Assessment Program's ensemble of climate models to simulate very heavy daily precipitation and its supporting processes, comparing simulations that used observation-based boundary conditions with observations. The analysis includes regional climate models and a time-slice global climate model that all used approximately half-degree resolution. Analysis focuses on an upper Mississippi River region for winter (December-February), when it is assumed that resolved synoptic circulation governs precipitation. All models generally reproduce the precipitation-versus-intensity spectrum seen in observations well, with a small tendency toward producing overly strong precipitation at high-intensity thresholds, such as the 95th, 99th, and 99.5th percentiles. Further analysis focuses on precipitation events exceeding the 99.5th percentile that occur simultaneously at several points in the region, yielding so-called widespread events. Examination of additional fields shows that the models produce very heavy precipitation events for the same physical conditions seen in the observations.
... At the same time, sea surface temperature anomalies over the North Atlantic have also been associated with mediating streamflow near the outlet of the Mississippi River basin through its influence on the position and strength of the North Atlantic Subtropical High (Enfield et al., 2001;Muñoz et al., 2018). These features and related atmospheric and oceanic mechanisms have been variously ascribed to historic flood events on the Missouri (Parrett et al. 1993;Kunkel et al., 1994;Arritt et al. 1997;Dirmeyer and Kinter 2009;Hoerling et al. 2013), Ohio and lower Mississippi Rivers (Lott and Meyers 1956;Nakamura et al. 2013;Therrell and Bialecki 2015;Smith and Baeck 2015), although the importance and timing of different mechanisms on these tributaries remains unclear. As a result, a unified model describing how hydrologic extremes on the major tributaries of the Mississippi River basin are mediated by ocean-atmosphere variability has yet to emerge. ...
The Mississippi River basin drains nearly half of the contiguous United States, and its rivers serve as economic corridors that facilitate trade and transportation. Flooding remains a perennial hazard on the major tributaries of the Mississippi River basin, and reducing the economic and humanitarian consequences of these events depends on improving their seasonal predictability. Here, we use climate reanalysis and river gage data to document the evolution of floods on the Missouri and Ohio Rivers — the two largest tributaries of the Mississippi River — and how they are influenced by major modes of climate variability centered in the Pacific and Atlantic Oceans. We show that the largest floods on these tributaries are preceded by the advection and convergence of moisture from the Gulf of Mexico following distinct atmospheric mechanisms, where Missouri River floods are associated with heavy spring and summer precipitation events delivered by the Great Plains Low-Level Jet, while Ohio River floods are associated with frontal precipitation events in winter when the North Atlantic subtropical high is anomalously strong. Further, we demonstrate that the El Niño-Southern Oscillation can serve as a precursor for floods on these rivers by mediating antecedent soil moisture, with Missouri River floods often preceded by a warm eastern tropical Pacific (El Niño) and Ohio River floods often preceded by a cool eastern tropical Pacific (La Niña) in the months leading up peak discharge. Finally, we use recent floods in 2019 and 2021 to demonstrate how linking flood hazard to sea surface temperature anomalies holds potential to improve seasonal predictability of hydrologic extremes on these rivers.
... At present, the most destructive Midwestern flood events generally occur during the mid-to-late spring when precipitation is heaviest and watershed soils are saturated due to spring snowmelt (Kunkel et al., 1994;Andresen et al., 2012). Based on these observations, it has been hypothesized that pre-industrial flood recurrence during the late Holocene would have been most common when zonal mean-state atmospheric circulation over the Midwest produced clockwise atmospheric flow that advected warm, moist vapor masses from the Gulf of Mexico and increased spring and summer precipitation (Knox, 2000). ...
Late Holocene flood frequencies on the lower Ohio River were investigated using 14 C-based sedimen-tation rates from three floodplain lakes located in Illinois (Avery Lake), Kentucky (Grassy Pond), and Indiana (Goose Pond). Changes in sediment accumulation rates were attributed to variability in the delivery of overbank sediment to each site as controlled by the frequency of Ohio River flooding. Sedi-mentation rates reached their lowest values in all three lakes between 400 and 1230 CE, indicating a regional reduction in flood frequencies on the lower Ohio River during a period that included the Medieval Climate Anomaly (MCA; ca. 950e1250 CE). Sedimentation rates increased after ca. 1230 CE and remained moderately high through the Little Ice Age (LIA; 1350e1820 CE) until the onset of extensive land clearance during the early 1800s CE. After 1820 CE, sedimentation rates increased further and were higher than any other time during the late Holocene. A comparison of regional paleoclimatic proxies with the above floodplain sedimentation records shows that Ohio River flooding during the late Holocene was responsive to mean-state changes in atmospheric circulation. During the MCA, when clockwise mean-state atmospheric circulation advected southerly moisture from the Gulf of Mexico into the Ohio River Valley primarily in the form of convective rainstorms, flooding on the Ohio River was least frequent. During the LIA, meridional mean-state atmospheric circulation increased the proportion of mid-continental moisture that was sourced from the northern Pacific and Arctic and delivered as snowfall, hence increasing flooding on the Ohio River. We attribute the increase in Ohio River flooding during the LIA to an increase in snowpack volume across the Ohio River Valley and the watershed-scale integration of runoff during spring snowmelt. Following Euro-American land clearance in the early 1800s, flood frequencies decoupled from this relationship and the lower Ohio River became susceptible to frequent flooding, despite a return to southerly and clockwise synoptic atmospheric conditions. These modern climate-flood dynamics are fundamentally different than those of the paleo-record and suggest that land-use changes e such as deforestation, tile draining, and landscape conversion to intensive row crop agriculture e have fundamentally altered the modern Midwestern hydrologic cycle.
... They found three major sources contributing uniquely to these anomalies: the SST anomalies in the tropical Pacific (Trenberth and Guillemot 1996;National Research Council 1998), the SST anomalies in the mid-and highlatitude North Pacific (Namias 1983(Namias , 1991, and the lowlevel southerly jet (LLJ) from the Gulf of Mexico (Helfand and Schubert 1995;Bell and Janowiak 1995;Mo et al. 1995Mo et al. , 1997Paegle et al. 1996). Additional studies also showed that local moisture supplies and surface conditions sometimes can have a sizeable contribution VOLUME 14 J O U R N A L O F C L I M A T E to enhancing summer rainfall anomalies (Brubaker et al. 1993;Kunkel et al. 1994Kunkel et al. , 1996. These results showed the very different and competing processes important in causing summer rainfall variations in the central United States. ...
Summer rainfall in the central United States has singular interannual variations of a 3–6-yr period. Identifying the causes of these variations assures improvement in predictions of summer rainfall in the region. A review of previous studies revealed a puzzling situation: the outstanding interannual variations of the summer rainfall in the central United States showed no persistent correlations with known influential interannual variations in the Northern Hemisphere and the El Nin ̃o–Southern Oscillation (ENSO). This study was undertaken to identify the cause of this situation and ultimately explain the causes of the observed interannual summer rainfall variations. Its results showed a teleconnection of the ENSO with the summer rainfall in the central United States. The intensity of which has varied over the last 125 years. The teleconnection was active in two epochs, 1871–1916 and 1948–78, and absent in the two epochs 1917–47 and 1979–present. This variation was associated with a multidecadal variation in both sea surface temperature and sea level pressure in the mid- and high-latitude North Pacific. In the epochs of active teleconnection, the circulation in the warm phase of ENSO favored a deformation field in the lower troposphere in the central United States causing wet summers and a reversed circulation in cold phase of ENSO yielding dry summers, a process that partially explains the interannual summer rainfall variations. The result also showed that the variations of the teleconnection were ‘‘in phase’’ with the variation in the average surface temperature of the Northern Hemisphere. When the ‘‘abrupt warming’’ of the surface temperature developed in 1917–47 and the most recent two decades, the teleconnection broke down. Because of the limitation in data record length, this observed relationship and the persistence of the variation in the teleconnection need further investigations when additional data are available.
... If so, a wide range of influence and large cumulative intensity will contribute to a large amount of runoff on the surface, easily causing serious floods, landslides, mud-rock flows, and other meteorological and geological disasters, even with serious national economic losses. For example, the 1993 summer flooding in the Upper Mississippi River Basin is the most devastating flood of modern times, with damage estimates in the range of $15-20 billion (National Oceanic and Atmospheric Administration, 1993;Kunkel et al., 1994). In 1998, the persistent extreme rainfall event in the Yangtze River basin caused great losses over wide areas, and it is very severe and rare in recent decades. ...
