Figure - available from: Geophysical Research Letters
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Budget of moisture in the Tropical Tropopause Layer (TTL). A layer of the TTL between altitude levels z and 19 km and latitudes 30°N and 30°S is considered. Moisture enters the layer through vertical advection of vapor and ice (corresponding fluxes: Vadv and Iadv) and is also imported through horizontal exchanges (the corresponding flux Vhoriz is negative for most altitudes with our sign convention). Moisture leaves the layer by sedimentation (corresponding flux: Ised) and as vapor enters the stratosphere (corresponding flux: Vstrat).
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In the tropics, the tropopause is exceptionally cold and air entering the stratosphere is dehydrated down to a few parts per million leading to the extreme dryness of Earth’s stratosphere. Deep convection typically detrains a few kilometers below the tropopause, but the few storms that may reach up to the tropopause could have an outsize effect on...
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Citations
... The advent of global storm-resolving models (GSRM, [12][13][14]) allows to investigate the importance of individual frozen hydrometeor pathways within a single, coherent model explicitly resolving many of the dynamical processes involved such as deep convection [15,16], dynamically forced slow upwelling [4], cloud lofting from cloud radiative heating [17] as well as in-cloud turbulence and gravity waves [18,19]. GSRMs are also able to simulate both the large-scale mean flow and dynamically forced slow ascent [20], which can not be captured by limited-domain models. ...
... The analysis shows that cold-point overshooting convection is the most important transport pathway for frozen hydrometeors in the tropical coldpoint tropopause, both in PTB and CTL. The prominence of convective events in the tropical cold-point tropopause is in line with the findings by a modeland an observation-based study [36,16]. ...
The exceptionally low temperature in the tropical tropopause layer (TTL) restricts the amount of water vapor entering the stratosphere.
However, moisture may also enter the stratosphere in its frozen state, and the amount thereof depends on hydrometeor sedimentation and air vertical velocity. We investigate the sensitivity of frozen hydrometeor transport pathways to substantial perturbations
of the TTL temperature structure in global storm-resolving model simulations. A special focus is laid on the question which process - convection, slow upwelling within the background velocity field, in-cloud radiative processes, gravity waves or turbulence - is responsible for most of the transport. The study shows that the main contribution to the frozen hydrometeor flux is cold-point overshooting convection in both the control and perturbed scenario. The average convective event transports an increased amount of frozen hydrometeors at the cold-point tropopause, when the later is warmed. This finding can be explained by scaling of frozen moisture content with Clausius Clapeyron in a saturated environment.
... Dauhut and Hohenegger (2022) report a frozen moisture contribution of 11%, which is the only estimate based on direct quantification using a global storm-resolving simulation. Additionally, Bolot and Fueglistaler (2021) presented the first global estimate based on observational data with a frozen contribution of around 18%. ...
... The base simulation is in line with previously published data on moisture flux partitioning in non-frozen and frozen contributions. Our 20% contribution is in agreement with the 18% frozen contribution found in the observational based study by Bolot and Fueglistaler (2021). ICON was used by Dauhut and Hohenegger (2022) as well, their estimates are 11%, when only considering the deepest convection, and 29%, when considering all hydrometeors. ...
Plain Language Summary
The stratosphere is a dry region since moisture entering it from below has to pass the cold‐point, a temperature minimum between troposphere and stratosphere. The low temperatures lead to ice formation and sedimentation of moisture. Frozen moisture within clouds rising above the cold‐point tropopause can pass this temperature barrier and be injected into the stratosphere, where temperatures increase again, promoting the melting and sublimation of ice crystals. However, little is known about the sensitivity of the split of moisture entering the stratosphere into frozen and non‐frozen moisture, especially under external influences, like heating by volcanic aerosol or stratospheric geoengineering efforts. Convective parameterizations in conventional simulations can lead to biases. The emerging km‐scale simulations, which explicitly resolve the physical processes, offer the unique possibility to study moisture fluxes under external forcing while circumventing the downsides of parameterizations. Here, the sensitivity of the moisture flux partitioning into non‐frozen and frozen components to an additional heating source is studied for the first time in global storm‐resolving simulations. The analysis reveals an unaltered flux partitioning even at an average cold‐point warming exceeding 8 K. In the control and perturbed simulations, water vapor contributes around 80% of the moisture entering the stratosphere.
... The violin plots made using publicly available code from https://doi.org/10.5281/zenodo.4559847 Bechtold (2016). ...
