AS2.4

Advancing understanding of the coupling between clouds, convection and circulation

The uncertain response of clouds to global warming is a major contributor to the spread in climate sensitivity across climate models. Cloud feedback uncertainty is related to a limited understanding of the coupling between clouds, convection and the large-scale circulation across various spatial and temporal scales. This session focuses on efforts to advance our understanding of the cloud-circulation coupling and its role in climate change. Contributions from dedicated field campaigns such as EUREC4A, from various observing platforms like ground-based and satellite remote sensing or in situ measurements, as well as modelling and theoretical studies are welcomed. We also invite abstracts focusing on the role of convective organization and precipitation in modulating the cloud-circulation coupling and cloud feedbacks.

Convener: Raphaela VogelECSECS | Co-conveners: Claudia AcquistapaceECSECS, Leif DenbyECSECS, Isabel L. McCoyECSECS, Ann Kristin NaumannECSECS
Presentations
| Wed, 25 May, 15:10–18:30 (CEST)
 
Room 0.11/12

Presentations: Wed, 25 May | Room 0.11/12

Chairpersons: Ann Kristin Naumann, Leif Denby, Isabel L. McCoy
15:10–15:15
Topic 1: Cloud-circulation coupling & cloud feedbacks
15:15–15:20
|
EGU22-8819
|
ECS
|
On-site presentation
Emily Van de Koot, Michael P. Byrne, and Tim Woollings

Significant challenges in modelling clouds render observational data an important resource for quantifying cloud feedbacks. Here, we use data from satellite and reanalysis products to estimate tropical cloud feedbacks over a wide range of circulation regimes. We use two distinct methods, month-to-month variability and linear multi-decadal trends, to gain insight as to whether short-term feedbacks are representative of feedbacks associated with CO2-induced warming. We also investigate the extent to which cloud feedbacks are circulation-driven by decomposing the relative contributions of circulation versus thermodynamic changes to the feedbacks in each regime. The influence of thermodynamic processes on cloud feedbacks has been shown to be dominant at large spatial scales in global climate models (Byrne and Schneider, 2018), but it is unclear whether observed feedbacks are consistent with model behaviour. A particular focus of our analysis is the effect of circulation on the tropical anvil cloud area feedback in ascending regions, as this feedback constitutes the largest source of uncertainty in the overall cloud feedback yet is relatively understudied (Sherwood et al. 2020).   

References:

  • Byrne, M. P., & Schneider, T. (2018). Atmospheric dynamics feedback: Concept, simulations, and climate implications. Journal of Climate, 31(8), 3249-3264.
  • Sherwood, S. C., Webb, M. J., Annan, J. D., Armour, K. C., Forster, P. M., Hargreaves, J. C., ... & Zelinka, M. D. (2020). An assessment of Earth's climate sensitivity using multiple lines of evidence. Reviews of Geophysics, 58(4), e2019RG000678.

How to cite: Van de Koot, E., Byrne, M. P., and Woollings, T.: Observed relationships between circulation and cloud feedbacks in the tropics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8819, https://doi.org/10.5194/egusphere-egu22-8819, 2022.

15:20–15:25
|
EGU22-9916
|
Virtual presentation
Peter Hill and Chris Holloway

Cloud feedbacks remain the dominant source of uncertainty in climate model predictions of the surface warming response to increasing carbon dioxide. One cause of this uncertainty is the intimate coupling between clouds and circulation: cloud responses to circulation changes are poorly understood, the circulation changes are themselves uncertain, and the potential for changes in cloud to further influence circulation contributes further uncertainty.

Motivated by the need to better understand the coupling between clouds and circulation, this presentation describes the relationship between cloud radiative effects and circulation regime (based on vertical velocity at 500 hPa) over the tropical Pacific Ocean. Based on a combination of vertical velocity from state-of-the-art reanalyses with satellite radiation measurements, we examine how the relationship between cloud and circulation changes with spatial and temporal scale, season, and ENSO index. We then examine whether these relationships are reproduced in a range of models, ranging from high resolution idealised cloud resolving simulations to the latest CMIP6 climate simulations.

How to cite: Hill, P. and Holloway, C.: Relationships between clouds and circulation in reanalyses and climate models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9916, https://doi.org/10.5194/egusphere-egu22-9916, 2022.

15:25–15:30
|
EGU22-5366
|
Virtual presentation
Levi Silvers, Kevin Reed, and Allison Wing

Previous work has found that as the surface warms the large-scale tropical circulations weaken, convective anvil cloud fraction decreases, and atmospheric static stability increases. Circulation changes inevitably lead to changes in the humidity and cloud fields which influence the surface energetics.  The exchange of mass between the boundary layer and the midtroposphere has also been shown to weaken in global climate models.    What has remained less clear is how robust these changes in the circulation are to different representations of convection, clouds, and microphysics in numerical models.  We use simulations from the Radiative‐Convective Equilibrium Model Intercomparison Project (RCEMIP) to investigate the interaction between overturning circulations, surface temperature, and atmospheric moisture.  We analyze the underlying mechanisms of these relationships using a 21-member model ensemble that includes both general circulation models and cloud resolving models.  We find a large spread in the change of intensity of the overturning circulation.  Both the range of the circulation intensity, and its change with warming can be explained by the range of the mean upward vertical velocity.  There is also a consistent decrease in the exchange of mass between the boundary layer and the midtroposphere.  However, the magnitude of the decrease varies substantially due to the range of responses in both mean precipitation and mean precipitable water.   This work implies that despite well understood thermodynamic constraints, there is still a considerable ability for the cloud fields and the precipitation efficiency to drive a substantial range of tropical convective responses to warming.

How to cite: Silvers, L., Reed, K., and Wing, A.: The Response of the Large-Scale Tropical Circulation to Warming, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5366, https://doi.org/10.5194/egusphere-egu22-5366, 2022.

