TABLE 2 - uploaded by Andreas Muhlbauer
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Local budgets for upslope, crest, downslope, and total precipitation as well as the spillover factor after 10 h. Units are mm (10 h) 1 for the precipitation values. All precipitation values are rounded to the first decimal place.
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Aerosols serve as a source of cloud condensation nuclei (CCN) and influence the microphysical properties of clouds. In the case of orographic clouds, it is suspected that aerosol–cloud interactions reduce the amount of precipitation on the upslope side of the mountain and enhance the precipitation on the downslope side when the number of aerosols i...
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Context 1
... contrast to the clean case, the polluted case shows an inhibition of the upslope precipitation together with a shift of the precipitation maximum of roughly 6 km farther toward the leeward side. As a consequence, the spillover factor increases nearly to 1 in the polluted case (see also Table 2). Hence, almost all precipitation that is generated by the mountain wave falls on the leeward side. ...
Context 2
... up- stream shift of the precipitation pattern is also evident in the polluted case but the maximum precipitation re- mains located on the downstream side of the mountain and the amount of upslope precipitation is reduced. Again, the spillover factor increases with increased aerosol load from 0.58 in the clean case to 0.89 in the polluted case (see also Table 2) but is generally lower than for the narrow mountain, which is in agreement with the findings of Jiang (2003). Here, the effects of adding aerosols to the mountain flow can be seen as a decrease of the effective mountain half-width since, generally, more precipitation is advected downstream and the spillover factor is increased. ...
Context 3
... the effects of adding aerosols to the mountain flow can be seen as a decrease of the effective mountain half-width since, generally, more precipitation is advected downstream and the spillover factor is increased. This decrease in the spillover factor with increased mountain half-width is consistently found for all simulations in experiment A and the details are summarized in Table 2. Figure 7 shows the spatial precipitation differences for different mountain widths. ...
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Citations
... This phenomenon was found in observations as well as model simulations (Givati and Rosenfeld, 2004;Zubler et al., 2011a;Saleeby et al., 2013), and is commonly referred to as the spillover effect. In an orographic setting, this spillover effect implies a decrease in windward precipitation while moisture transport over the mountain ridge is enhanced, which will eventually result in an increase of precipitation on the leeward side (Lynn et al., 2007;Muhlbauer and Lohmann, 2008;Xiao et al., 2014). ...
... In both CCN perturbation simulations, the increased CCN concentrations cause more numerous, but smaller, cloud droplets which delay the precipitation process. This decreased loss of cloud water on the southern side of the Alpine ridge leads to a build-up in QLIQ on the northern side of the Alpine ridge which in turn increases precipitation formation according to the spillover effect (Figures 7g-i, 8g-i and Jiang, 2003;Muhlbauer and Lohmann, 2008). Increased riming rates in the perturbed simulations may further contribute to the spillover effect and the increase in surface precipitation north of the Alpine ridge ( Figure S4a). ...
... Elevated CCN concentrations substantially increase QLIQ, and decrease QICE between the melting and the homogeneous freezing levels south of the Alpine ridge during Peak 1. The build-up of QLIQ is carried towards the north of the Alpine ridge where surface precipitation is increased according to the spillover effect (Jiang, 2003;Muhlbauer and Lohmann, 2008). In addition, the position of deep convective cells shift in the CCN perturbation simulations as compared to CTRL, which may also cause locally large differences in surface precipitation. ...
