How model uncertainties influence tropical humidity in global storm-resolving simulations

We conduct a series of eight 45-day experiments with a global storm-resolving model (GSRM) to test the sensitivity of relative humidity R in the tropics to changes in model resolution and parameterizations. These changes include changes in horizontal and vertical grid spacing as well as in the parameterizations of microphysics and turbulence, and are chosen to capture currently existing differences among GSRMs. To link the R distribution in the tropical free troposphere with processes in the deep convective regions, we adopt a trajectory-based assessment of the last-saturation paradigm. The perturbations we apply to the model result in tropical mean R changes ranging from 0.5 to 8 (absolute) in the mid troposphere. The generated R spread is similar to that in a multi-model ensemble of GSRMs and smaller than the spread across conventional general circulation models, supporting that an explicit representation of deep convection reduces the uncertainty in tropical R. The largest R changes result from changes in parameterizations, suggesting that model physics represent a major source of humidity spread across GSRMs. The R in the moist tropical regions is disproportionately sensitive to vertical mixing processes within the tropics, which impact R through their effect on the last-saturation temperature rather than their effect on the evolution of the humidity since last-saturation. In our analysis the R of the dry tropical regions strongly depends on the exchange with the extra-tropics. The interaction between tropics and extratropics could change with warming and presage changes in the radiatively sensitive dry regions

Temporal evolution of stable water isotopologues in cloud droplets in a hill cap cloud in central Europe (HCCT-2010)

In this work, we present the first study resolving the temporal evolution of δ2H and δ18O values in cloud droplets during 13 different cloud events. The cloud events were probed on a 937 m high mountain chain in Germany in the framework of the Hill Cap Cloud Thuringia 2010 campaign (HCCT-2010) in September and October 2010. The δ values of cloud droplets ranged from −77‰ to −15‰ (δ2H) and from −12.1‰ to −3.9‰ (δ18O) over the whole campaign. The cloud water line of the measured δ values was δ2H=7.8×δ18O+13×10−3, which is of similar slope, but with higher deuterium excess than other Central European Meteoric Water Lines. Decreasing δ values in the course of the campaign agree with seasonal trends observed in rain in central Europe. The deuterium excess was higher in clouds developing after recent precipitation revealing episodes of regional moisture recycling. The variations in δ values during one cloud event could either result from changes in meteorological conditions during condensation or from variations in the δ values of the water vapor feeding the cloud. To test which of both aspects dominated during the investigated cloud events, we modeled the variation in δ values in cloud water using a closed box model. We could show that the variation in δ values of two cloud events was mainly due to changes in local temperature conditions. For the other eleven cloud events, the variation was most likely caused by changes in the isotopic composition of the advected and entrained vapor. Frontal passages during two of the latter cloud events led to the strongest temporal changes in both δ2H (≈ 6‰ per hour) and δ18O (≈ 0.6‰ per hour). Moreover, a detailed trajectory analysis for the two longest cloud events revealed that variations in the entrained vapor were most likely related to rain out or changes in relative humidity and temperature at the moisture source region or both. This study illustrates the sensitivity of stable isotope composition of cloud water to changes in large scale air mass properties and regional recycling of moisture

Stable water isotopologue ratios in fog and cloud droplets of liquid clouds are not size-dependent

In this work, we present the first observations of stable water isotopologue ratios in cloud droplets of different sizes collected simultaneously. We address the question whether the isotope ratio of droplets in a liquid cloud varies as a function of droplet size. Samples were collected from a ground intercepted cloud (= fog) during the Hill Cap Cloud Thuringia 2010 campaign (HCCT-2010) using a three-stage Caltech Active Strand Cloud water Collector (CASCC). An instrument test revealed that no artificial isotopic fractionation occurs during sample collection with the CASCC. Furthermore, we could experimentally confirm the hypothesis that the δ values of cloud droplets of the relevant droplet sizes (μm-range) were not significantly different and thus can be assumed to be in isotopic equilibrium immediately with the surrounding water vapor. However, during the dissolution period of the cloud, when the supersaturation inside the cloud decreased and the cloud began to clear, differences in isotope ratios of the different droplet sizes tended to be larger. This is likely to result from the cloud's heterogeneity, implying that larger and smaller cloud droplets have been collected at different moments in time, delivering isotope ratios from different collection times

Temporal evolution of stable water isotopologues in cloud droplets in a hill cap cloud in central Europe (HCCT-2010)