15:30–15:35
|
EGU22-12604
|
ECS
|
Virtual presentation
Monisha Natchiar Subbiah Renganathan, Hugo Lambert, and Geoffrey Vallis

The intermodel spread in the equilibrium climate sensitivity (ECS) determined from GCM simulations has been majorly ascribed to the spread in the cloud feedback, wherein the multimodel-mean net cloud feedback is found to be positive. It was previously identified that the largest source of intermodel spread in the net cloud feedback comes from the low cloud amount (> 680 hPa) modelled by the GCMs (1). However, recent evidence points to the importance of understanding the processes contributing to the tropical high-cloud feedback in order to constrain the uncertainty in the total cloud feedback (2). One of the key challenges that remains is understanding the coupling between the clouds and the large-scale circulation. In this work, we focus on the subtropical low (stratocumulus) clouds and tropical high (anvil) clouds. We perform idealised GCM simulations using the Met Office Unified Model with different prescribed sea-surface temperature gradients in the tropics and extratropics that emulate the sea-surface temperature response to increases in atmospheric CO2. We also perform idealised simulations with an interactive slab ocean setup. Investigation of the influence of circulation changes on the tropical cloud feedback is done using a combination of simple mathematical frameworks. We then compare our GCM simulation results with those obtained using long-channel cloud-resolving model (CRM) simulations. Our results corroborate previous results that indicate that the cloud feedback at the tropics-wide scale is dominated by the local thermodynamical changes than by dynamical changes. However, interestingly, we find a decrease in the tropical low cloud amount in some of the GCM simulations with a slab ocean setup. The processes causing the decrease in the low cloud amount and/or the robustness of this result remains to be investigated.

References:

  • Ceppi, P., Brient, F., Zelinka, M.D. and Hartmann, D.L., 2017. Cloud feedback mechanisms and their representation in global climate models. Wiley Interdisciplinary Reviews: Climate Change8(4), p.e465.
  • Sherwood, S.C., Webb, M.J., Annan, J.D., Armour, K.C., Forster, P.M., Hargreaves, J.C., Hegerl, G., Klein, S.A., Marvel, K.D., Rohling, E.J. and Watanabe, M., 2020. An assessment of Earth's climate sensitivity using multiple lines of evidence. Reviews of Geophysics58(4), p.e2019RG000678.

 

How to cite: Subbiah Renganathan, M. N., Lambert, H., and Vallis, G.: Influence of dynamical changes on the tropical cloud feedback using extratropical forcing, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12604, https://doi.org/10.5194/egusphere-egu22-12604, 2022.

15:35–15:40
|
EGU22-6952
|
ECS
|
Virtual presentation
Christoph Heim and Christoph Schär

Even though the complexity and resolution of global climate models has increased over the last decades, the inter-model spread in equilibrium climate sensitivity has not narrowed. The representation of tropical and subtropical marine clouds remains a major source of uncertainty in climate models. Going to higher model resolution to explicitly resolve a larger fraction of the underlying convective circulations is the most direct way towards reducing the uncertainty associated with these clouds. Convection-resolving models (CRMs) are therefore an attractive complementary tool to study tropical cloud feedbacks. Even though decade-long global CRM simulations are not yet computationally feasible, CRMs can be used in climate applications for selected limited-area domains and periods.

Here we run 3-year-long CRM simulations with the COSMO model over the tropical-to-subtropical Atlantic. We run a control simulation to evaluate the model’s capability of representing the clouds and the radiative balance. We also run a climate change scenario simulation using the pseudo-global warming (PGW) approach to study cloud-radiative feedbacks at convection-resolving resolution.

We find a good agreement between the simulated and observed annual cycle in the top-of-the-atmosphere radiative fluxes, despite a mean bias that should be possible to reduce through model calibration. There are pronounced improvements in the CRM simulation compared to the CMIP6 models over the ITCZ and the trade-wind cumulus region, while the representation of stratocumulus clouds remains challenging also in the CRM simulation. The simulated cloud-radiative feedback is at the upper end of what the CMIP6 models predict due to a pronounced positive longwave feedback at the ITCZ caused by an increase in high clouds. The shortwave cloud-radiative feedback is moderately positive and lies well within the range of the CMIP6 models with a reduction in the low-level cloud fraction over the subtropics and a partly compensating increase in the cloud fraction at the ITCZ.

How to cite: Heim, C. and Schär, C.: Convection-resolving climate simulation of the tropical-to-subtropical Atlantic, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6952, https://doi.org/10.5194/egusphere-egu22-6952, 2022.

15:40–15:45
|
EGU22-10302
|
ECS
|
On-site presentation
Raphaela Vogel, Anna Lea Albright, Jessica Vial, Geet George, Bjorn Stevens, and Sandrine Bony

Shallow trade cumulus clouds cool the planet and fuel the large-scale circulation. Their unknown response to climate change is a major source of uncertainty in climate projections. Differing changes in cloudiness near the base of the cumulus layer with warming control the spread in simulated trade cumulus cloud feedbacks in models, with high climate sensitivity models showing a strong negative coupling between lower-tropospheric mixing and cloudiness. However, such a mixing-desiccation mechanism has never been tested with observations. Here we present novel measurements of the convective mass flux, cloud fraction and relative humidity at cloud base from the recent EUREC4A field campaign and find the dynamical control of cloudiness through the mass flux to overwhelm the thermodynamic response to humidity. Because the mesoscale vertical velocity controls the mass flux as much as entrainment does, the mass flux ends up being uncorrelated to relative humidity, which opposes the mixing-desiccation hypothesis. The magnitude, variability, and coupling of mass flux and cloudiness differs drastically between climate models and the EUREC4A observations. Models that have particularly strong trade cumulus feedbacks tend to exaggerate the dependence of cloudiness on humidity rather than the mass flux, and also exaggerate variability in cloudiness. The process-based constraints presented here render those strongly positive trade cumulus feedbacks unrealistic, for the first time supporting and explaining a weak trade cumulus feedback at the relevant process scale.

 

How to cite: Vogel, R., Albright, A. L., Vial, J., George, G., Stevens, B., and Bony, S.: Mesoscale dynamics protect trade-cumulus clouds from mixing-induced desiccation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10302, https://doi.org/10.5194/egusphere-egu22-10302, 2022.

15:45–15:50
|
EGU22-9021
|
Presentation form not yet defined
Susan van den Heever, Ziad Haddad, Simone Tanelli, Graeme Stephens, Derek Posselt, Yunjin Kim, Shannon Brown, Scott Braun, Leah Grant, Pavlos Kollias, Zhengzhao Johnny Luo, Gerald Mace, Peter Marinescu, Sharmila Padmanabhan, Philip Partain, Walter Petersent, Sai Prasanth, Kristen Rasmussen, Steven Reising, and Courtney Schumacher and the INCUS Mission team

The INvestigation of Convective UpdraftS (INCUS) is a recently selected NASA Earth Ventures Mission. The overarching goal of INCUS is to enhance our understanding of why, when and where tropical convective storms form, and why only some storms produce extreme weather. Life on Earth is bound to convective storms, from the fresh water they supply to the extreme weather they produce. Much of the vertical transport of water and air between Earth’s surface and the upper troposphere is facilitated by convective storms. This vertical transport of water and air, referred to as convective mass flux (CMF), plays a critical role in the weather and climate system through its influence on storm intensity, precipitation rates, upper tropospheric moistening, high cloud feedbacks, and the large-scale circulation. Recent studies have also suggested that CMF may change with changing climates. In spite of the critical role of this vertical transport of water and air within the weather and climate system, much is not understood regarding the way in which various environmental factors govern this mass transport, nor the subsequent impacts of CMF on high clouds and extreme weather. Representation of CMF is also a major source of error in weather and climate models, thereby limiting our ability to predict convective storms and their associated feedbacks on weather through climate timescales.