Changes in the ambient aerosol concentration are known to affect the microphysical properties of clouds. Especially regarding precipitation formation, increasing aerosol concentrations are assumed to delay the precipitation onset, but may increase precipitation rates via convective invigoration and orographic spillover further downstream. In this study, we analyse the effect of increased aerosol concentrations on a heavy precipitation event observed in summer 2017 over northeastern Switzerland, an event which was considerably underestimated by the operational weather forecast model. Preceding the precipitation event, Saharan dust was advected towards the Alps, which could have contributed to increased precipitation rates north of the Alpine ridge. To investigate the potential impact of the increased ambient aerosol concentrations on surface precipitation, we perform a series of sensitivity simulations using the Consortium for Small‐scale Modeling (COSMO) model with different microphysical parametrizations and prognostic aerosol perturbations. The results show that the choice of the microphysical parametrization scheme in terms of a one‐ or two‐moment scheme has the relatively largest impact on surface precipitation rates. In the one‐moment scheme, surface precipitation is strongly reduced over the Alpine ridge and increased further downstream. Simulated changes in surface precipitation in response to aerosol perturbations remain smaller in contrast to the impact of the microphysics scheme. Elevated cloud condensation nuclei (CCN) concentrations lead to increased cloud water and decreased cloud ice mass, especially in regions of high convective activity south of the Alps. These altered cloud properties indeed increase surface precipitation further downstream, but the simulated change is too small to explain the observed heavy precipitation event. Additional ice‐nucleating particles (INPs) increase cloud ice mass, but only trigger local changes in downstream surface precipitation. Thus, increased aerosol number concentrations during the Saharan dust outbreak are unlikely to have caused the heavy precipitation event in summer 2017.
... In this regard, the work of Choudhury et al. (2019) performs a thorough review on the state-of-the-art literature on the Aerosol-Orography-Precipitation (AOP) interaction, in which an overall orographic precipitation reduction is concluded for increased aerosol conditions. Accompanied by this overall reduction is also an spillover factor (leeward over windward precipitation) increase when increasing aerosol, according to Muhlbauer and Lohmann (2008). This may be due to the formation of smaller size cloud droplets showing a longer lifetime before growing into raindrops, leading to an enhancement of the cloud LWP, an effect usually referred to as the lifetime effect or the second kind of indirect effect (Albrecht, 1989). ...
The present study aims at disentangling the role of marine aerosols in extreme precipitation events of orographic nature over near-maritime locations. Being the hazardous character of these events mainly associated to runoffs and river floodings, their destructive potential makes them subjects of special interest. In pursuit of a deep understanding on the development mechanisms of such events, this study reproduces the conditions and development of an event ranging from 20 to 27 of December 2009 that took place in Southern Spain. During this event, a series of Atlantic fronts swept the southernmost part of the Iberian Peninsula which, along with water vapor entrainment in the form of an atmospheric river, produced rainfall accumulations up to 900 mm in some specific mountainous locations. With this purpose, a number of simulations are conducted to study the microphysical processes and aerosol-cloud interactions governing the rain production. The WRF model is utilized with two different configurations of the Morrison double-moment microphysics scheme: (1) with prescribed type and concentrations of aerosols (PA); and (2) a setup in which a chemistry module is coupled to the meteorological core (Interactive Aerosols, IA), allowing for the consideration of prognostic aerosol concentrations and other relevant aspects such as the hygroscopicity and dry size of the different chemical species. According to the validation performed with a database constructed from measurements of rain gauges, results show that using prognostic aerosols concentrations for natural aerosols, namely sea salt and dust, improves the model's skill for reproducing extreme rainfall with respect to the setup using the default aerosols distribution prescribed in the WRF-alone simulations. Additionally, IA simulations increase windward precipitation and reduce leeward rainfall, thus largely amplifying the spatial variability of precipitation along the direction of the moisture flux. Besides, it is observed that this effect is independent of the model resolution. Consequently, it is concluded that a realistic aerosols concentration, along with a detailed treatment of both the size and hygroscopic properties of aerosols, have a crucial impact on the microphysical processes involved in this type of orographically-induced extreme precipitation events. Specifically, under a regime of low concentration of cloud condensation nuclei originating from natural aerosols, the formed cloud droplets are more prone to grow to precipitating size, thus enhancing the auto-conversion rate.