Persistent extreme precipitation covering a large area usually causes severe flooding disasters in China, but how to depict it and what are the possible causes are still open questions. With Climate Prediction Center global unified gauge-based analysis of daily precipitation and NCEP/NCAR daily reanalysis dataset from 1979 to 2019, summer regional pentad extreme precipitation (RPEP) is defined according to the threshold of the 95th percentile of pentad precipitation with more than 5% land grids coverage in eastern China. While the definition of RPEP highlights the climate features of both the persistence and the regionality of extreme precipitation, it is distinctly different from the previous definitions that mainly reflect the synoptic aspects with daily data and have strictly temporal-spatial constraints. Four categories of RPEPs are objectively identified by K-means cluster analysis, i.e., South China (SC), South of Yangtze River (SYR), Jianghuai River (JHR), and North China (NC). Along the Yangtze River (SYR and JHR), intensity and area of RPEP are positively correlated with each other, and with the increase of RPEP intensity, its center of gravity tends to move eastward in all the four cluster regions and southward in Jianghuai River and North China, respectively, and vice versa. The RPEPs mostly persist for one pentad but can reach up to two to three pentads at most, and along with the duration of RPEP, its intensity and area are both enhanced accordingly. Furthermore, the frequency of RPEP increased significantly since the late 1990s in SYR, JHR, and SC. Associated with RPEP, strong pentad-mean convergence and ascending motion occur in the middle-lower troposphere, and except for SC that is dominated by the local low-pressure and cyclone anomalies, the other three cluster regions are all forced by the western Pacific subtropical high to the southeast and weak low-pressure trough to the north, and the low-level anticyclone anomaly to the southeast transports abundant water vapors to the RPEP regions accordingly. Besides, all the RPEPs are closely in accordance with obvious subseasonal oscillations, especially the 10–30-day and 30–60-day oscillations, which can be regarded as the potential sources of RPEP predictability in eastern China.
... moisture exerts a primary control on flood occurrence on the upper Mississippi River basin (Dirmeyer & Kinter, 2009;Knox, 1988;Kunkel et al., 1994;Wise et al., 2018)-consistent with precipitation anomalies observed during historical floods (Figure 4h). The close coupling we find between our reconstruction of relative warm-season precipitation and flood occurrence on the upper Mississippi and Missouri Rivers during the Common Era supports model-based projections of increased flood hazard in these basins under continued greenhouse gas forcing (Milly et al., 2002;Qiao et al., 2014;Wise et al., 2018), though changes in land-cover and river management also play a fundamental role in modulating flood hazard Pinter et al., 2008;Tao et al., 2014). ...
Floods and droughts in the Mississippi River basin are perennial hazards that cause severe economic disruption. Here we develop and analyze a new lipid biomarker record from Horseshoe Lake (Illinois, USA) to evaluate the climatic conditions associated with hydroclimatic extremes that occurred in this region over the last 1,800 years. We present geochemical proxy evidence of temperature and moisture variability using branched glycerol dialkyl glycerol tetraethers (brGDGTs) and plant leaf wax hydrogen isotopic composition (δ²Hwax) and use isotope‐enabled coupled model simulations to diagnose the controls on these proxies. Our data show pronounced warming during the Medieval era (CE 1000–1,600) that corresponds to midcontinental megadroughts. Severe floods on the upper Mississippi River basin also occurred during the Medieval era and correspond to periods of enhanced warm‐season moisture. Our findings imply that projected increases in temperature and warm‐season precipitation could enhance both drought and flood hazards in this economically vital region.
... Extreme precipitation occurring over shorter durations and/or smaller spatial scales is also caused by other phenomena, such as mesoscale convective systems and air mass convection, which do not have multiday lifetimes. Also, extreme precipitation persisting over multiweek timescales, such as the 1993 Upper Mississippi River flood, arises from multiple systems, often of different types (Kunkel et al., 1994). ...
The top 100 largest area‐averaged, multiday precipitation events in the U.S. historical record for the period 1949–2018 were identified by calculating box‐average precipitation using a network of observing stations with minimal missing data. Hurricane Harvey was the single largest event for an area sized 50,000 km² and a duration of 4 days. Rainfall associated with Hurricane Florence ranked seventh. Almost all of the top 100 events occurred in the southeastern United States or along the Pacific coast. The predominant meteorological cause (in 59% of the events) was fronts associated with extratropical cyclones, including 15% that were also associated with atmospheric rivers. Tropical cyclones were a significant cause, representing 25% of all events. The spatial locations, the seasonal distribution, and the spectrum of meteorological causes of these events are characteristics of the precipitation climatology that could be used as metrics to evaluate climate models.
... Climate has been previously implicated as having a strong effect on mosquito distribution [35]. There was a slight increase in total rainfall from 1975 to 2008, with 1993 being a notably higher-than-average year for summer precipitation corresponding with a massive, regional flood that occurred [36]. Yet, as a whole, total seasonal precipitation amounts did not significantly change over the 34-year period at either location. ...
The ecology and environmental conditions of a habitat have profound influences on mosquito population abundance. As a result, mosquito species vary in their associations with particular habitat types, yet long-term studies showing how mosquito populations shift in a changing ecological landscape are lacking. To better understand how land use changes influence mosquito populations, we examined mosquito surveillance data over a thirty-four-year period for two contrasting sites in central Iowa. One site displayed increasing levels of urbanization over time and a dramatic decline in Culex pipiens group (an informal grouping of Culex restuans, Culex pipiens, and Culex salinarius, referred to as CPG), the primary vectors of West Nile virus in central Iowa. Similar effects were also shown for other mosquito vector populations, yet the abundance of Aedes vexans remained constant during the study period. This is in contrast to a second site, which reflected an established urban landscape. At this location, there were no significant changes in land use and CPG populations remained constant. Climate data (temperature, total precipitation) were compiled for each location to see if these changes could account for altered population dynamics, but neither significantly influence CPG abundance at the respective site locations. Taken together, our data suggest that increased landscape development can have negative impacts on Culex vector populations, and we argue that long-term surveillance paired with satellite imagery analysis are useful methods for measuring the impacts of rapid human development on mosquito vector communities. As a result, we believe that land use changes can have important implications for mosquito management practices, population modeling, and disease transmission dynamics.
... Historically, flood studies have followed two distinct research lines: hydrometeorology of floods and flood frequency analysis. Flood hydrometeorology focuses on understanding (i) hydrodynamics of the rainfall-runoff process during flood events, (ii) spatial structure of local rainfall events that are associated with floods, (iii) soil-atmosphere response, and large-scale circulation patterns associated with the forecast and diagnosis of rainfall events ( Maddox, 1983;Kunkel et al., 1994;Pal and Eltahir, 2002;Schumacher and Johnson, 2005;Amengual et al., 2007;Viglione et al., 2010;Li et al., 2013). There is also extensive literature related to the statistical analysis and modeling of flood frequency from local and regional data of rainfall, streamflow, and water basin attributes, including nonstationary approaches (e.g., Thomas and Benson, 1970;Stedinger and Cohn, 1986;Stedinger et al., 1993;Kroll and Stedinger, 1998;Kwon et al., 2008;Lima and Lall, 2010;Cheng et al., 2014;Luke et al., 2017). ...
Floods are the main natural disaster in Brazil, causing substantial economic
damage and loss of life. Studies suggest that some extreme floods result
from a causal climate chain. Exceptional rain and floods are determined by
large-scale anomalies and persistent patterns in the atmospheric and oceanic
circulations, which influence the magnitude, extent, and duration of
these extremes. Moreover, floods can result from different generating
mechanisms. These factors contradict the assumptions of homogeneity, and
often stationarity, in flood frequency analysis. Here we outline a
methodological framework based on clustering using self-organizing maps (SOMs)
that allows the linkage of large-scale processes to local-scale observations. The
methodology is applied to flood data from several sites in the flood-prone
Upper Paraná River basin (UPRB) in southern Brazil. The SOM clustering
approach is employed to classify the 6-day rainfall field over the UPRB into
four categories, which are then used to classify floods into four types based
on the spatiotemporal dynamics of the rainfall field prior to the observed
flood events. An analysis of the vertically integrated moisture fluxes,
vorticity, and high-level atmospheric circulation revealed that these four
clusters are related to known tropical and extratropical processes,
including the South American low-level jet (SALLJ); extratropical cyclones;
and the South Atlantic Convergence Zone (SACZ). Persistent anomalies in the
sea surface temperature fields in the Pacific and Atlantic oceans are also
found to be associated with these processes. Floods associated with each
cluster present different patterns in terms of frequency, magnitude, spatial
variability, scaling, and synchronization of events across the sites and
subbasins. These insights suggest new directions for flood risk
assessment, forecasting, and management.