INCUS is a NASA class-D mission. Three RainCube-heritage Ka-band 5-beam scanning radars that are compatible with SmallSat platforms comprise the mission. The satellite platforms will be 30 and 90 seconds apart. Each SmallSat will carry one radar system each, and the middle SmallSat will house a single TEMPEST-D-heritage cross-track-scanning passive microwave radiometer with four channels between 150 and 190 GHz. Through its novel measurements of time-differenced profiles of radar reflectivity, INCUS is the first systematic investigation of the rapidly evolving CMF within tropical convective storms. The primary INCUS objectives are: (1) to determine the predominant environmental properties controlling CMF in tropical convective storms; (2) to determine the relationship between CMF and high anvil clouds; (3) to determine the relationship between CMF and the type and intensity of the extreme weather produced; and (4) to evaluate these relationships between CMF and environmental factors, high anvil clouds, and extreme weather within weather and climate models. The ground breaking observations of convective storms by INCUS are expected to significantly enhance our understanding and prediction of convective processes and extreme weather in current and future climates.

How to cite: van den Heever, S., Haddad, Z., Tanelli, S., Stephens, G., Posselt, D., Kim, Y., Brown, S., Braun, S., Grant, L., Kollias, P., Luo, Z. J., Mace, G., Marinescu, P., Padmanabhan, S., Partain, P., Petersent, W., Prasanth, S., Rasmussen, K., Reising, S., and Schumacher, C. and the INCUS Mission team: The INCUS Mission, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9021, https://doi.org/10.5194/egusphere-egu22-9021, 2022.

15:50–15:55
Topic 2: Cloud microphysics, precipitation and cold pools
15:55–16:00
|
EGU22-12948
|
Virtual presentation
Gary Lloyd, Thomas Choularton, Alan Blyth, Martin Gallagher, keith bower, Zhiquang Cui, and Leif Denby

EUREC4A is an international project that aims to address the current lack of understanding of the processes controlling the response of trade-wind cumulus clouds to changing environmental conditions in a warmer climate. The radiative properties of the trade-wind cumulus clouds have a major influence on the Earth's radiation budget. The response to global warming of these clouds is therefore critical for global mean cloud feedbacks. The EUREC4A field campaign took place in the vicinity of Barbados during January and February, 2020. The BAS Twin Otter aircraft was deployed in the project to make measurements of aerosols, cloud microphysics and boundary-layer processes in the life cycle of the clouds. In-situ measurements were made of the cloud droplet size distributions and the development of warm rain in multiple cases at different altitudes. We found significant variability in the development of precipitation between cases. The cloud structure appeared to have a significant impact on the precipitation, while the aerosol concentrations in the boundary layer were strongly related to the initial droplet number concentration at cloud base. We will present these findings by highlighting a number of cases with different cloud types and aerosol properties.

How to cite: Lloyd, G., Choularton, T., Blyth, A., Gallagher, M., bower, K., Cui, Z., and Denby, L.: Cloud Microphysics Measurements and the Development of Precipitation During EUREC4A, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12948, https://doi.org/10.5194/egusphere-egu22-12948, 2022.

16:00–16:05
|
EGU22-7436
|
Virtual presentation
Zhiqiang Cui, Alan Blyth, Steve Boeing, and Ralph Burton

The global radiative budget is strongly linked to cloud microphysical processes. In numerical models, the liquid cloud microphysics is usually parameterised, and the radiative forcing from shortwave radiation due to liquid cloud is controlled overall by liquid water path, effective radius, cloud droplet number concentration, solar zenith angle, and surface albedo. Previous studies have shown that many cloud liquid optical property computations in weather and climate models have uncertainties due to not accounting for the drop size distribution and from averaging single scattering properties over wide spectral bands. Low-level clouds are the primary cause of uncertainty in cloud feedback in climate model projections. EUREC4A is a coordinated international effort that aims to address the current lack of understanding of the processes controlling the response of trade-wind cumulus clouds to changing environmental conditions in a warmer climate. The EUREC4A field campaign took place in the vicinity of Barbados during January and February, 2020 since clouds at Barbados are representative of clouds across the trade wind regions in observations and climate models. A flower, i.e., circular clumped features surrounded by large areas of clear air, cloud system formed on 2 February 2020. The Twin Otter aircraft of the British Antarctic Survey made airborne measurements of aerosol and cloud microphysics of the cloud system and its environment. We present here the detrainment layer analysis of the cloud system using the in-situ and the satellite data. The aircraft flew close to cloud top and across a comma-shaped area with effective radius exceeding 30 µm. The area had cloud optical depth greater than 50, indicating that the area was associated with active convection and strong warm-rain processes. The drop number concentrations were less than 140 cm-3 along the leg, with the concentrations being less than 40 cm-3 across the comma-shape. The concentrations of drops larger than 500 µm were ~ 3 L-1. A reasonably good agreement was achieved between the GOES-16 retrieved effective radius and the calculated effective radius along the leg. The high values of effective radius calculated from the in-situ data were found in places where the concentrations were not great but had a reasonable amount of large drops, not the places where the largest drops existed but the concentrations of all drops were higher. The drop size distributions along the leg displayed the variations. These observations will be compared and contrasted with others made in similar cloud types.

How to cite: Cui, Z., Blyth, A., Boeing, S., and Burton, R.: An observational study of the detrainment layer of a flower system during EUREC4A, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7436, https://doi.org/10.5194/egusphere-egu22-7436, 2022.