... Previous studies often used BC as an unreactive tracer to evaluate these cloud-relevant microphysics processes. These processes are sensitive to the assumed BC mixing state, e.g., an increased externally mixed hydrophobic BC concentration may enhance the ice nucleation and promote the riming process and cloud loss 169 . Long-term measurements 166 at an alpine site influenced by orographic clouds showed that the scavenging efficiency of BC was reduced from 60 to 5-10% when the ice mass fraction of clouds increased from 0 to >0.2. ...
Light-absorbing carbonaceous aerosols (LACs), including black carbon and light-absorbing organic carbon (brown carbon, BrC), have an important role in the Earth system via heating the atmosphere, dimming the surface, modifying the dynamics, reducing snow/ice albedo, and exerting positive radiative forcing. The lifecycle of LACs, from emission to atmospheric evolution further to deposition, is key to their overall climate impacts and uncertainties in determining their hygroscopic and optical properties, atmospheric burden, interactions with clouds, and deposition on the snowpack. At present, direct observations constraining some key processes during the lifecycle of LACs (e.g., interactions between LACs and hydrometeors) are rather limited. Large inconsistencies between directly measured LAC properties and those used for model evaluations also exist. Modern models are starting to incorporate detailed aerosol microphysics to evaluate transformation rates of water solubility, chemical composition, optical properties, and phases of LACs, which have shown improved model performance. However, process-level understanding and modeling are still poor particularly for BrC, and yet to be sufficiently assessed due to lack of global-scale direct measurements. Appropriate treatments of size-and composition-resolved processes that influence both LAC microphysics and aerosol-cloud interactions are expected to advance the quantification of aerosol light absorption and climate impacts in the Earth system. This review summarizes recent advances and up-to-date knowledge on key processes during the lifecycle of LACs, highlighting the essential issues where measurements and modeling need improvement.
... Observational and modeling studies show that cloud droplet size and collision efficiency between droplets are decreased with increasing aerosol loading, and then orographic precipitation will be suppressed and shift to the downwind side of mountain (Lynn et al., 2007;Muhlbauer and Lohmann, 2008;Xiao et al., 2014). When including icephase particles, more small droplets can be transferred above the freezing level and participate more in ice-phase or mixed-phase microphysical processes, leading to more uncertain in precipitation (Zubler et al., 2011;Fan et al., 2014;Xiao et al., 2016). ...
Autoconversion is an important role in describing initial formation of raindrops through droplets collision-coalescence, especially in discussing aerosol-cloud interactions. Seven autoconversion schemes have been adopted to investigate aerosol particles severing as cloud condensation nuclei (CCN) effect on mixed-phase orographic cloud. As CCN represented by initial droplet concentration is increased, more cloud droplets caused by a suppression of autoconversion rate benefit for snow growth by accreting droplets, leading to a delay in precipitation. Moreover, the loss of rain growth induced by a decrease in the accretion of cloud droplets by rain with increasing CCN can, to some extent, be offset by an enhancement of snow growth. As the freezing level is decreased, ice-phase particles become more effective, and then the decrease in precipitation becomes less obvious. Compared analysis finds that sensitive degrees of surface precipitation, hydrometeors and their related microphysical processes vary from different autoconversion schemes. Among them, the decrease in precipitation induced by increasing CCN is found to range from 3.2% to 36.3% under different schemes, while the increase in spillover is changed from 2.9% to 86.4%. The decrease in the accretion of cloud droplets by rain varies from 4.47% to 62.5%, and then the increase in snow melting can be changed from 7.32% to 31.8%. Hence, it should be pay attention to autoconversion scheme in estimating aerosol-cloud-precipitation interactions. Finally, based on the SCE (the stochastic collection equation) scheme, Berry scheme and Seifert and Beheng scheme are more appropriate for clean background condition, while Khairoutdinov and Kogan scheme and Liu and Daum scheme are applicable to polluted condition.