... During the warm season in the central United States, mesoscale convective systems (MCSs) are the highest contributors to surface accumulated precipitation (Fritsch et al. 1986). Under certain atmospheric conditions, individual MCS events can also produce widespread and intense precipitation that leads to flooding (e.g., Doswell et al. 1996;Schumacher and Johnson 2005;Stevenson and Schumacher 2014), such as the extreme 1993 floods in the Mississippi valley (Kunkel et al. 1994). As such, understanding changes to MCS precipitation due to perturbations in the environment is important. ...
Simulations of two leading-line, trailing-stratiform mesoscale convective system (MCS) events that occurred during the Midlatitude Continental Convective Clouds Experiment (MC3E) have been used to understand the relative microphysical impacts of lower- versus midtropospheric aerosol particles (APs) on MCS precipitation. For each MCS event, four simulations were conducted in which the initial vertical location and concentrations of cloud droplet nucleating APs were varied. These simulations were used to determine the precipitation response to AP vertical location. Importantly, the total integrated number and mass of the initial aerosol profiles used in the sensitivity simulations remained constant, such that differences in the simulations could be directly attributable to changes in the vertical location of cloud droplet nucleating APs. These simulations demonstrate that lower-tropospheric APs largely influenced the precipitation response directly rearward of the leading cold pool boundary. However, farther rearward in the MCS, the relative impact of lower- versus midtropospheric APs largely depended on the MCS structure, which varied between the two events because of differences in line-normal wind shear. Midtropospheric APs were able to activate new cloud droplets in the midtropospheric levels of convective updrafts and to enhance mixed-phase precipitation through increased cloud riming, and this microphysical pathway had a more significant impact on mixed-phase precipitation in weaker line-normal wind shear conditions. This result exposes the importance of properly representing midtropospheric APs when assessing aerosol effects on clouds. This study also demonstrates the utility of assessing aerosol effects within the different regions of MCSs.
... Recent examples include the October 2015 South Carolina flooding resulting from a series of prolific rainfall events that shattered or surpassed countless records throughout the state causing 17 casualties and about $12 billion in damage [Leberfinger, 2015;Mizzell et al., 2016], the August 2016 Louisiana flooding as extreme rainfall of more than 0.6 m fell over many areas sending rivers to record levels [Watson et al., 2017], and the January-February 2017 California flooding as heavy rainfall associated with atmospheric rivers [Rosen, 2017] pounded the state threatening the safety of reservoir dams and causing record-setting flooding and mudslides [Branson-Pott and Hamilton, 2017]. Unfortunately, regions of the U.S. are likely to see more of these catastrophic flooding events as there has been an increasing trend over the recent decades in both the frequency and particularly the intensity of extreme precipitation, and this trend is likely to continue into the future [Karl et al., 1995;Karl and Knight, 1998;Kunkel et al., 1994Kunkel et al., , 1999Kunkel et al., , 2003Kunkel et al., , 2007Easterling et al., 2000;[2010] suggested that during positive phases of the NPI, the number of heavy precipitation events is reduced over the Pacific coast, the southwestern U.S., Texas, the High Plains, and Florida and increased over the Mountain West and the Ohio-Mississippi Basin. Florida and the coastal southeastern U.S. experience an increase in the intensity of warm-season (cold-season) precipitation during the warm (cold) phase of the AMO [Curtis, 2008;Teegavarapu et al., 2013;Goly and Teegavarapu, 2014], with the increase in the warmseason precipitation intensity attributed to an increase in the number of major hurricanes during the AMO warm phase [Goldenberg et al., 2001]. ...
Intense precipitation over a short duration is a major cause of flash floods. Using hourly rainfall data from the North America Land Data Assimilation System Phase 2 (NLDAS-2) from 1979-2013, we compared the differences in the response of the sub-daily extreme precipitation occurrences across the contiguous U.S. to strong anomalous warming over the eastern equatorial Pacific known as El Niño, and over the central equatorial Pacific known as El Niño Modoki. For both types of anomalous equatorial Pacific warming, the teleconnection is much stronger in the cold season (November through April) than warm season (May through October). During the warm season, while both types correspond to an increase in sub-daily extreme precipitation in areas of Texas and a decrease in some areas of the northern Plains, El Niño is associated with a significant increase in the northern Rockies, and El Niño Modoki is associated with a decrease in the Intermountain West. During the cold season, the overall patterns are similar between the two types, with positive anomalies over much of the southern and negative anomalies over the northern U.S. However, large regional differences exist, with El Niño exerting a much stronger influence on sub-daily extreme precipitation in the Atlantic and Gulf tates as well as California, while the influence of El Niño Modoki is slightly broader over the Midwest. These differences can be largely explained by the vertical velocity, moisture convergence, and jet stream patterns induced by El Nino and El Nino Modoki episodes.
... Ten years later, the "Great Ohio and Mississippi River Flood of 1937" resulted from a prolonged period of early snow melt and high precipitation and covered more than 200,000 square miles with water depths above 11 inches (Welky, 2011). This "thousand year flood" was the benchmark event in the region until summer 1993 saw "unprecedentedly persistent" precipitation throughout the Mississippi River Basin (Kunkel et al., 1994). historical benchmarks for similar current events. ...
Agriculture is a complex human-natural system with intricate and continuous feedback
loops that bring the past forward into the present and the future. Like all humans, farmers learn
from the past. Intergenerational narratives and experiences with recent past extreme weather
events and variable climate patterns frequently become analog years used as benchmarks to build
knowledge of the natural environment and guide decisions. However, there is a lack of
knowledge about how individuals plan and structure the timescales between decision, action, and
outcome. For example, why do some people seem to act on “shorter” timescales without regard
for long-term consequences of their actions to themselves, others, or the environment? And why
do others make decisions based on “longer” timescales in order to preserve resources for the sake
of future use? Although agricultural and climate sciences are continuously advancing our
understanding of crop management, fewer investments have been made to understand the crucial
human element. There is a need to better understand the timescales of social and cultural factors
which influence reception (or rejection) of advances in scientific knowledge. How do time
perspectives—the orientation to time and pathways of time—influence interpretation of
information and decisions made to implement conservation practices on agricultural lands? What
are the disjunctures between how humans perceive and reference long-term timescales of
changing climatic conditions and short-term timescales of annual crop production?
This dissertation seeks to expand understanding of farmer decision-making as it relates
to timescales, climate change, corn-based cropping systems, and advances in science for
agricultural decision support. First a temporal reference framework is developed to explain the
processes by which past experiences and intergenerational narratives are brought forward in time
to inform current agricultural management decisions. Then, this theory is elaborated and
empirically tested in Chapters 4 and 5. A purposeful sample of interviews with corn farmers
(N=159) and climatologists (N=22) in nine upper Midwest states (Wisconsin, Indiana, Iowa,
Minnesota, Michigan, Ohio, Illinois, South Dakota, and Illinois) and a random sample farmer
survey (N=1,159) from the 2015 Iowa Farm and Rural Life Poll provide data for these analyses.
Chapter 4 conducts binary logistic regression on the survey of Iowa farmers to evaluate the
influence of previous generations and social pressures on decisions to decrease fall tillage,
increase no tillage, and increase the use of cover crops on their farm. Family-level norms and
pressures are shown to reinforce traditional crop production practices such as the action of post-harvest soil tillage. Chapter 5 explores the weight that farmers and climatologists give to
historical experiences in interpreting climate conditions and their effects on production systems
by analyzing in-person farmer interview data. Inductive reasoning is utilized to detect common
themes involving temporal orientations and temporal pathways that influence agricultural
decision-making.