16:05–16:10
|
EGU22-6462
|
ECS
|
Virtual presentation
Mampi Sarkar and Adriana Bailey

Sub-cloud rain evaporation in trade-wind regions significantly contributes to the boundary layer mass and energy budgets. However, parameterizing marine rain evaporation is difficult due to the sparse availability of well-resolved rain observations and the challenges of sampling short-lived marine cumulus clouds. In this study, 1-Hz raindrop size distribution (RSD) observations, sampled during the Atlantic Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign (ATOMIC) held in January-February 2020, are used to initialize a one-dimensional evaporation model to evaluate rain evaporation flux (Fe) in the sub-cloud layer for 22 case studies. Fe varies from 1 to 70 Wm−2 for 7 out of 22 cases where rain is sampled within ±100 m of ceilometer-based cloud base (700 m). These Fe values are comparable to radiative and surface fluxes in previous modeling and observational shallow cloud studies. The remaining cases where rain is sampled 800-1300 m above the cloud base have less reliable Fe due to unaccounted collision-coalescence growth as raindrops fall from sampling height to cloud base. The role of collision-coalescence growth is evident from the lower total raindrop concentration (N0) and slightly higher geometrical mean diameter (Dg) near cloud base compared to those sampled at higher altitudes. These microphysical parameters are found to not only influence the magnitude of vertically integrated Fe but also impact its vertical distribution. Comparatively, thermodynamic factors only influence the vertical distribution of Fe and not its vertically integrated magnitude. The rain evaporation is also detected by the modeled enrichment of stable isotope ratios of deuterium and oxygen in precipitation (dDp and d18Op, respectively). The enriched dDp and d18Op modeled at surface closely match observations from three independent surface sources, validating our isotope model. The enrichment modeled in both dDp and d18Op is proportional to Fe for the 7 cases close to cloud base. Compared to precipitation isotope ratios, water vapor isotope ratios cannot resolve the evaporation signals due to small ratio of evaporated to background water vapor. This increases our confidence in using rainwater isotope sampling to study sub-cloud rain evaporation in future campaigns. The substantial Fe in these shallow precipitating cumulus clouds also confirms the importance of rain evaporation in boundary layer energy budgets.

How to cite: Sarkar, M. and Bailey, A.: Sub-cloud Rain Evaporation in the North Atlantic Ocean during ATOMIC Campaign, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6462, https://doi.org/10.5194/egusphere-egu22-6462, 2022.

16:10–16:15
|
EGU22-8627
|
ECS
|
On-site presentation
Claudia Acquistapace, Diego Lange, Nils Risse, and Florian Späth

During the EUREC4A campaign, a synergy of ship-based remote sensing instruments deployed onboard the research vessel (RV) Maria S. Merian collected high-resolution observations of clouds, precipitation, and atmospheric boundary layer (ABL). Various data papers describe in detail the datasets collected. This work uses data from the W-band cloud radar, the Micro Rain Radar (MRR-PRO), the Atmospheric Raman Temperature and Humidity Sounder (ARTHUS), the wind lidar, and the radiosoundings. 

We statistically characterize clouds and precipitation properties by looking at specific observables collected during the campaign. We derive the W-band radar moments statistics (CFADs), the rain rate, and virga radar reflectivity profiles. We also display the relation between the W-band radar reflectivity and the radar skewness, revealing insights into the precipitation onset.

We investigate how the statistical distributions obtained for each of the observables mentioned above vary as a function of some environmental parameters like the columnar humidity, the turbulence, quantified in terms of eddy dissipation rate, and the vertical air motion.

The analysis aims to identify conditions and parameters that alter the cloud properties and precipitation characteristics to foster scientific knowledge of such processes and improve future model evaluations.

 

How to cite: Acquistapace, C., Lange, D., Risse, N., and Späth, F.: Investigation on the impact of environmental parameters on ship-based observations of trade wind shallow cumuli and precipitation., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8627, https://doi.org/10.5194/egusphere-egu22-8627, 2022.

16:15–16:20
|
EGU22-10285
|
ECS
|
Virtual presentation
Florian Tornow, Andrew Ackerman, Ann Fridlind, Brian Cairns, Ewan Crosbie, Simon Kirschler, Richard Moore, Claire Robinson, Chellappan Seethala, Michael Shook, Christiane Voigt, Edward Winstead, Luke Ziemba, Paquita Zuidema, and Armin Sorooshian

Mid-latitude marine cold air outbreaks (CAOs) occur in the post-frontal sector of extratropical cyclones. Once advected over the ocean, the marine boundary layer (MBL) quickly deepens and hosts near-overcast clouds that transition into an open-cellular cloud field downwind, mediated by a reduction in aerosol concentrations. Typically, the MBL experiences strong large-scale subsidence that is often associated with free-tropospheric (FT) dry intrusions. Apart from being relatively warm and dry, FT air may have substantially different aerosol properties and, thus, different cloud condensation nuclei (CCN) concentrations compared to the MBL.

In this study, we examine the difference between MBL and FT air by using in-situ and remote sensing observations collected during NASA's ACTIVATE (Aerosol Cloud Meteorology Interactions over the Western Atlantic Experiment) field campaign in the northwest Atlantic. Analysis of the 8 CAO flights in 2020 reveals predominantly far lesser CCN concentrations in the FT than in the MBL. We investigate one representative flight more deeply, through a fetch-dependent MBL CCN budget that has contributions from sea-surface fluxes, hydrometeor collision-coalescence, and entrainment of FT air. We find a dominant role of FT entrainment in reducing MBL CCN concentrations upwind of strong precipitation that results in cloud regime transition, consistent with satellite-retrieved gradients in droplet number concentration upwind of precipitation.

The FT circulation and its relative lack of CCN can accelerate overcast-to-broken cloud transitions, especially where MBL air is CCN-rich (e.g., near continents), and thereby dramatically reduce regional albedo.

 

How to cite: Tornow, F., Ackerman, A., Fridlind, A., Cairns, B., Crosbie, E., Kirschler, S., Moore, R., Robinson, C., Seethala, C., Shook, M., Voigt, C., Winstead, E., Ziemba, L., Zuidema, P., and Sorooshian, A.: Dilution of boundary layer cloud condensation nucleus concentrations by free tropospheric entrainment during marine cold air outbreaks, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10285, https://doi.org/10.5194/egusphere-egu22-10285, 2022.

16:20–16:25
|
EGU22-5072
|
ECS
|
Virtual presentation
Jannik Höller, Jan O. Härter, and Romain Fiévet

Cold pools are known to mediate the interactions between convective rain cells. Cold pool dynamics thus constitutes an important organizing mechanism for thunderstorms, in particular mesoscale convective systems and extreme rainfall events. Unfortunately, the observational detection of cold pools on a large scale has so far been hampered by the lack of relevant large-scale near-surface data. Unlike in numerical studies, where high-resolution near-surface fields of relevant quantities such as virtual temperature and winds are available and frequently used to detect cold pools, in observational studies cold pools are mainly identified based on surface time series. Since research vessels or weather stations measure these time series locally, the characterization of cold pools from observations is limited to regional or station-based studies. To eventually enable studies on a global scale, we here develop and evaluate a methodology for the detection of cold pools that relies only on data that (i) is globally available and (ii) has high spatio-temporal resolution. We trained convolutional neural networks to segment cold pools in cloud and rainfall fields from high-resolution cloud resolving simulation output. Such data is not only available from simulations, but also from geostationary satellites that fulfill both (i) and (ii). The networks feature a U-Net architecture, a common choice for image segmentation due to its strength in learning spatial correlations at different scales. Based on cloud and rainfall fields only, the trained networks systematically identify cold pool pixels in the simulation output. Our methodology may thus open for reliable global cold pool detection from space-borne sensors. As it also provides information on the spatial extent and the relative positioning of cold pools over time, our method may offer new insight into the role of cold pools in convective organization.