... In the case of 100 cm −3 , the formation time of raindrops advances from 24 min in the weak thundercloud to 18 min in the severe thundercloud. This is explained by a stronger convection intensity that leads to a quicker warm rain production by auto-conversion of cloud droplet-rain and the collection of cloud droplets by raindrops [47,[49][50][51][52]. Additionally, a higher concentration of CCN reduces raindrop number concentration and mixing ratio, and stronger thundercloud leads to a more obvious reduction (see Figure 5g,j-l). ...
Numerical simulations are performed to investigate the effect of varying CCN (cloud condensation nuclei) concentration on dynamic, microphysics, electrification, and charge structure in weak, moderate, and severe thunderstorms. The results show that the response of electrification to the increase of CCN concentration is a nonlinear relationship in different types of thunderclouds. The increase in CCN concentration leads to a significant enhancement of updraft in the weak thunderclouds, while the high CCN concentration in moderate and severe thunderclouds leads to a slight reduction in maximum updraft speed. The increase of the convection promotes the lift of more small cloud droplets, which leads to a faster and stronger production of ice crystals. The production of graupel is insensitive to the CCN concentration. The content of graupel increases from low CCN concentration to moderate CCN concentration, and slightly decreases at high CCN concentration, which arises from the profound enhancement of small ice crystals production. When the intensity of thundercloud increases, the reduction of graupel production will arise in advance as the CCN concentration increases. Charge production tends to increase as the aerosol concentration rises from low to high in weak and moderate thundercloud cases. However, the magnitude of charging rates in the severe thundercloud cases keeps roughly stable under the high CCN concentration condition, which can be attributed to the profound reduction of graupel content. The charge structure in the weak thundercloud at low CCN concentrations keeps as a dipole, while the weak thunderclouds in the other cases (the CCN concentration above 100 cm−3) change from a dipole charge structure to a tripole charge structure, and finally disappear with a dipole. In cases of moderate and severe intensity thunderclouds, the charge structure depicts a relatively complex structure that includes a multilayer charge region.
... By comparing two scenarios of maritime and continental aerosols, Lynn et al. (2007) found that simulations with maritime aerosols with relatively lower aerosol number concentrations yielded 30 % more precipitation than continental aerosols over a mountain slope. Such local effects can translate into large spatial shifts in clouds and precipitation, as aerosol-cloud interactions (ACIs) inducing suppression of precipitation upwind could give rise to the enhancement of precipitation downwind (Muhlbauer and Lohmann, 2008), thus modifying the spatial distribution of orographic precipitation, transferring precipitation from one watershed to another, and strongly influencing the local and regional hydrology. Yang et al. (2016) examined the reasons for warm rain suppression due to increased air pollution in the Mt. ...
A new cloud parcel model (CPM) including activation,
condensation, collision–coalescence, and lateral entrainment processes is
used to investigate aerosol–cloud interactions (ACIs) in cumulus development
prior to rainfall onset. The CPM was applied with surface aerosol
measurements to predict the vertical structure of cloud development at early
stages, and the model results were evaluated against airborne observations of
cloud microphysics and thermodynamic conditions collected during the
Integrated Precipitation and Hydrology Experiment (IPHEx) in the inner region
of the southern Appalachian Mountains (SAM). Sensitivity analysis was
conducted to examine the model response to variations in key ACI
physiochemical parameters and initial conditions. The CPM sensitivities
mirror those found in parcel models without entrainment and
collision–coalescence, except for the evolution of the droplet spectrum and
liquid water content with height. Simulated cloud droplet number
concentrations (CDNCs) exhibit high sensitivity to variations in the initial
aerosol concentration at cloud base, but weak sensitivity to bulk aerosol
hygroscopicity. The condensation coefficient ac plays a governing
role in determining the evolution of CDNC, liquid water content (LWC), and
cloud droplet spectra (CDS) in time and with height. Lower values of
ac lead to higher CDNCs and broader CDS above cloud base, and
higher maximum supersaturation near cloud base. Analysis of model simulations
reveals that competitive interference among turbulent dispersion, activation,
and droplet growth processes modulates spectral width and explains the
emergence of bimodal CDS and CDNC heterogeneity in aircraft measurements from
different cloud regions and at different heights. Parameterization of
nonlinear interactions among entrainment, condensational growth, and
collision–coalescence processes is therefore necessary to simulate the
vertical structures of CDNCs and CDSs in convective clouds. Comparisons of
model predictions with data suggest that the representation of lateral
entrainment remains challenging due to the spatial heterogeneity of the
convective boundary layer and the intricate 3-D circulations in mountainous
regions.