Findings suggest that farmers are influenced by historical intergenerational narratives of
family farm management practices. Higher weights are often placed upon personally
experienced past events and narratives of analogous historical conditions than predictions or
expectations of future environmental conditions. Farmers are more likely to consider decisions
relative to a past time orientation which reinforces pathways of time as socially-referenced to
cyclical intergenerational events. This may result in farmers perceiving environmental
conditions as maintaining stability through reoccurrence of environmental weather and climate
risks. This suggests that scientific information describing early warning signals of future climate
disruptions and opportunities for agricultural management adaption may not be resonating with
the farming population.
This research offers a contribution to further understand the role of timescales—
temporal perspectives, orientations, and pathways—associated with decisions about agricultural
production and climate. Implications of these findings may be helpful for scientists, educators,
and other agricultural stakeholders who seek to connect advances in climate science with
opportunities for agricultural adaptation. Recommendations involve building the capacity of
information facilitators, or individuals skilled in communicating and framing science in
messaging which resonates to intergenerational narratives of farm and soil conservation.
Scientists should find ways to involve farmers in the co-production of knowledge to increase
understanding of timescale perspectives in the interpretation of scientific knowledge. As
agriculture adapts to changing climate and environmental conditions, decision-makers may need
to continually assess and reconsider the trajectory of predominant corn-based cropping
management.
... Recent floods in the United States (e.g., Mississippi River flooding in 1993) were caused primarily by unusually high precipitation combined with soil saturation from earlier precipitation (Kunkel et al., 1994). In the United States, flash floods currently are the leading cause of weather-related mortality. ...
... Ten years later, the "Great Ohio and Mississippi River Flood of 1937" resulted from a prolonged period of early snow melt and high precipitation and covered more than 200,000 square miles with water depths above 11 inches (Welky, 2011). This "thousand year flood" was the benchmark event in the region until summer 1993 saw "unprecedentedly persistent" precipitation throughout the Mississippi River Basin (Kunkel et al., 1994). The flood of 1993 then became the new analog flood risk event to which current and future climate risks were assessed. ...
Climate scientists rely on observed historical weather and climate data to inform current and future climate model projections. Similarly, agricultural producers use historical events—recent past experiences and historical narratives—to construct local knowledge to assess, quantify, and manage current and future risks. These historical data, events, and experiences become reference points or analogs when compared to a current phenomenon that exhibits similar characteristics specific to past conditions. In this paper we utilize a lens of agricultural traditions and past experiences to posit a temporal reference framework. In-person interviews with 159 Midwest farmers illustrate how the past influences farmers’ perceptions of current and future risks and is used to integrate scientific climate information to inform decision-making. Qualitative analysis provides support for the temporal reference framework, but more empirical testing is needed to further validate the model.
... In recent years, the duration of extreme precipitation events has been increasing at several places around the world (Kunkel et al., 1994(Kunkel et al., , 1999Bell and Janowiak, 1995;Grazzini and van der Grijin, 2003;Trenberth et al., 2003;Schumacher, 2011). Though heavy rainfall events over South China and the Yangtze-Huaihe valley (Bei et al., 2002) have been extensively studied, most studies of rainstorms over the SCS and the islands south of 20 • N have focused on typhoon systems (Hogsett and Zhang, 2010;Wangwongchai et al., 2010), and only a few studies have investigated non-typhoon induced rainstorms (Qiao et al., 2015). ...
A case study is presented of the multiscale characteristics that produced the record-breaking persistent heavy rainfall event (PHRE) over Hainan Island, northern South China Sea (SCS), in autumn 2010. The study documents several key weather systems, from planetary scale to mesoscale, that contributed to the extreme rainfall during this event. The main findings of this study are as follows. First, the convectively active phase of the MJO was favorable for the establishment of a cyclonic circulation and the northward expansion of the Intertropical Convergence Zone (ITCZ). The active disturbances in the northward ITCZ helped direct abundant moisture from adjacent oceans towards Hainan Island continuously throughout the event, where it interacted with cold air from the midlatitudes and caused heavy rain. Second, the 8-day-long PHRE can be divided into three processes according to different synoptic systems: peripheral cloud clusters of a tropical depression-type disturbance over the central SCS in process 1; interactions between the abnormally far north ITCZ and the invading cold air in process 2; and the newly formed tropical depression near Hainan Island in process 3. In the relatively stable synoptic background of each process, meso-α- and meso-β-scale cloud clusters repeatedly traveled along the same path to Hainan Island. Finally, based on these analyses, a conceptual model is proposed for this type of PHRE in autumn over the northern SCS, which demonstrates the influences of multiscale systems.
... Contrary to our expectations, hydrology of the Mississippi River (river levels) did not appear to affect extinction and colonization rates. We assumed that the floodplain site would be more influenced by regional weather (precipitation upstream and elsewhere in the river basin that affects river levels; Kunkel et al. 1994) than the mining site. Yet, the apparent effects of river hydrology on turnover rates were inconsistent. ...
Key factors affecting metapopulation dynamics of animals include patch size, isolation, and patch quality. For wetland-associated species, hydrology can affect patch availability, connectivity, and potentially habitat quality; and therefore drive metapopulation dynamics. Wetlands occurring on natural river floodplains typically have more dynamic hydrology than anthropogenic wetlands. Our overall objective was to assess the multiyear spatial and temporal variation in occupancy and turnover rates of a semi-aquatic small mammal at two hydrologically distinct wetland complexes. We live-trapped marsh rice rats (Oryzomys palustris) for 3 yr and >50 000 trap nights at nine wetland patches on the Mississippi River floodplain and 14 patches at a reclaimed surface mine in southern Illinois. We used dynamic occupancy modeling to estimate initial occupancy, detection, colonization, and extinction rates at each complex. Catch per unit effort (rice rats captured/1000 trap nights) was markedly higher at the floodplain site (28.1) than the mining site (8.1). We found no evidence that temperature, rainfall, or trapping effort affected detection probability. Probability of initial occupancy was similar between sites and positively related to patch size. Patch colonization probability at both sites was related negatively to total rainfall 3 weeks prior to trapping, and varied across years differently at each site. We found interacting effects of site and rainfall on extinction probability: extinction increased with total rainfall 3 months prior to trapping but markedly more at the floodplain site than at the mining site. These site-specific patterns of colonization and extinction are consistent with the rice rat metapopulation in the floodplain exhibiting a habitat-tracking dynamic (occupancy dynamics driven by fluctuating quality), whereas the mineland complex behaved more as a classic metapopulation (stochastic colonization & extinction). Our study supports previous work demonstrating metapopulation dynamics in wetland systems being driven by changes in patch quality (via hydrology) rather than solely area and isolation.
... The most costly effect of extreme precipitation results from flooding, which can cause infrastructure damage, soil erosion, environmental pollution, ecosystem destruction, and even loss of lives (Spierre and Wake 2010). For example, the Great Flood of 1993 that was caused by persistent storms with voluminous rainfall over the upper Mississippi River Basin resulted in an estimated $18 billion in damage, making it one of the most costly natural disasters in the US (Kunkel et al 1994, Changnon 1996. It has been reported in recent literature, based on analyses of various datasets and over different time periods, that there has been an increase in extreme precipitation events across much of the contiguous US (Karl et al 1995, Karl and Knight 1998, Kunkel et al 1999, Kunkel 2003, Groisman et al 2005, Alexander et al 2006, Pryor et al 2009, Matonse and Frei 2013, Muschinski and Katz 2013. ...
The mean global climate has warmed as a result of the increasing emission of greenhouse gases induced by human activities. This warming is considered the main reason for the increasing number of extreme precipitation events in the US. While much attention has been given to extreme precipitation events occurring over several days, which are usually responsible for severe flooding over a large region, little is known about how extreme precipitation events that cause flash flooding and occur at sub-daily time scales have changed over time. Here we use the observed hourly precipitation from the North American Land Data Assimilation System Phase 2 forcing datasets to determine trends in the frequency of extreme precipitation events of short (1 h, 3 h, 6 h, 12 h and 24 h) duration for the period 1979–2013. The results indicate an increasing trend in the central and eastern US. Over most of the western US, especially the Southwest and the Intermountain West, the trends are generally negative. These trends can be largely explained by the interdecadal variability of the Pacific Decadal Oscillation and Atlantic Multidecadal Oscillation (AMO), with the AMO making a greater contribution to the trends in both warm and cold seasons.