How to cite: Höller, J., Härter, J. O., and Fiévet, R.: U-Net Segmentation for the Detection of Convective Cold Pools From Cloud and Rainfall Fields, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5072, https://doi.org/10.5194/egusphere-egu22-5072, 2022.

16:25–16:30
|
EGU22-3682
|
Presentation form not yet defined
Nicolas Rochetin, Ludovic Touzé-Peiffer, and Raphaëla Vogel

A new method is developed to detect cold pools from atmospheric soundings over tropical oceans and applied to sounding data from the EUREC4A field campaign, which took place south and east of Barbados in January-February 2020. The proposed method uses soundings to discriminate cold pools from their surroundings: cold pools are defined as regions where the mixed-layer height is smaller than 400 m. The method is first tested against 2D surface temperature and precipitation fields in a realistic high-resolution simulation over the western tropical Atlantic. Then, the method is applied to a data set of 1068 atmospheric profiles from dropsondes (launched from two aircrafts) and 1105 from radiosondes (launched from an array of four ships and the Barbados Cloud Observatory). We show that 7 % of the EUREC4A soundings fell into cold pools. Cold pools soundings coincide with i) mesoscale cloud arcs, ii) temperature drops of about 1 K compared to the environment and moisture increases of about 1 g kg -1. Furthermore, cold pool moisture profiles exhibit a "moist layer" close to the surface, topped by a "dry layer" until the cloud base level, and followed by another moist layer in the cloud layer. In the presence of wind shear, the spreading of cold pools is favored downshear, suggesting downward momentum transport by unsaturated downdrafts. The results support the robustness of our detection method in diverse environmental conditions and its simplicity makes the method a promising tool for the characterization of cold pools, including their vertical structure. The applicability of the method to other regions and convective regimes is discussed.

How to cite: Rochetin, N., Touzé-Peiffer, L., and Vogel, R.: Cold pools observed during EUREC4A: detection and characterization from atmospheric soundings, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3682, https://doi.org/10.5194/egusphere-egu22-3682, 2022.

16:30–16:35
|
EGU22-7264
|
ECS
|
On-site presentation
Jule Radtke, Ann Kristin Naumann, Raphaela Vogel, Martin Hagen, and Felix Ament

Trade wind convection organises into a rich spectrum of spatial patterns, often in conjunction with precipitation development. This raises the question of the role of precipitation for spatial organization and vice versa. Using rain radar measurements during the EUREC4A field campaign we find that precipitation rates vary mainly independently from the spatial arrangement of precipitating cells. Mean precipitation increases with the size or number of cells, as it is closely related to the precipitating area. The cells’ degree of clustering, contrary, is typically greatest where the mean cell size is large and the cell number small. Consequently, scenes with a quite different spatial structure – with larger, more clustered convective structures at one time or with more numerous and distributed convective structures at another time – can have similar precipitation rates. Could spatial organization be a process to maintain precipitation rates in very different environments? We exploit large-domain realistic large eddy simulations to investigate scenes of trade wind convection that exhibit similar precipitation rates but different spatial structures. We discuss how the environment and circulation differ in these scenes and how this might necessitate different spatial structures to rain.

How to cite: Radtke, J., Naumann, A. K., Vogel, R., Hagen, M., and Ament, F.: On the relationship between precipitation and the spatial structure of trade wind convection, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7264, https://doi.org/10.5194/egusphere-egu22-7264, 2022.

16:35–16:40
Coffee break
Chairpersons: Raphaela Vogel, Claudia Acquistapace
Topic 3: (Mesoscale) convective organization
17:00–17:10
|
EGU22-11607
|
solicited
|
On-site presentation
Caroline muller, Da Yang, George Craig, Timothy Cronin, Benjamin Fildier, Jan Haerter, Cathy Hohenegger, Brian Mapes, David Randall, Sara Shamekh, and Steven Sherwood

Idealized simulations of the tropical atmosphere have predicted that clouds can spontaneously clump together in space, despite perfectly homogeneous settings. This phenomenon has been called self-aggregation, and results in a state where a moist cloudy region with intense deep convective storms is surrounded by extremely dry subsiding air devoid of deep clouds. We review here the main findings from theoretical work and idealized models, highlighting the physical processes believed to play a key role in convective self-aggregation. We also review the growing literature on the importance and implications of this phenomenon for the atmosphere, notably for the hydrological cycle and for precipitation extremes, in our current and in a warming climate.

How to cite: muller, C., Yang, D., Craig, G., Cronin, T., Fildier, B., Haerter, J., Hohenegger, C., Mapes, B., Randall, D., Shamekh, S., and Sherwood, S.: Spontaneous aggregation of convective storms, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11607, https://doi.org/10.5194/egusphere-egu22-11607, 2022.

17:10–17:15
|
EGU22-3836
|
ECS
|
Presentation form not yet defined
Leif Denby

The representation of shallow tradewind cumulus clouds in climate models accounts for the majority of inter-model spread in climate projections,
highlighting an urgent need to understand these clouds better. In particular, their spatial organisation appears to cause a strong impact of their radiative properties and dynamical evolution. The precise mechanisms driving different forms of convective organisation which arise both in nature and in simulations are, however, currently unknown.

Using unsupervised learning for identifying regimes of convective organisation in the tropical Atlantic, we will show results from analysing: a) what the
radiative properties of different forms of organisation are, b) what atmospheric characteristics coincide with different forms of organisation and c) what transitions occur when following air-masses along Lagrangian trajectories.  Specifically, we find: a) net radiation changes significantly between different forms of organisation, b) agreement with previous studies on the importance of boundary layer wind-speed and to some degree atmospheric stability, and c) we are able to succinctly capture what transitions occur between regimes.

How to cite: Denby, L.: Properties and transitions of mesoscale convective organisation during EUREC4A using unsupervised learning, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3836, https://doi.org/10.5194/egusphere-egu22-3836, 2022.

