Ubiquity of shallow mesoscale circulations in the trades and their influence on moisture variance

Abstract

Understanding drivers of cloud organization is crucial for accurately estimating clouds' feedback to a warming climate. Shallow mesoscale circulations are thought to play an important role in cloud organization, but they have not been observed. Here, we present observational evidence for shallow mesoscale overturning circulations (SMOCs) from divergence measurements made during the EUREC4A field campaign in the north-Atlantic trades. Meteorological reanalyses reproduce the observed lowlevel divergence well and confirm SMOCs to be mesoscale features (ca. 200 km). Large mesoscale variability, five-fold the mean, is shown to be associated with the ubiquity of SMOCs. Furthermore, time-lag correlations suggest that SMOCs amplify mesoscale moisture variance at cloud-base and in the sub-cloud layer. Through their modulation of cloud-base moisture, SMOCs influence the drying efficiency of entrainment, thus yielding moist ascending branches and dry descending branches. The observed moisture variance differs from expectations from large-eddy simulations, which show largest variance near cloud top and negligible sub-cloud variance. The ubiquity of SMOCS and their coupling to moisture and cloud fields suggest that the strength and scale of mesoscale circulations are important in determining how clouds couple to climate, something which is not considered by present theories.

Evaluating large-domain, hecto-meter, large-eddy simulations of trade-wind clouds using EUREC4A data

“…In the past, there had been the concern that the large-scale vertical winds from the reanalysis winds would not be representative of the observed conditions. George et al (2022) demonstrates that the mean large-scale vertical motion observed across the EUREC 4 A-circle agrees well with the analysis, also when the sondes were not included, giving confidence in the ability of the analysis to capture the large-scale conditions.…”

Cloud transition across the daily cycle illuminates model responses of trade cumuli to warming

“…Most of the climate models considered here simulate unrealistic cloud behaviors at the subdaily timescale. These shortcomings call for a community effort to i) analyze high-frequency model outputs to assess fast physical processes in which many uncertainties of the current climate and future projections are rooted, ii) develop parameterizations that better represent the cloud–convection coupling, the main populations of shallow cumuli observed in nature and their transition on the daily timescale, and iii) better understand the factors that influence the strength of shallow convection, including the role of shallow mesoscale circulations that are ubiquitous in the tropics and have the potential to influence trade cumulus dynamics ( 21 , 26 , 36 ). Addressing these gaps with observations that can be collected every day stands a good chance to ultimately increase the credibility of climate projections.…”

Spatial organisation affects the pathway to precipitation in simulated trade-wind convection

“…We investigate spatial organisation in scenes of 4 x 4 • (about 450 x 450 km), an area extent similar to previous studies (Radtke et al, 2022;George et al, 2022). Figure 1a shows three example scenes with different degrees of organisation.…”

“…enhanced moisture transport into anomalously moist patches by mesoscale circulations (Narenpitak et al, 2021;George et al, 2022), larger cloud clusters that are less prone to dilution (Tian & Kuang, 2016), as well as clouds pre-conditioning the environment for subsequent convection (Kuang & Bretherton, 2006). The more favourable thermodynamic conditions allow clouds to develop under less favourable dynamic conditions, leading to a less efficient rain production.…”