... The connection between ARs and flooding over the central United States was recently assessed by Lavers and Villarini (2013b). They found that during the period 1979-2011, in over 60% of the basins studied, five or more of the top 10 annual maxima were associated with ARs, including the two most destructive events, the great flood of 1993 (e.g., Kunkel et al. 1994) and the June 2008 flood (e.g., Budikova et al. 2010;Dirmeyer and Kinter 2010;Smith et al. 2013). Mesoscale convective systems (MCSs) can often produce heavy rainfall events over the central United States in spring and summer Johnson 2005, 2006). ...
Atmospheric rivers (ARs) play a major role in causing extreme precipitation and flooding over the central United States (e.g., Midwest floods of 1993 and 2008). The goal of this study is to characterize rainfall associated with ARs over this region during the Iowa Flood Studies (IFloodS) campaign that took place in April-June 2013. Total precipitation during IFloodS was among the five largest accumulations recorded since the mid-twentieth century over most of this region, with three of the heavy rainfall events associated with ARs. As a preliminary step, the authors evaluate how well different remote sensing-based precipitation products captured the rainfall associated with the ARs and find that stage IV is the product that shows the closest agreement to the reference data. Two of the three ARs during IFloodS occurred within extratropical cyclones, with the moist ascent associated with the presence of cold fronts. In the third AR, mesoscale convective systems resulted in intense rainfall at many locations. In all the three cases, the continued supply of warm water vapor from the tropics and subtropics helped sustain the convective systems. Most of the rainfall during these ARs was concentrated within similar to 100 km of the AR major axis, and this is the region where the rainfall amounts were highly positively correlated with the vapor transport intensity. Rainfall associated with ARs tends to be larger as these events mature over time. Although no major diurnal variation is detected in the AR occurrences, rainfall amounts during nocturnal ARs were higher than for ARs that occurred during the daytime.
... Large fluvial flood events in Asia, Europe, Australia and North America during the past 15 years (e.g. Kunkel et al., 1994;Munich Re, 1997;CEH Wallingford/Met Office, 2001;Wang & Plate, 2002;Yeo, 2002;Saurí et al., 2003) have received much attention in the media. At the same time, many climate change simulations, where river flows are calculated from output from global climate models, suggest that high river flows will increase in a greenhouse gas-induced warmer future climate (e.g. ...
Floods are of great concern in many areas of the world, with the last decade seeing major fluvial flood events in, for example, Asia, Europe and North America. This has focused attention on whether or not these are a result of a changing climate. River flows calculated from outputs from global climate models often suggest that high river flows will increase in a warmer, future climate. However, the future projections are not necessarily in tune with the records collected so far - the observational evidence is more ambiguous. A recent study of trends in long time series of annual maximum river flows at 195 gauging stations worldwide suggests that the majority of these flow records (70%) do not exhibit any statistically significant trends. Trends in the remaining records are almost evenly split between having a positive and a negative direction. This paper discusses factors that influence the results of trend estimates of floods, and that contribute to the general lack of compelling observational evidence of any long-term increase in extreme river flows. Recent results of trend analysis of observed floods are outlined. Expected impacts of indirect anthropogenic climate change are discussed, and a summary is given of the direct impact of man's influence on river flows in terms of catchment and river management. Different methodologies to detect trends are briefly outlined, and examples are given of how the choice of method can interact with climatological features to result in different estimates of trend. The examples illustrate the effects of using different types of flow indices and different periods of record. The effects on trend estimates of decadal-scale oscillations that have been shown to occur in many river flow records are discussed. Oscillations compound the problem of untangling trends from normal climatic variability as the cycles of the underlying climatic phenomena (e.g. the North Atlantic Oscillation) may also be predicted to change in a greenhouse gas-induced warmer climate. Initiatives to compile networks of pristine catchments with long river flow records are welcomed as a means of keeping scientific objectivity at the forefront of studies of change detection, an area of research riddled by uncertainty and speculation.
... This was a rain-driven flood. Rainfall totals for most of the region were the largest of the 20th century for the 2-, 3-, 4-and 12-month periods that encompassed peak summer rainfall months (Kunkel, et al., 1994). Estimated return periods for most of these totals were over 200 years. ...
... Record and near-record summer precipitation fell on soil saturated from previous seasonal precipitation and spring snowmelt, resulting in flooding along major rivers and their tributaries (USACE 1994). Rainfall totals for the UMRB were by far the largest of this century in summer 1993 (Kunkle et al. 1994). Precipitation over central Iowa during January through July 1993 was as much as 200% of the 30-year precipitation normal for those months from 1961 through 1990 (Parrett et al. 1993). ...
... The Central United States is the world's most productive, agriculture region. During the summer, heavy rainfall events frequently occur over this region and cause severe floods with devastating damages and considerable socioeconomic consequences (Kunkel et al. 1994;Smith et al. 2013;Nakamura et al. 2013). These extreme heavy rainfall events have been identified with complicated physical mechanisms at different scales. ...
This study comprehensively evaluates the effects of twelve cumulus parameterization (CUP) schemes on simulations of 1993 and 2008 Central US summer floods using the regional climate-weather research and forecasting model. The CUP schemes have distinct skills in predicting the summer mean pattern, daily rainfall frequency and precipitation diurnal cycle. Most CUP schemes largely underestimate the magnitude of Central US floods, but three schemes including the ensemble cumulus parameterization (ECP), the Grell-3 ensemble cumulus parameterization (G3) and Zhang-McFarlane-Liang cumulus parameterization (ZML) show clear advantages over others in reproducing both floods location and amount. In particular, the ECP scheme with the moisture convergence closure over land and cloud-base vertical velocity closure over oceans not only reduces the wet biases in the G3 and ZML schemes along the US coastal oceans, but also accurately reproduces the Central US daily precipitation variation and frequency distribution. The Grell (GR) scheme shows superiority in reproducing the Central US nocturnal rainfall maxima, but others generally fail. This advantage of GR scheme is primarily due to its closure assumption in which the convection is determined by the tendency of large-scale instability. Future study will attempt to incorporate the large-scale tendency assumption as a trigger function in the ECP scheme to improve its prediction of Central US rainfall diurnal cycle.
... The floods of record occurred in the summers of 1993 and 2008 and resulted from large rainfall events preceded by extended periods of anomalously wet weather that saturated the basin. This evidence, along with previous research (Kunkel et al., 1994;Coleman and Budikova, 2010;Nakamura et al., 2013), suggests that the worst flooding in the area is not caused by short-duration, high-intensity storms in isolation, but rather long periods of wet weather that lead to highly saturated antecedent basin conditions. It is under these conditions that large rainstorms can result in extreme floods that challenge the flood risk reduction capabilities of the dam. ...
This work examines future flood risk within the context of integrated climate and hydrologic modeling uncertainty. The research questions investigated are 1) whether hydrologic uncertainties are a significant source of uncertainty relative to other sources such as climate variability and change, and 2) whether a statistical characterization of uncertainty from a lumped, conceptual hydrologic model is sufficient to account for hydrologic uncertainties in the modeling process. To investigate these questions, an ensemble of climate simulations are propagated through hydrologic models and then through a reservoir simulation model to delimit the range of flood protection under a wide array of climate conditions. Uncertainty in mean climate changes and internal climate variability are framed using a risk-based methodology and are explored using a stochastic weather generator. To account for hydrologic uncertainty, two hydrologic models are considered, a conceptual, lumped parameter model and a distributed, physically-based model. In the conceptual model, parameter and residual error uncertainties are quantified and propagated through the analysis using a Bayesian modeling framework. The approach is demonstrated in a case study for the Coralville Dam on the Iowa River, where recent, intense flooding has raised questions about potential impacts of climate change on flood protection adequacy. Results indicate that the uncertainty surrounding future flood risk from hydrologic modeling and internal climate variability can be of the same order of magnitude as climate change. Furthermore, statistical uncertainty in the conceptual hydrological model can capture the primary structural differences that emerge in flood damage estimates between the two hydrologic models. This article is protected by copyright. All rights reserved.