17:15–17:20
|
EGU22-2408
|
ECS
|
Virtual presentation
Leo Saffin, John Marsham, Alan Blyth, and Doug Parker

Tropical shallow convection exhibits strong variations in mesoscale organisation. Bretherton and Blossey (2017) used an LES to show how this convective organisation can be produced due to an advective feedback with mesoscale vertical motion and adjustment to a weak temperature gradient. Narenpitak et al. (2021) used an LES with forcings following a boundary-layer trajectory to simulate a case study from the EUREC4A field campaign where the clouds transition from disorganised small cumulus (sugar) to deeper more organised clouds with large detrainment layers (flowers). The LES produced a transition in cloud organisation and the main driving process was shown to be the advective feedback in mesoscale vertical motion seen in Bretherton and Blossey (2017).For comparison, we have looked at the same case study using high-resolution nested simulations with the Met Office's weather model, the UM. The UM reproduces the transition from sugar to flowers. Consistent with Narenpitak et al. (2021), the UM shows that the transition is associated with an increase in organisation and the mesoscale advective feedback is an important driving process. However, unlike the LES, the UM simulations show that the sugar clouds are already associated with a large amount of organisation. Because the mesoscale organisation is already present in the UM, the advective dispersion of mesoscale aggregation is an important process opposing aggregation during the transition from sugar to flowers, unlike in the LES.

How to cite: Saffin, L., Marsham, J., Blyth, A., and Parker, D.: Mesoscale organisation transition during the 2nd Feb EUREC4A case study simulated by a high-resolution weather model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2408, https://doi.org/10.5194/egusphere-egu22-2408, 2022.

17:20–17:25
|
EGU22-10489
|
ECS
|
Presentation form not yet defined
Pierre-Etienne Brilouet, Dominique Bouniol, Fleur Couvreux, Alex Ayet, Carlos Granero-Belinchon, Marie Lothon, and Alexis Mouche

Studying the marine atmospheric boundary layer (MABL) processes through satellite products is challenging. Here, we propose an innovative approach to investigate the MABL turbulent structures thanks to the spaceborne Synthetic Aperture Radar (SAR) images combined with the Geostationary Operational Environmental Satellite (GOES) images.

Due to access difficulties, the number of field campaigns carried out over the sea is limited. In this framework, the intensive EUREC4A field campaign that took place over the Western Tropical Atlantic Ocean, in Jan-Feb 2020, provides a relevant context with reference in situ measurements to evaluate the spaceborne observations. Especially the turbulence measurements of the French ATR-42 research aircraft, which include fine scale measurements of air motion, provide a valuable support to validate the hypothesis of a sea surface roughness signature of atmospheric coherent structures in the SAR images.

The February 13, 2020 day was chosen as a case study, given the good spatial and temporal colocalization between the airborne measurements and the satellite overpass. Two types of atmospheric processes are investigated: convective rolls in clear sky regions and cold pools characterizing the convective activity areas. The size and the orientation of the convective rolls has been characterize through the correlation function of the surface roughness and provides a very good correlation with the characteristics deduced from the airborne in situ data. Also, an object identification method is used to segregate the cold pools within the SAR image. Their characteristics such as their size, age and spreading rate can then be estimated with respect to the cloud field evolution provided by the GOES data.

How to cite: Brilouet, P.-E., Bouniol, D., Couvreux, F., Ayet, A., Granero-Belinchon, C., Lothon, M., and Mouche, A.: Combining satellite and in situ data to investigate the marine atmospheric boundary-layer structure and trade-wind cumuli organization, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10489, https://doi.org/10.5194/egusphere-egu22-10489, 2022.

17:25–17:30
|
EGU22-3254
|
ECS
|
Virtual presentation
Isabel L. McCoy, Paquita Zuidema, Sunil Baidar, Jessica Vial, Hauke Schulz, and Alan Brewer

The diurnal cycle in trade wind cloudiness has been observed to be driven by the diurnal cycle in the relative frequency of occurrence of mesoscale morphologies (i.e., Vial et al. 2021). These morphologies have been grouped based on their distinct appearance and cloud size into four categories, from small to large sizes: Sugar, Gravel, Flowers, and Fish. The diurnal cycle in cloudiness is associated with a late afternoon maximum in the smallest (Sugar) clouds which give way to clouds of larger size and vertical extent (Gravel, then Flowers) throughout the night. A remaining question is how sub-cloud dynamics evolve diurnally to facilitate this diurnal cycle in cloud morphology and thus cloudiness.

We examine the daily evolution of trade wind mesoscale morphologies with in situ observations from the 2020 joint campaign, EUREC4A (Elucidating the Role of Clouds–Circulation Coupling in Climate) and ATOMIC (Atlantic Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign), that took place in January and February in the Northwest tropical Atlantic. Measurements from the Ronald H. Brown research vessel allow us to analyze differences in the daily evolution of boundary layer structure and dynamics between morphologies. We decompose Doppler lidar-derived mass fluxes into their vertical velocity and cloud fraction contributions and examine their effect on diurnal cloud evolution as well as their relationship to environmental controls such as surface wind speeds, energy and moisture fluxes, stability, and near-surface air properties. Relationships between environmental controls and morphologies are further extended with the long-term recorded observations at the nearby moored NTAS (Northwest Tropical Atlantic Station) buoy.

How to cite: McCoy, I. L., Zuidema, P., Baidar, S., Vial, J., Schulz, H., and Brewer, A.: The Diurnal Evolution of Controls on Trade Wind Mesoscale Morphologies, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3254, https://doi.org/10.5194/egusphere-egu22-3254, 2022.

17:30–17:35
|
EGU22-5574
|
ECS
|
Presentation form not yet defined
Benjamin Fildier, Caroline Muller, Robert Pincus, and Stephan Fueglistaler

In models, a local maximum of clear-sky radiative cooling in the lower troposphere often appears as a necessary condition for the development and persistence of convective organization. However, no robust understanding has been provided for the emergence and disappearance of lower-tropospheric cooling in the atmosphere. Here we propose a theoretical characterization of clear-sky radiative cooling peaks, recently calculated from over 2,000 soundings launched during the EUREC4A field campaign in various patterns of shallow organization. A suite of scaling approximations are developed from simplified spectral theory to connect the longwave cooling peak to the vertical humidity structure set by convection. Its height is controlled by local maxima in the vertical gradients of water vapor path, and its magnitude is mainly controlled by the ratio between column relative humidity above and below the peak. In contrast, the value of the Planck function and the spectral width of emission only weakly vary across soundings.  Water vapor spectroscopy implies that upper-level intrusions of moist air detrained from lower latitudes can substantially dim these peaks, possibly by reducing the range of the spectrum that effectively cools to space at the level of the peak. This work motivates future modeling work, formulating the hypothesis that "Fish" patterns, which embed the widest persisting dry areas, may be the most favorable conditions for radiative processes to organize convection. If at play, this radiative feedback would maintain these patterns that are efficient at cooling the tropics, a type of "dry radiator fins" which could mitigate the risk of runaway climate states.