... Previous studies by [27][28][29][30] over the central Himalayas in Nepal have shown that the space-time distribution of rainfall and the terrain are strongly intertwined in the region. Depending upon the type of cloud systems and synoptic conditions, changes in aerosol number concentration and chemical properties influence the microphysical pathways of ACPI in different ways, resulting in suppression of rainfall, storm invigoration, and even spatial displacement of rainfall [10,13,[31][32][33][34][35]. The particle sizes measured during the Joint Aerosol Monsoon Experiment (JAMEX09) in Central Nepal indicate that the dominant aerosol mode is around 100 nm [26], which is also consistent with the predominance of fine aerosol (< 350 nm) found by [36] in the Himalayan foothills using MISR (Multi Imaging Spectro-Radiometer) observations. ...
... Microphysics budgets and tracking of dynamical feedbacks can be more easily carried out in simulations of isolated storm events [54] or under idealized and controlled conditions [33]. The present study consists of a succession of multiple cells and airflow over complex topography, and thus the approach is to conduct the analysis over selected subdomains within d04. ...
In mountainous regions, the nonlinear thermodynamics of orographic land-atmosphere interactions (LATMI) in organizing and maintaining moisture convergence patterns on the one hand, and aerosol-cloud-precipitation interactions (ACPI) in modulating the vertical structure of precipitation and space-time variability of surface precipitation on the other, are difficult to separate unambiguously because the physiochemical characteristics of aerosols themselves exhibit large sub-regional scale variability. In this chapter, ACPI in the Central Himalayas are examined in detail using aerosol observations during JAMEX09 (Joint Aerosol Monsoon Campaign 2009) to specify CCN activation properties for simulations of a premonsoon convective storm using the Weather Research and Forecasting (WRF) version 3.8.1. The focus is on contrasting AIE during episodes of remote pollution run-up from the Indo-Gangetic Plains and when only local aerosols are present in Central Nepal. This study suggests strong coupling between the vertical structure of convection in complex terrain that governs the timescales and spatial organization of cloud development, CCN activation rates, and cold microphysics (e.g. graupel production is favored by slower activation spectra) that result in large shifts in the spatial distribution of precipitation, precipitation intensity and storm arrival time.
... By comparing two scenarios of maritime and continental aerosols, Lynn et 5 al. (2007) found that simulations with maritime aerosols with relatively lower aerosol number concentration yielded 30% more precipitation than continental aerosols over a mountain slope. Such local effects can translate into large spatial shifts in clouds and precipitation as aerosol-cloud interactions (ACI) inducing suppression of precipitation upwind could give rise to the enhancement of precipitation downwind (Muhlbauer and Lohmann, 2008), thus modifying the spatial distribution of orographic precipitation, transferring precipitation from one watershed to another and thus strongly influencing the local and 10 regional hydrology. Yang et al. (2016) examined the reasons for warm rain suppression due to increased air pollution in the Mt. ...