Lagrangian trajectories are frequently used to trace air parcels from the troposphere to the stratosphere through the tropical tropopause layer (TTL), and the coldest temperatures of these trajectories have been used to reconstruct water vapor variability in the lower stratosphere, where water vapor's radiative impact on Earth's surface is strongest. As such, the ability of these trajectories to accurately capture temperatures encountered by parcels in the TTL is crucial to water vapor reconstructions and calculations of water vapor's radiative forcing. A potential source of error for trajectory calculations is the resolution of the input data. Here, we explore how improving the spatial and temporal resolution of model input data impacts the temperatures measured by Lagrangian trajectories that cross the TTL during boreal winter using ERA5 reanalysis data. We do so by comparing the temperature distribution of trajectories computed with data downsampled in either space or time to those computed with ERA5's maximum resolution. We find that improvements in temporal resolution from 6 to 3 and 1 h lower the cold point temperature distribution, with the mean cold point temperature decreasing from 185.9 to 185.0 and 184.5 K for reverse trajectories initialized at the end of February for each year from 2010 to 2019, while improvements to vertical resolution from that of MERRA2 data (the GEOS5 model grid) to full ERA5 resolution also lower the distribution but are of secondary importance, and improvements in horizontal resolution from 1∘ × 1∘ to 0.5∘ × 0.5∘ or 0.25∘ × 0.25∘ have negligible impacts to trajectory cold points. We suggest that this is caused by excess vertical dispersion near the tropopause when temporal resolution is degraded, which allows trajectories to cross the TTL without passing through the coldest regions, and by undersampling of the four-dimensional temperature field when either temporal or vertical resolution is reduced.
... These hypotheses for the source of additional water vapor have flaws though: isotopic constraints have been used to suggest that ice lofting brings water vapor 85 to the upper troposphere but not across the cold point tropopause, which means that ice lofting cannot inject significant water vapor to the lower stratosphere (Dessler et al., 2007). A similar conclusion was found using dynamic constraints by Bolot and Fueglistaler (2021), who showed that convective ice lofting supplies water up to the cold point tropopause, but that the transition to the slow ascent transport regime above that layer prevents further upward motion of significant amounts of ice. ...
Lagrangian trajectories are frequently used to trace air parcels from the troposphere to the stratosphere through the tropical tropopause layer (TTL), and the coldest temperatures of these trajectories have been used to reconstruct water vapor variability in the lower stratosphere, where water vapor’s radiative impact on Earth’s surface is strongest. As such, the ability of these trajectories to accurately capture temperatures encountered by parcels in the TTL is crucial to water vapor reconstructions and calculations of water vapor’s radiative forcing. A potential source of error for trajectory calculations is the resolution of the input data. Here, we explore how improving the temporal and spatial resolution of model input data impacts the temperatures measured by Lagrangian trajectories that cross the TTL during boreal winter using ERA5 reanalysis data. We do so by comparing the temperature distribution of trajectories computed with data downsampled in either space or time to those computed with ERA5's maximum resolution. We find that improvements in temporal resolution from 6 hour to 3 or 1 hour lower the cold point temperature distribution, with the mean cold point temperature decreasing from 185.9 K to 185.0 K or 184.5 K for trajectories run during boreal winters of 2010 to 2019, while improvements to vertical resolution from that of MERRA2 data (the GEOS5 model grid) to full ERA5 resolution also lower the distribution but are of secondary importance, and improvements in horizontal resolution from 1° x 1° to 0.5° x 0.5° or 0.25° x 0.25° have negligible impacts. We suggest that this is caused by excess vertical dispersion near the tropopause when temporal resolution is degraded, which allows trajectories to cross the TTL without passing through the coldest regions, and by undersampling of the four--dimensional temperature field when either temporal or vertical resolution is reduced.
... In contrast, the frozen mass flux distribution (Figure 13d, purple line) is weighted toward updrafts exceeding 1 m s −1 because they contain much more frozen water than nearly quiescent air. In all w bins, the frozen mass flux is much larger than the water vapor flux, consistent with the observationally based estimates from Bolot and Fueglistaler (2021). Similarly, the integrated mass flux values given in the lower legends of Figures 13 and S7 in Supporting Information S1 show that in both regions, the upward frozen mass flux in NICAM at 14 km is much larger than the vapor mass flux and thus is the principal source of water in the TTL. ...