How to cite: Fildier, B., Muller, C., Pincus, R., and Fueglistaler, S.: Low-level radiative cooling peaks in regimes of shallow convective organization, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5574, https://doi.org/10.5194/egusphere-egu22-5574, 2022.

17:35–17:40
|
EGU22-11839
|
ECS
|
Virtual presentation
Martin Janssens, Franziska Glassmeier, A. Pier Siebesma, Stephan R. de Roode, Chiel C. van Heerwaarden, and Jordi Vilà-Guerau de Arellano

A growing body of evidence suggests that shallow circulations play a key role in organising trade-wind clouds at the mesoscales. In turn, many of these mesoscale circulations appear to emerge directly from the shallow convection itself. We infer a very simple model for explaining this feedback from Large-Eddy Simulations of a classical numerical experiment with minimal physics (BOMEX), which depends only on the turbulent transport of liquid water in cumulus clouds and the mean environment. Since the dominant scales of the cumulus convection driving the organisation are constrained around a kilometer, we hypothesise that simulations of the development of mesoscale cloud patterns through this mechanism are sensitive to choices in the numerical representation of the cumulus convection. We show that the timescale over which mesoscale cloud structures develop can more than double in our model, merely by modifying its grid spacing or advection schemes. Hence, rather high resolutions (<100m) or significantly improved unresolved scales models may be required to faithfully represent certain forms of trade-wind mesoscale cloud patterns in models, and to understand their influence on the cloud feedback more accurately.

How to cite: Janssens, M., Glassmeier, F., Siebesma, A. P., R. de Roode, S., C. van Heerwaarden, C., and Vilà-Guerau de Arellano, J.: Numerical choices in Large-Eddy Simulations influence their ability to represent shallow convective organisation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11839, https://doi.org/10.5194/egusphere-egu22-11839, 2022.

17:40–17:45
Topic 4: Boundary layer processes and circulations
17:45–17:50
|
EGU22-12975
|
Virtual presentation
Jan H. Schween and Ulrich Löhnert

Marine stratocumulus clouds above the eastern Pacific form one of the largest permanent cloud fields of the planet.
They play an essential role in the Earth's energy and radiation budget. At the west coast of South America they reach the continent and provide a major water source for the hyperarid Atacama desert.  As part of the DFG collaborative research center 'Earth evolution at the dry limit' we observed these clouds over one year with state of the art remote sensing instruments from the coastal town of Iquique at 20.5°S.  The instruments provide vertical profiles of wind, turbulence and temperature, as well as integrated values of water vapor and liquid water. The cloudnet algorithm is used to exploit instrument synergy and provides vertical cloud structure information. 

The stratocumulus shows here a distinct diurnal behaviour with the cloud dissolving in the morning, and recurring in the afternoon. The observations show that the clouds dissolve from the surface. Comparison with surface measurements reveals that this is the result of an interplay between surface heating and a somewhat delayed advection of dry air from the desert during night and early morning and moist air from the ocean during daytime. 

The annual course with stratocumulus at nearly all times in austral winter and less frequent and higher clouds in austral summer shows a strong connection to sea surface temperature (SST): During winter stratification in the maritime boundary layer (MBL) is neutral and temperature is about that of the ocean surface. In contrast hereto stratification in summer is slightly stable and the MBL is warmer than the ocean. This inhibits moisture transport into the MBL and thus does not allow a persistent stratocumulus cloud. Interestingly the temperature of the coastal MBL would be in equilibrium with the SST some 50 km off the coast. The low coastal SST is related to upwelling of ocean water along the coast, while the warmer waters off the coast are result of a displacement of the cold waters of Humboldt current in summer. 
This points to a rather complex interplay of ocean dynamics and atmospheric circulation.

 

How to cite: Schween, J. H. and Löhnert, U.: The connection of Stratocumulus Clouds at the West Coast of South America to environmental parameters., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12975, https://doi.org/10.5194/egusphere-egu22-12975, 2022.

17:50–17:55
|
EGU22-7904
|
ECS
|
On-site presentation
Marc Prange, Manfred Brath, and Stefan A. Buehler

Satellite based hyperspectral infrared (IR) sounders like IASI, AIRS and CrIS offer a wealth of information about the atmospheric composition and vertical structure. A key quantity these instruments are able to capture is the vertical profile of water vapor in clear-sky and partly cloudy conditions. The work presented here revolves around mid tropospheric layers of increased humidity, so called Elevated Moist Layers (EMLs). EMLs frequently emerge in the vicinity of deep convection in the tropics as they are thought to originate from detrained moisture of convective plumes near the stable freezing level at around 5 km altitude. Previous retrieval case studies indicate limited retrievability of EMLs based on hyperspectral IR observations depending on the exact retrieval method, retrieval setup and the atmospheric conditions. Since EMLs severely influence the local radiation budget of the atmosphere, we need to understand what operational retrievals capture and what they may miss about EMLs.

As a starting point, we present an EML case study from the NARVAL-2 measurement campaign to directly compare IASI and AIRS retrieval products to in-situ soundings. We also introduce ERA5 as an additional reference to assess whether limitations in the retrieval product propagate to the reanalysis. As a next step, we conduct a first systematic statistical assessment of EML retrievability based on long term operational retrieval data. As reference, we use radiosonde data from the GRUAN database and ERA5. The EMLs in the different datasets are first identified by introducing smooth reference humidity profiles. The EMLs are then characterized by their layer averaged anomalous humidity, their thickness and altitude. These EML characteristics are compared statistically to assess what type of EMLs the retrievals capture well and where there might be systematic issues. We also calculate radiative heating profiles and assess the impact of EML retrievability on radiative heating.

How to cite: Prange, M., Brath, M., and Buehler, S. A.: Retrieval of Elevated Moist Layers using Hyperspectral Infrared Sounders, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7904, https://doi.org/10.5194/egusphere-egu22-7904, 2022.