A new cloud parcel model (CPM) including activation, condensation, collision-coalescence, and lateral entrainment processes is presented here to investigate aerosol-cloud interactions (ACI) in cumulus development prior to rainfall onset. The CPM was applied with surface aerosol measurements to predict the vertical structure of cloud development at early stages, and the model results were compared with airborne observations of cloud microphysics and thermodynamic conditions collected during the Integrated Precipitation and Hydrology Experiment (IPHEx) in the inner region of the Southern Appalachian Mountains (SAM). Sensitivity analysis was conducted to examine the model response to variations in key ACI physical parameters. The condensation coefficient ac plays a governing role in determining the cloud droplet number concentration (CDNC), liquid water content (LWC), and droplet size distribution. Lower values of ac lead to higher cloud droplet number concentrations, broader droplet spectra, and higher maximum supersaturation near cloud base. The simulated vertical structure of CDNC exhibits strong nonlinear sensitivity to entrainment strength and condensation efficiency illustrative of competitive interference between turbulent dispersion and activation processes. Further, simulated CDNC exhibits high sensitivity to variations in initial aerosol concentration at cloud base, but weak sensitivity to aerosol hygroscopicity. These findings provide new insights into determinant factors of convective cloud formation leading to mid-day warm season rainfall in complex terrain.
... In Jha [4], we examined the combined effect of dust and aerosol pollution on orographic precipitation in the CRB for 2004-2005 winter season. It was found that dust tends to enhance precipitation primarily by acting as IN, while aerosol pollution reduces water resources in the CRB through the "spillover" effect [5][6][7], by enhancing cloud droplet concentrations and reducing riming rates. It was found that the combined response to dust and aerosol pollution is a net reduction of water resources in the CRB. ...
In this study, we examine the cumulative e ect of pollution aerosol and dust acting as cloud nucleating aerosol;cloud condensation nuclei (CCN), giant cloud condensation nuclei, and ice nuclei (IN), on orographic precipitation in the Rocky Mountains. We analyze the results of sensitivity studies for speci c cases in 2004-2005 winter season to analyze the relative impact of aerosol pollution and dust acting as CCN and IN on precipitation in the Colorado River Basin. Dust is varied from 3 to 10 times in the experiments, and the response is found to be nonmonotonic and depends on various environmental factors. e sensitivity studies show that adding dust in a wet system increases precipitation when IN e ects are dominant. For a relatively dry system high concentrations of dust can result in overseeding the clouds and reductions in precipitation. However, when adding dust to a system with warmer cloud bases where drizzle formation is active, the response is nonmonotonic.
... Furthermore, the pristine air mass across the Caliafornia Mountains produce 30% more precipitation than the aged polluted air [25]. An increasing aerosol number concentrations over the Swiss Alps (e.g., in the region of the Jungfraujoch Mountain) seemed to suppress warm cloud processes, leading to a decrease in orographic precipitation on the upslope of the mountains [26]. However, the relation between aerosols and precipitation is not always linear, which may depend on environmental conditions of the study area [24]. ...
Increasing the amount of anthropogenic aerosols over the Himalayas modulate cloud properties, thereby altering cloud phase and cloud height, consequently influence formation and distribution of orographic precipitation. Moreover, further rises in global temperature may influence cloud properties by the ‘Clausius – Clapeyron effect’, which increases moisture holding capacity of air.
This study presents sensitivity of simulated cloud properties to aerosol and temperature perturbations using the Weather Research and Forecasting (WRF) model, coupled with a bulk microphysics scheme, in a convection permitting configuration applied to a complex topographical region, the Nepal Himalayas. We find that the effect of aerosol on the simulated rainfall is nonlinear, ranging from -3% to +4% depending on the investigated aerosol perturbation scenarios. The model results highlight a realistic simulation of the 1st indirect (Twomey) effect. However, the rainfall was not overly sensitive to the aerosol perturbations and not statistically significant at the 95% confidence interval. The oversimplified parameterization of ice phase processes, a dominant cloud formation process over the Himalayas, appears to play a crucial role in buffering the sensitivity to increased aerosol loading. Our results, however, show that aerosol perturbations may modify shape, size and spatial distribution of individual cloud and their precipitation production. In contrast, the impact of temperature perturbations is more than the aerosol effect, ranging from -17% to +93%, which is statistically significant at the 95% confidence interval, suggesting that more intense rain events are likely as the climate warms in this region.