We present a comparative analysis of atmospheric rivers (ARs) and Great Plains low-level jets (GPLLJs) in the central U.S. during April–September 1901–2010 using ECMWF’s CERA-20C. The analysis is motivated by a perceived need to highlight overlap and synergistic opportunities between traditionally disconnected AR and GPLLJ research. First, using the Guan–Walliser integrated vapor transport (IVT)-based AR classification and Bonner–Whiteman-based GPLLJ classification, we identify days with either an AR and/or GPLLJ spanning 15% of the central U.S. These days are grouped into five event samples: 1) all GPLLJ, 2) AR GPLLJ, 3) non-AR GPLLJ, 4) AR non-GPLLJ, and 5) all AR. Then, we quantify differences in the frequency, seasonality, synoptic environment, and extreme weather impacts corresponding to each event sample. Over the 20th century, April–September AR frequency remained constant whereas GPLLJ frequency significantly decreased. Of GPLLJ days, 36% are associated with a coincident AR. Relative to ARs that are equally probable from April–September, GPLLJs exhibit distinct seasonality, with peak occurrence in July. A 500 hPa geopotential height comparison shows a persistent ridge over the central U.S for non-AR GPLLJ days, whereas on AR GPLLJ days, a trough and ridge pattern is present over western to eastern CONUS. AR GPLLJ days have 34% greater 850 hPa windspeeds, 53% greater IVT, and 72% greater 24-hour precipitation accumulation than non-AR GPLLJ days. In terms of 95th percentile 850 hPa windspeed, IVT, and 24-hour precipitation, that of AR GPLLJs is 25%, 45%, and 23% greater than non-AR GPLLJs, respectively.
Summertime heavy rainfall and its resultant floods are among the most harmful natural hazards in the US Midwest, one of the world's primary crop production areas. However, seasonal forecasts of heavy rain, currently based on preseason sea surface temperature anomalies (SSTAs), remain unsatisfactory. Here, we present evidence that sea surface salinity anomalies (SSSAs) over the tropical western Pacific and subtropical North Atlantic are skillful predictors of summer time heavy rainfall one season ahead. A one standard deviation change in tropical western Pacific SSSA is associated with a 1.8 mm day⁻¹ increase in local precipitation, which excites a teleconnection pattern to extratropical North Pacific. Via extratropical air‐sea interaction and long memory of midlatitude SSTA, a wave train favorable for US Midwest heavy rain is induced. Combined with soil moisture feedbacks bridging the springtime North Atlantic salinity, the SSSA‐based statistical prediction model improves Midwest heavy rainfall forecasts by 92%, complementing existing SSTA‐based frameworks.
Floods caused by extreme precipitation events, in the context of climate warming, are one of the most serious natural disasters in monsoon region societies. The great flood in the Yangtze River Basin in 1849, in Eastern China, was a typical extreme flood event. According to historical archives, local chronicles, diaries, and historical hydrological survey data, this study reconstructed the temporal and spatial patterns of extreme precipitation in 1849, and the flood process of the Yangtze River. We found four major precipitation events at the middle and lower reaches of the Yangtze River, from 18 May to 18 July 1849. The torrential rainfall area showed a dumbbell-like structure along the Yangtze River, with two centers distributed separately in the east and west. For the specific flood process of the Yangtze River, many tributaries of the Yangtze River system entered the flood season consecutively since April, and the mainstream of the Yangtze River experienced tremendous pressure on flood prevention with the arrival of multiple rounds of heavy rainfall. In mid-to-late July, the water level and flow rate of many stations along the mainstream and tributaries had reached their record high. The record-breaking peak flow rate at many stations along the mainstream and tributaries in the middle reaches of the Yangtze River indicated intense precipitation in the area. The heavy rainfall disaster in the Yangtze River Basin could be driven by these reasons. First, the cold air in North China was extraordinary active in 1849, which made it difficult for the subtropical high pressure to move northward. Second, the rain belt stagnated in the Yangtze River Basin for a long time, and the Meiyu period reached 42 days, 62% longer than normal years. Third, the onset of a southwest monsoon was earlier and more active, which provided abundant moisture to the Yangtze River Basin. The great flood disaster was caused by heavy precipitation at the middle reaches, which made it quite different from the other three great floods in the Yangtze River in the 20th century. At present, the large water conservancy projects in the Yangtze River are mainly designed for flood problems caused by rainstorms in the upper reaches of the Yangtze River. The middle reaches of the Yangtze River, however, are facing the weakening of flood diversion capacity, caused by social and economic development. Therefore, future flood prevention measures in the Yangtze River should pay great attention to the threat of this flood pattern.
Based on hourly precipitation from national surface stations, persistent heavy rainfall events (PHREs) over the Sichuan Basin (SCB) are explored during the warm season (May to September) from 2000 to 2015 to compare synoptic circulations and maintenance mechanisms between different PHRE types. There are two main types of PHREs: one is characterized by a rain belt west of 106°E over the SCB (WSB-PHREs), and the other features a rain belt east of 106°E over the SCB (ESB-PHREs). In total, there are 18 ESB-PHREs and 10 WSB-PHREs during the study period. Overall, the rain belts of WSB-PHREs are along the terrain distribution east of the Tibetan Plateau, while the precipitation intensity of ESB-PHREs is stronger. For the two types of PHREs, the shortwave trough over the SCB and the western Pacific subtropical high act as their favorable background environments, particularly for ESB-PHREs. The water vapor of WSB-PHREs is mainly transported from the South China Sea, whereas for ESB-PHREs the South China Sea and Bay of Bengal are their main moisture sources. The composite vorticity budgets of southwest vortices during their mature stage indicate that the convergence effect is a dominant factor for maintaining the two types of PHREs, and the strong vertical vorticity advection is also favorable, but the relative contribution of vertical advection is larger for WSB-PHREs.
摘要
本文筛选出四川盆地西部 (盆西型) 和盆地东部 (盆东型) 持续性暴雨个例, 深入对比两类持续性暴雨的大气环流特征和直接造成持续性暴雨的西南低涡维持的机理.四川盆地的短波槽和西太平洋副热带高压的配置有利于持续性暴雨的维持, 盆东型的降水强度较盆西型个例强, 高空急流位置偏南, 南亚高压的强度更强, 高层辐散更强, 对流层中层副热带高压偏东偏南.盆西型的水汽输送主要来自南海, 而盆东型的水汽输送主要来自南海和孟加拉湾.合成涡度收支的结果表明散度项是两类持续暴雨中西南涡维持的主要原因, 但盆西型中, 垂直平流的作用更强.
This study investigates the responses of the hydroclimate and extremes in the US Midwest to global warming, based on ensemble projections of the Coupled Model Intercomparison Project Phase 6 and the Multi-model Initial-condition Large-ensemble Simulations. The precipitation response features a seasonally dependent change with increased precipitation in April-May but reduced precipitation in July-August. The late-spring wetting is attributed to the enhanced low-level moisture-transporting southerlies, which are induced by regional sea level pressure anomaly linked to the poleward shift of the North America westerly jet (NAWJ). The late-summer drying is attributed to the weakened storm track, which is also linked to the poleward NAWJ shift. The seasonally dependent future changes of the Midwest precipitation are analogous to its climatological seasonal progression, which increases over late spring as the NAWJ approaches the Midwest and decreases over late summer as the NAWJ migrates away. In response to the mean precipitation changes, the extremely wet late springs (April-May precipitation above the 99th percentile of the historical period) and the extremely dry late summer (below the 1st percentile) will occur much more frequently, implying increased late-spring floods and late-summer droughts. Future warming in the Midwest is amplified in late summer due to the reduced precipitation. With amplified background warming and increased occurrence, future late-summer droughts will be more devastating. Our results highlight that, under a time-invariant poleward jet shift, opposite precipitation changes arise before and after the peak rainy month, leading to substantial increases in the subseasonal extremes. The severity of such climate impacts is obscured in projections of the rainy season mean.
Streamflow gives rivers and streams their distinctive character, influencing virtually every physical, chemical, and biological process within fluvial ecosystems. Measurement of streamflow characterizes stream or river size, variability across all time scales and among seasons and river types, and the response to human intervention. Because rivers have enormous value to society as a source of drinking water and means of transport, as well as for hydropower, waste removal, and irrigation, humans have extracted, diverted, and impounded river flows since earliest civilizations. Yet, freshwater ecosystems also need enough water, in the right amounts and at the right time, to remain ecologically intact and provide economically valuable commodities and services to society. Stream gages exist at many locations, some with daily records extending a century or more, providing a rich database for analysis of streamflow variability, quantification of natural variation, and detection of human alteration due to dams, land-use change, and climate. From knowledge of the relationship between streamflow and ecology over a wide range of flows and species, river scientists are beginning to develop the tools to recommend a hydrologic regime that can achieve desired ecological outcomes within a framework that also includes social and economic responses.