17:55–18:00
|
EGU22-6458
|
ECS
|
Virtual presentation
Anna Lea Albright, Sandrine Bony, Bjorn Stevens, and Raphaela Vogel

The transition layer in the trades has long been observed and simulated, but its origins remain little investigated. It is often associated with an about 150 m deep layer at the top of the subcloud layer that acts as a barrier to overlying convection. Using extensive observations from the EUREC4A field campaign, we propose a reconceptualization of the transition layer. Strong jumps at the mixed layer top, as expected from the theory of cloud-free convective boundary layers, are only found rarely and when they occur, they tend to occur in large cloud-free areas. We show that small clouds with their bases around 600 m maintain the transition layer, in analogy with the maintenance of the trade-wind inversion by deeper clouds. From this analysis also emerges the potential for an alternate view of entrainment mixing, which is based on the ability to detrain condensate into the transition layer and induce gentle sinking motion through negative buoyancy. Mixed layer theory and Paluch mixing diagrams are also used to gain inferences into entrainment mixing.

How to cite: Albright, A. L., Bony, S., Stevens, B., and Vogel, R.: A new conceptual picture of the transition layer, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6458, https://doi.org/10.5194/egusphere-egu22-6458, 2022.

18:00–18:05
|
EGU22-8080
|
Virtual presentation
Louise Nuijens, Alessandro Savazzi, Gijs de Boer, Pierre-Etienne Brilouet, Marie Lothon, Geet George, and Dongxiao Zhang

Profiles of eddy momentum flux divergence are calculated as the residual in the momentum budget constructed from from airborne circular dropsonde arrays (~ 220 km) for thirteen days during the EUREC4A/ATOMIC field study east of Barbados. The observed dynamical forcing averaged over all flight days agrees broadly with ECMWF IFS forecasts. They suggest a flux divergence, or friction on the mean flow, over a 1.5 km deep layer in the prevailing wind direction. Assuming only vertical flux divergence that is zero near a local wind maximum, the observed friction corresponds to a 10 m momentum flux of ~ 0.1 Nm-2, comparable to in-situ turbulence measurements by a Saildrone. Between 1 - 1.5 km the momentum flux divergence is counter-gradient and vertical wind shear exceeds the observed thermal wind. An averaged momentum flux divergence in the cross-wind direction is also observed and corresponds to a veering of the wind that promotes flow parallel to the isobars. 

The along- and cross-wind flux divergence differ substantially between days, whereby a number of flights capture ascending branches of shallow circulations where only weak flux divergence near the surface is found and flux convergence (an acceleration of the mean flow) in the cloud and inversion layer. Budget-derived and in-situ measured momentum fluxes disagree on individual days. Turbulence measurements on board the SAFIRE ATR-42 (ATR) aircraft and the UAV CU RAAVEN reveal pronounced spatial variability (5 - 60 km) of momentum flux, which suggests that convectively-driven (mesoscale) flows can compensate turbulence-induced friction within the dropsonde array.

How to cite: Nuijens, L., Savazzi, A., de Boer, G., Brilouet, P.-E., Lothon, M., George, G., and Zhang, D.: The frictional layer in the observed momentum budget of the trades, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8080, https://doi.org/10.5194/egusphere-egu22-8080, 2022.

18:05–18:10
|
EGU22-8166
|
ECS
|
On-site presentation
Alessandro Carlo Maria Savazzi, Louise Nuijens, Wim de Rooy, and Pier Siebesma

The transport of horizontal momentum takes place at various spatial and temporal scales: from small-scale turbulence to cloud- and meso-scale circulations. This study focuses on the role of convective momentum transport (CMT) in the momentum budget in trade-wind cloud regimes with different patterns of cloud organization. Observations of the momentum budget during EUREC4A suggest that in early February, deeper convection and larger cloud structures are associated with a different profile of eddy momentum flux divergence than days with shallower cumulus humilis. Using large eddy simulation hindcasts and a mesoscale weather model, we study the profiles of eddy momentum flux associated with turbulence, convection and mesoscale flows in different cloud scenes during EUREC4A. Are turbulent, convective or mesoscale circulations responsible for a deceleration or acceleration of the mean flow? Are along-wind or cross-wind circulations more pronounced? Do the models show evidence of countergradient flux  production in the cloud layer?

 

We select a ten-day period for which the Dutch Atmospheric Large-Eddy Simulation (DALES) model is run on a 150 km x 150 km domain with a resolution of 100 m. Its boundaries are forced hourly with dynamical tendencies from the mesoscale weather model (HARMONIE), which is initialized every 24 hours from ERA5. HARMONIE is also run in a climatological mode on a 3200 km x 2000 km domain with 2.5 km resolution, in runs with shallow convective momentum transport on and off.

 

In this presentation, we first evaluate the models’ ability to reproduce the mean and evolution of the wind profiles and the momentum fluxes during the ten days, as well as the cloud organization. Second, we present and discuss the eddy momentum flux divergence that is carried by flows on different scales and evaluate its role in the momentum budget. Third, we discuss the relationship between shallow convective momentum transport and cloud organization.

How to cite: Savazzi, A. C. M., Nuijens, L., de Rooy, W., and Siebesma, P.: Unveiling Convective Momentum Transport at different scales during EUREC4A, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8166, https://doi.org/10.5194/egusphere-egu22-8166, 2022.

18:10–18:15
|
EGU22-13444
|
ECS
|
Virtual presentation
Geet George, Bjorn Stevens, Sandrine Bony, Raphaela Vogel, and Ann Kristin Naumann

We find an abundance of low-level, mesoscale circulations in the atmosphere below the trade-wind inversion layer based on observations of the mesoscale atmospheric circulation taken during the EUREC4A campaign. Over time-means of 3-6 hours, the mean sub-cloud divergence anomaly correlates negatively with the mean divergence anomaly in the cloud layer. Here, the term anomaly means the deviation from the EUREC4A-wide month-long mean of divergence at the corresponding altitude. Additionally, sub-cloud divergence anomaly correlates negatively with specific humidity anomaly in the sub-cloud and cloud layers, indicating moist, convergent regimes and dry, divergent regimes. We hypothesise that the presence of shallow circulations below the inversion layer explains these associations. Our proposed mechanism of shallow circulations is that regions of ascending air are balanced by neighbouring cells of subsidence, thus creating and maintaining moist and dry regions, which reinforce the shallow circulations. We use mixed-layer theory to estimate the time-scales at which the sub-cloud layer moisture would respond to such divergence patterns. The observed relationships are also evident in reanalysis data, which further reinforce that these are indeed mesoscale features and not large-scale signals captured by the observations.

How to cite: George, G., Stevens, B., Bony, S., Vogel, R., and Naumann, A. K.: The ubiquity of shallow circulations in the trades, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13444, https://doi.org/10.5194/egusphere-egu22-13444, 2022.

18:15–18:20
18:20–18:30