... This pattern agrees qualitatively with Bolot and Fueglistaler (2021), who found that deep convection is more important for moisture transport than large-scale advection (i.e., our Category 3 mass fluxes) below the cold point (∼16-17 km). However, the frozen mass flux values in NICAM are larger by a factor of ∼100 in both regions than the observationally based, tropics-wide estimates from Bolot and Fueglistaler (2021). We normalize the estimates of ice flux from Figure 3 in Bolot and Fueglistaler (2021) by the area of their 30°S-30°N domain as well as the 38-day average Sahel or TWP precipitation rate. ...
... However, the frozen mass flux values in NICAM are larger by a factor of ∼100 in both regions than the observationally based, tropics-wide estimates from Bolot and Fueglistaler (2021). We normalize the estimates of ice flux from Figure 3 in Bolot and Fueglistaler (2021) by the area of their 30°S-30°N domain as well as the 38-day average Sahel or TWP precipitation rate. These estimates are then plotted alongside the NICAM values in Figures 15 and S9 in Supporting Information S1. ...
Pervasive cirrus clouds in the upper troposphere and tropical tropopause layer (TTL) influence the climate by altering the top‐of‐atmosphere radiation balance and stratospheric water vapor budget. These cirrus are often associated with deep convection, which global climate models must parameterize and struggle to accurately simulate. By comparing high‐resolution global storm‐resolving models from the Dynamics of the Atmospheric general circulation Modeled On Non‐hydrostatic Domains (DYAMOND) intercomparison that explicitly simulate deep convection to satellite observations, we assess how well these models simulate deep convection, convectively generated cirrus, and deep convective injection of water into the TTL over representative tropical land and ocean regions. The DYAMOND models simulate deep convective precipitation, organization, and cloud structure fairly well over land and ocean regions, but with clear intermodel differences. All models produce frequent overshooting convection whose strongest updrafts humidify the TTL and are its main source of frozen water. Intermodel differences in cloud properties and convective injection exceed differences between land and ocean regions in each model. We argue that, with further improvements, global storm‐resolving models can better represent tropical cirrus and deep convection in present and future climates than coarser‐resolution climate models. To realize this potential, they must use available observations to perfect their ice microphysics and dynamical flow solvers.
Tropical ice clouds have an important influence on the Earth’s radiative balance. They often form as a result of tropical deep convection, which strongly affects the water budget of the tropical tropopause layer. Ice cloud formation involves complex interactions on various scales, which are not fully understood yet and lead to large uncertainties in climate predictions. In this study, we investigate the formation of tropical ice clouds related to deep convection in the West African monsoon, using stable water isotopes as tracers of moist atmospheric processes. We perform simulations using the regional isotope-enabled model COSMOiso with different resolutions and treatments of convection for the period of June–July 2016. First, we evaluate the ability of our simulations to represent the isotopic composition of monthly precipitation through comparison with GNIP observations, and the precipitation characteristics related to the monsoon evolution and convective storms based on insights from the DACCIWA field campaign in 2016. Next, a case study of a mesoscale convective system (MCS) explores the isotope signatures of tropical deep convection in atmospheric water vapour and ice. Convective updrafts within the MCS inject enriched ice into the upper troposphere leading to depletion of vapour within these updrafts due to the preferential condensation and deposition of heavy isotopes. Water vapour in downdrafts within the same MCS are enriched by non-fractionating sublimation of ice. In contrast to ice within the MCS core regions, ice in widespread cirrus shields is isotopically in approximate equilibrium with the ambient vapour, which is consistent with in situ formation of ice. These findings from the case study are supported by a statistical evaluation of isotope signals in the West African monsoon ice clouds. The following five key processes related to tropical ice clouds can be distinguished based on their characteristic isotope signatures: (1) convective lofting of enriched ice into the upper troposphere, (2) cirrus clouds that form in situ from ambient vapour under equilibrium fractionation, (3) sedimentation and sublimation of ice in the mixed-phase cloud layer in the vicinity of convective systems and underneath cirrus shields, (4) sublimation of ice in convective downdrafts that enriches the environmental vapour, and (5) the freezing of liquid water in the mixed-phase cloud layer at the base of convective updrafts. Importantly, the results show that convective systems strongly modulate the humidity budget and the isotopic composition of the lower tropical tropopause layer. They contribute to about 40 % of the total water and 60 % of HDO in the 175–125 hPa layer in the African monsoon region according to estimates based on our model simulations. Overall, this study demonstrates that isotopes can serve as useful tracers to disentangle the role of different processes in the Earth’s water cycle, including convective transport, the formation of ice clouds, and their impact on the tropical tropopause layer.