In this work, we present the first study resolving the temporal evolution of δ<sup>2</sup>H and δ<sup>18</sup>O values in cloud droplets during 13 different cloud events. The cloud events were probed on a 937 m high mountain chain in Germany in the framework of the Hill Cap Cloud Thuringia 2010 campaign (HCCT-2010) in September and October 2010. The δ values of cloud droplets ranged from −77‰ to −15‰ (δ<sup>2</sup>H) and from −12.1‰ to −3.9‰ (δ<sup>18</sup>O) over the whole campaign. The cloud water line of the measured δ values was δ<sup>2</sup>H=7.8×δ<sup>18</sup>O+13×10<sup>−3</sup>, which is of similar slope, but with higher deuterium excess than other Central European Meteoric Water Lines. Decreasing δ values in the course of the campaign agree with seasonal trends observed in rain in central Europe. The deuterium excess was higher in clouds developing after recent precipitation revealing episodes of regional moisture recycling. The variations in δ values during one cloud event could either result from changes in meteorological conditions during condensation or from variations in the δ values of the water vapor feeding the cloud. To test which of both aspects dominated during the investigated cloud events, we modeled the variation in δ values in cloud water using a closed box model. We could show that the variation in δ values of two cloud events was mainly due to changes in local temperature conditions. For the other eleven cloud events, the variation was most likely caused by changes in the isotopic composition of the advected and entrained vapor. Frontal passages during two of the latter cloud events led to the strongest temporal changes in both δ<sup>2</sup>H (≈ 6‰ per hour) and δ<sup>18</sup>O (≈ 0.6‰ per hour). Moreover, a detailed trajectory analysis for the two longest cloud events revealed that variations in the entrained vapor were most likely related to rain out or changes in relative humidity and temperature at the moisture source region or both. This study illustrates the sensitivity of stable isotope composition of cloud water to changes in large scale air mass properties and regional recycling of moisture

EUREC⁴A

The science guiding the EUREC⁴A campaign and its measurements is presented. EUREC⁴A comprised roughly 5 weeks of measurements in the downstream winter trades of the North Atlantic – eastward and southeastward of Barbados. Through its ability to characterize processes operating across a wide range of scales, EUREC⁴A marked a turning point in our ability to observationally study factors influencing clouds in the trades, how they will respond to warming, and their link to other components of the earth system, such as upper-ocean processes or the life cycle of particulate matter. This characterization was made possible by thousands (2500) of sondes distributed to measure circulations on meso- (200 km) and larger (500 km) scales, roughly 400 h of flight time by four heavily instrumented research aircraft; four global-class research vessels; an advanced ground-based cloud observatory; scores of autonomous observing platforms operating in the upper ocean (nearly 10 000 profiles), lower atmosphere (continuous profiling), and along the air–sea interface; a network of water stable isotopologue measurements; targeted tasking of satellite remote sensing; and modeling with a new generation of weather and climate models. In addition to providing an outline of the novel measurements and their composition into a unified and coordinated campaign, the six distinct scientific facets that EUREC⁴A explored – from North Brazil Current rings to turbulence-induced clustering of cloud droplets and its influence on warm-rain formation – are presented along with an overview of EUREC⁴A's outreach activities, environmental impact, and guidelines for scientific practice. Track data for all platforms are standardized and accessible at https://doi.org/10.25326/165 (Stevens, 2021), and a film documenting the campaign is provided as a video supplement

EUREC⁴A

The science guiding the EUREC⁴A campaign and its measurements is presented. EUREC⁴A comprised roughly 5 weeks of measurements in the downstream winter trades of the North Atlantic – eastward and southeastward of Barbados. Through its ability to characterize processes operating across a wide range of scales, EUREC⁴A marked a turning point in our ability to observationally study factors influencing clouds in the trades, how they will respond to warming, and their link to other components of the earth system, such as upper-ocean processes or the life cycle of particulate matter. This characterization was made possible by thousands (2500) of sondes distributed to measure circulations on meso- (200 km) and larger (500 km) scales, roughly 400 h of flight time by four heavily instrumented research aircraft; four global-class research vessels; an advanced ground-based cloud observatory; scores of autonomous observing platforms operating in the upper ocean (nearly 10 000 profiles), lower atmosphere (continuous profiling), and along the air–sea interface; a network of water stable isotopologue measurements; targeted tasking of satellite remote sensing; and modeling with a new generation of weather and climate models. In addition to providing an outline of the novel measurements and their composition into a unified and coordinated campaign, the six distinct scientific facets that EUREC⁴A explored – from North Brazil Current rings to turbulence-induced clustering of cloud droplets and its influence on warm-rain formation – are presented along with an overview of EUREC⁴A's outreach activities, environmental impact, and guidelines for scientific practice. Track data for all platforms are standardized and accessible at https://doi.org/10.25326/165 (Stevens, 2021), and a film documenting the campaign is provided as a video supplement