Although significant improvements have been made to the prediction and understanding of extreme precipitation events in recent decades, there is still much to learn about these impactful events on the subseasonal timescale. This study focuses on identifying synoptic patterns and precursors ahead of an extreme precipitation event over the contiguous United States (CONUS). First, we provide a robust definition for 14-day “extreme precipitation events” and partition the CONUS into six different geographic regions in order to compare and contrast the synoptic patterns associated with events in those regions. Then, several atmospheric variables from ERA-Interim (e.g., geopotential height and zonal winds) are composited to understand the evolution of the atmospheric state before and during a 14-day extreme precipitation event. Common synoptic signals seen during events include significant zonally-oriented trough-ridge patterns, an energized subtropical jet stream, and enhanced moisture transport into the affected area. Also, atmospheric river activity increases in the specific region during these events. Modes of climate variability and lagged composites are then investigated for their potential use in lead time prediction. Key findings include synoptic-scale anomalies in the North Pacific and regional connections to modes such as the Pacific North American pattern and the North Pacific Oscillation. Taken together, our results represent a significant step forward in understanding the evolution of 14-day extreme precipitation events for potential damage and casualty mitigation.
Flash floods have long been common in Asian cities, with recent increases in urbanization and extreme rainfall driving increasingly severe and frequent events. Floods in urban areas cause significant damage to infrastructure, communities and the environment. Numerical modelling of flood inundation offers detailed information necessary for managing flood risk in such contexts. This study presents a calibrated flood inundation model using referenced photos, an assessment of the influence of four extreme rainfall events on water depth and inundation area in the Hanoi central area. Four types of historical and extreme rainfall were input into the inundation model. The modeled results for a 2008 flood event with 9 referenced stations resulted in an R2 of 0.6 compared to observations. The water depth at the different locations was simulated under the four extreme rainfall types. The flood inundation under the Probable Maximum Precipitation presents the highest risk in terms of water depth and inundation area. These results provide insights into managing flood risk, designing flood prevention measures, and appropriately locating pump stations.
Regional climate models (RCMs) from the North American Regional Climate Change Assessment Program (NARCCAP) are compared with the two gridded precipitation data sets [Climate Prediction Center (CPC) and the University of Washington (UW)] and the North American regional reanalysis (NARR) to examine if RCMs are able to reproduce very heavy precipitation under similar physical conditions seen in observations. The analysis focuses on contemporary climate (1982–1999) in an upper Mississippi region during the summer (June–July–August) months and utilizes output from NARCCAP RCMs forced with a reanalysis and atmosphere–ocean global climate models (AOGCMs).
The NARCCAP models generally reproduce the precipitation frequency versus intensity spectrum seen in observations up to around 25 mm day ⁻¹ , before producing overly strong precipitation at high intensities. CRCM simulations produce lower precipitation amounts than the rest of the models and observations past the 25 mm day ⁻¹ threshold. Further analysis focuses on precipitation events exceeding the 99.5th percentile that occur simultaneously at several points in the region, yielding ‘widespread events’. Apart from the CRCM and EPC2 simulations, models and observations produce peaks in widespread events during 0300–0900 UTC, although the models typically produce slightly weaker intensities compared to observations. Widespread precipitation falls too frequently throughout the day, especially between 1500 and 2100 UTC, compared to observations. Composite precipitation shows inter‐model differences in magnitude and location of widespread events. Examination of additional fields shows that NARCCAP models produce credible representations of very heavy precipitation and their supporting environments when compared to the NARR.
Convective systems can, in addition to generating strong winds, hail, and lightning, produce large amounts of rainfall. In some cases, it is the heavy rain itself that distinguishes the storm, even when the rain is accompanied by one or more of the other defining features of severe storms (Maddox et al. 1979; Changnon 1999). Whereas rain is usually a favorable product of moist convection in the atmosphere, excessive amounts or rates of rainfall can lead to surface erosion, property or crop damage, and devastating floods (e.g., Pontrelli et al. 1999). Rain production carries with it atmospheric implications, as well, so it is important that we understand the physical processes that give rise to precipitation-size hydrometeors in convective systems (Ludlam 1963).
Proxy variables from palaeolimnological studies of lakes in the Prairie Pothole Region of North America have been used to infer large oscillations during the late Holocene between longer periods of high-salinity–dry conditions and shorter periods of low-salinity–wet conditions producing a normative pattern marked by the absence of hydrological stability. Studies of the historical rise in lake level at Devils Lake have identified 1980 as a transition point between two such hydroclimatic modes. This study uses multiple datasets to characterize the mean hydroclimatological and hydrological conditions of these two climatic modes. Mode 1 is a cool and dry phase, and mode 2 is a warmer and wetter phase. Precipitation onto the lake increased by 24% from mode 1 to mode 2. This small, but sustained, increase produced significant changes in the mean hydroclimatic and hydrological states for the basin, including a 383% increase in surface run-off to the lake, and a 282% increase in the basin run-off ratio. Devils Lake Basin is located along a hydrotone (region of strong hydroclimatic gradients) where small changes in hydrological drivers are amplified into large changes in regional moisture. The effects of the fluctuating climatic modes and strong hydroclimatic gradients are probably further amplified by the unique fill–spill hydrology of the northern glaciated plains, which can result in nonlinear precipitation–run-off relationships. This natural pattern of extreme hydrological variations for Devils Lake produces enormous challenges for lake management.
A coupled atmosphere–land-surface mesoscale model is used to assess the responses of precipitation to soil-moisture anomalies in two regions: (1) the core region of the North American Monsoon (NAM; 105°–112°W, 24°–36°N); (2) the central–southern United States (CS-US; 85°–95°W, 30°–36°N). Results from a series of numerical experiments integrated from July to September 2000 show that precipitation increases in the NAM region in July with a prescribed wet soil-moisture anomaly; meanwhile, precipitation decreases in the CS-US region. In the following months, when the prescribed wet soil-moisture anomaly in the NAM region was removed, the increase in precipitation in the NAM region becomes weaker and shifts eastward to the CS-US region. By September, an inverse precipitation seesaw in these two regions is built up. Except for local evaporation, the transportation of atmospheric moisture affects the interaction between soil moisture and precipitation, especially in the regions and periods without the prescribed soil-moisture anomaly. The soil-moisture anomaly in the NAM region is only partially responsible for the precipitation seesaw in the southern United States.
Flood type classification is an optimal tool to cluster floods with similar meteorological triggering conditions. Under climate change these flood types may change differently as well as new flood types develop. This paper presents a new methodology to classify flood types, particularly for use in climate change impact studies. A weather generator is coupled with a conceptual rainfall-runoff model to create long synthetic records of discharge to efficiently build an inventory with high number of flood events. Significant discharge days are classified into causal types using k-means clustering of temperature and precipitation indicators capturing differences in rainfall amount, antecedent rainfall and snow-cover and day of year. From climate projections of bias-corrected temperature and precipitation, future discharge and associated change in flood types are assessed. The approach is applied to two different Alpine catchments: the Ubaye region, a small catchment in France, dominated by rain-on-snow flood events during spring, and the larger Salzach catchment in Austria, affected more by rainfall summer/autumn flood events. The results show that the approach is able to reproduce the observed flood types in both catchments. Under future climate scenarios, the methodology identifies changes in the distribution of flood types and characteristics of the flood types in both study areas. The developed methodology has potential to be used flood impact assessment and disaster risk management as future changes in flood types will have implications for both the local social and ecological systems in the future.