Deuterium excess as a proxy for continental moisture recycling and plant transpiration

Studying the evaporation process and its link to the atmospheric circulation is central for a better understanding of the feedbacks between the surface water components and the atmosphere. In this study, we use 5 months of deuterium excess (d) measurements at the hourly to daily timescale from a cavity ring-down laser spectrometer to characterise the evaporation source of low-level continental water vapour at the long-term hydrometeorological monitoring site Rietholzbach in northeastern Switzerland. To reconstruct the phase change history of the air masses in which we measure the d signature and to diagnose its area of surface evaporation we apply a Lagrangian moisture source diagnostic. With the help of a correlation analysis we investigate the strength of the relation between d measurements and the moisture source conditions. Temporal episodes with a duration of a few days of strong anticorrelation between d and relative humidity as well as temperature are identified. The role of plant transpiration, the large-scale advection of remotely evaporated moisture, the local boundary layer dynamics at the measurement site and recent precipitation at the site of evaporation are discussed as reasons for the existence of these modes of strong anticorrelation between d and moisture source conditions. We show that the importance of continental moisture recycling and the contribution of plant transpiration to the continental evaporation flux may be deduced from the d-relative humidity relation at the seasonal timescale as well as for individual events. The methodology and uncertainties associated with these estimates of the transpiration fraction of evapotranspiration are presented and the proposed novel framework is applied to individual events from our data set. Over the whole analysis period (August to December 2011) a transpiration fraction of the evapotranspiration flux over the continental part of the moisture source region of 62 % is found albeit with a large event-to-event variability (0 % to 89 %) for continental Europe. During days of strong local moisture recycling a higher overall transpiration fraction of 76 % (varying between 65 % and 86 %) is found. These estimates are affected by uncertainties in the assumptions involved in our method as well as by parameter uncertainties. An average uncertainty of 11 % results from the strong dependency of the transpiration estimates on the choice of the non-equilibrium fractionation factor. Other uncertainty sources like the influence of boundary layer dynamics are probably large but more difficult to quantify. Nevertheless, such Lagrangian estimates of the transpiration part of continental evaporation could potentially be useful for the verification of model estimates of this important land-atmosphere coupling parameter

Precipitation extremes in Ukraine from 1979 to 2019: climatology, large-scale flow conditions, and moisture sources

Understanding extreme precipitation events (EPEs) and their underlying dynamical processes and moisture transport patterns is essential to mitigating EPE-related risks. In this study, we investigate the dynamics of 82 EPEs (≥100 mm d−1) over the territory of Ukraine in the recent decades (1979–2019), of which the majority occurred in summer. The EPEs are identified based on precipitation observations from 215 meteorological stations and posts in Ukraine. The atmospheric variables for the case study analysis of selected EPEs and for climatological composites and trajectory calculations were taken from ERA5 reanalyses. Moisture sources contributing to the EPEs in Ukraine are identified with kinematic backward trajectories and the subsequent application of a moisture source identification scheme based on the humidity mass budget along these trajectories. The large-scale atmospheric circulation associated with EPEs was studied for a selection of representative EPEs in all seasons and with the aid of composites of all events per season. Results show that EPEs in summer occur all across Ukraine, but in other seasons EPE hotspots are mainly in the Carpathians and along the Black Sea and Sea of Azov. All EPEs were associated with a surface cyclone, with most having an upper-level trough, except for the winter events that occurred in situations with very strong westerly jets. Isentropic potential vorticity anomalies associated with EPEs in Ukraine show clear dipole structures in all seasons, however, interestingly with a different orientation of these anomaly dipoles between seasons. The analysis of moisture sources revealed a very strong case-to-case variability and often a combination of local and remote sources. Oceanic sources dominate in winter, but land evapotranspiration accounts for 60 %–80 % of the moisture that rains out in EPEs in the other seasons. Taken together, these findings provide a novel insight into large-scale characteristics of EPEs in Ukraine, a region with a unique geographical setting and with moisture sources as diverse as Newfoundland, the Azores, the Caspian Sea, and the Arctic Ocean.</p