Soil moisture and water balance for global and regional scales have been calculated using a land-surface process model (SiB2) forced by observed and model assimilated data. The simulated runoff for each grid cell has been provided as input to a global river routing model, in order to simulate river discharge rates. The simulated soil moisture and water balance have been compared with available observations for their annual mean and seasonal cycles and for global, basin and grid point scales. The global distributions of the annual-mean soil moisture and wetness have been reasonably simulated. There were large inter-annual variations of soil moisture in both the simulations and observations at local stations. The simulated annual discharges for major river basins agree reasonably well with observations, but with some underestimates for large discharges and some overestimates for small discharges. The seasonal cycle of river discharges has been well simulated for specific basins in the tropics, midlatitudes, and high latitudes, although for some basins the annual mean is underestimated. In the tropics, the seasonal cycles of soil moisture and the surface water balance are dominated by the precipitation cycle. In mid- and high latitudes, soil moisture and the water balance are affected by both the temperature and precipitation cycles, and by the snow accumulation/melting cycle. The range of seasonal soil moisture variations becomes smaller with increasing latitude. The seasonal cycles of soil moisture for selected grid points have been compared with selected station observations. Even though there are differences in forcing and in some specific surface boundary parameters at the stations, the simulated soil moisture agrees well with multiyear observations at a majority of the stations. However, for almost all the selected grid cells, the seasonal variations are smaller, the snow melt and soil drying processes are late by about one month, and the soil is relatively wet in summer, compared with observations. These errors can be partly attributed to the unrealistically cool temperatures provided to the model as forcing data, favoring less surface evaporation and a later seasonal cycle, especially for mid- and high latitudes.
Synopsis Changes in extreme weather and climate events have significant impacts and are among the most serious challenges to society in coping with a changing climate. Many extremes and their associated impacts are now changing. For example, in recent decades most of North America has been experiencing more unusually hot days and nights, fewer unusually cold days and nights, and fewer frost days. Heavy downpours have become more frequent and intense. Droughts are becoming more severe in some regions, though there are no clear trends for North America as a whole. The power and frequency of Atlantic hurricanes have increased substantially in recent decades, though North American mainland land-falling hurricanes do not appear to have increased over the past century. Outside the tropics, storm tracks are shifting northward and the strongest storms are becoming even stronger. It is well established through formal attribution studies that the global warming of the past 50 years is due primarily to human-induced increases in heat-trapping gases. Such studies have only recently been used to determine the causes of some changes in extremes at the scale of a continent. Certain aspects of observed increases in temperature extremes have been linked to human influences. The increase in heavy precipitation events is associated with an increase in water vapor, and the latter has been attributed to human-induced warming. No formal attribution studies for changes in drought severity in North America have been attempted. There is evidence suggesting a human contribution to recent changes in hurricane activity as well as in storms outside the tropics, though a confident assessment will require further study. In the future, with continued global warming, heat waves and heavy downpours are very likely to further increase in frequency and intensity. Substantial areas of North America are likely to have more frequent droughts of greater severity. Hurricane wind speeds, rainfall intensity, and storm surge levels are likely to increase. The strongest cold season storms are likely to become more frequent, with stronger winds and more extreme wave heights. Current and future impacts resulting from these changes depend not only on the changes in extremes, but also on responses by human and natural systems.
The 1927 flood in the Lower Mississippi River was the most destructive flood in American history, inundating more than 70,000 km2 of land, resulting in approximately 500 fatalities and leaving more than 700,000 people homeless. Despite the prominence of the 1927 flood, details on the flood, and the storms that produced the flood, are sparse. We examine the hydrometeorology and hydroclimatology of the 1927 flood in the Lower Mississippi River through downscaling simulations of the storms that were responsible for catastrophic flooding and through empirical analyses of rainfall and streamflow records. We use Twentieth Century Reanalysis fields as boundary conditions and initial conditions for downscaling simulations using the Weather Research and Forecasting (WRF) model. We place the hydrometeorological analyses of the 1927 storms in a hydroclimatological context through analyses of the Twentieth Century Reanalysis fields. Analyses are designed to assess the physical processes that control the upper tail of flooding in the Lower Mississippi River. We compare the 1927 flood in the Lower Mississippi River to floods in 1937 and 2011 that represent the most extreme flooding in the Lower Mississippi River. Key Points:: The 1927 flood in the Lower Mississippi River was the most damaging flood in U.S. history Rainfall over the Lower Mississippi River is related to water vapor flux from Gulf of Mexico WRF model results illustrate utility of simulations using Twentieth Century Reanalysis for floods
Ensembles of winter and summer seasonal hindcasts have been carried out with an 80-km resolution version of the National Centers for Environmental Prediction, Environmental Modeling Center Eta model over the North American region. The lateral boundary conditions for the Eta model are prescribed from Center for Ocean-Land-Atmosphere R40 atmospheric general circulation model integrations, which used observed atmospheric initial conditions and observed global sea surface temperature (SST). An examination of 15 seasonal winter hindcasts and 15 seasonal summer hindcasts shows that the nested model reduces the systematic errors in seasonal precipitation compared to the global model alone. The physical parameterizations, enhanced resolution, and better representation of orography in the Eta model produces better simulations of precipitation, and in some cases, its interannual variability. In particular, the precipitation difference between the 1988 drought and 1993 flood over the United States was much better simulated by the nested model. The predictions of circulation features were generally as good or better than those from the global model alone. Estimates of external (SST forced 'signal') and internal (dynamics generated 'noise') variability were made for both the global model and the nested model predictions. Contrary to the expectation that a higher-resolution model would have higher internal-dynamics-generated variability, the signal, noise, and signal-to-noise ratios of the near-surface temperature and precipitation fields were generally quite similar between the nested model and the global model predictions. In the winter season the nested model had larger signal-to-noise ratios in both temperature and precipitation than did the global model alone.
This paper undertakes a hydrometeorological analysis of flood events in the central United States. Vertically integrated horizontal water vapor transport over 1979-2011 is calculated in the ECMWF Interim Re-Analysis (ERA-Interim) and used in an algorithm to identify episodes of high moisture transport, or atmospheric rivers (ARs), over the central United States. The AR events are almost evenly divided among the seasons (143 during the winter, 144 during the spring, and 124 during the fall), with a minimum (40) during the summer. The annual maxima (AM) floods from 1105 basins over the period 1980-2011 are used as a measure of the hydrologic impact of the AR events. Of these basins, 470 (or 42.5%) had more than 50% of their AM floods linked to ARs. Furthermore, 660 of the 1105 basins (59.7%) had 5 or more of their top 10 AM floods related to ARs, indicating that ARs control the upper tail of the flood peak distribution over large portions of the study area. The seasonal composite average of mean sea level pressure anomalies associated with the ARs shows a trough located over the central United States and a ridge over the U.S. East Coast, leading to southerly winds and the advection of moisture over the study region. Based on the findings of this study, ARs are a major flood agent over the central United States.
The frequency and intensity of heavy rainfall events have increased in the Central U.S. over the last several decades and model projections from dynamical downscaling suggest a continuation with climate change. In this study, we examine how climate change might affect mechanisms related to the development of heavy rainfall events that occur on the scale of mesoscale convective systems (MCSs) over the Central U.S. To accomplish these goals, we incorporate dynamical downscaled simulations of two CMIP5 models in the Weather Research and Forecasting (WRF) model that accurately simulate heavy rainfall events. For each model, a set of heavy rainfall events that match the frequency, timing, and intensity of observed events are objectively identified in historical and future simulations. We then examine multi-model composites of select atmospheric fields during these events in simulations of historical and future scenarios, enabling an identification of possible physical mechanisms that could contribute to the intensification of heavy rainfall events with climate change. Simulations show that additional moisture is transported into convective updrafts during heavy rain events in future simulations, driving stronger evaporative cooling from the entrainment of drier mid-tropospheric air. This results in the formation of a stronger low-level cold pool, which enhances moisture convergence above the cold pool and increases rainfall rates during future heavy precipitation events. In addition, a warmer profile in future simulations might allow for heavier rainfall rates as a deeper atmospheric column can support additional collision-coalescence of liquid hydrometeors.
AbstractThe authors examine the hydroclimatology, hydrometeorology, and hydrology of extreme floods through analyses that center on the June 2008 flooding in Iowa. The most striking feature of the June 2008 flooding was the flood peak of the Cedar River at Cedar Rapids (3964 m3 s?1), which was almost twice the previous maximum from a record of 110 years. The spatial extent of extreme flooding was exceptional, with more U.S. Geological Survey stream gauging stations reporting record flood peaks than in any other year. The 2008 flooding was produced by a sequence of organized thunderstorm systems over a period of two weeks. The authors examine clustering and seasonality of flooding in the Iowa study region and link these properties to features of the June 2008 flood event. They examine the environment of heavy rainfall in Iowa during June 2008 through analyses of composite rainfall fields (15-min time interval and 1-km spatial resolution) developed with the Hydro-NEXRAD system and simulations using the W
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