The Morphology of Simulated Trade-Wind Convection and Cold Pools Under Wind Shear.

K. C. Helfer,L. Nuijens

doi:10.1029/2021jd035148

Abstract

A growing body of literature investigates convective organization, but few studies to date have sought to investigate how wind shear plays a role in the spatial organization of shallow (trade‐wind) convection. The present study hence investigates the morphology of precipitating marine cumulus convection using large‐eddy‐simulation experiments with zonal forward and backward shear and without shear. One set of simulations includes evaporation of precipitation, promoting cold‐pool development, and another set inhibits the evaporation of precipitation and thus cold‐pool formation. Without (or with only weak) subcloud‐layer shear, conditions are unfavorable for convective deepening, as clouds remain stationary relative to their subcloud‐layer roots so that precipitative downdrafts interfere with emerging updrafts. Under subcloud‐layer forward shear (FS), where the wind strengthens with height (a condition that is commonly found in the trades), clouds move at greater speed than their roots and precipitation falls downwind away from emerging updrafts. FS in the subcloud layer appears to promote the development of stronger subcloud circulations, with greater divergence in the cold‐pool area downwind of the original cell and larger convergence and stronger uplift at the gust front boundary. As clouds shear forward, a larger fraction of precipitation falls outside of clouds, leading to more moistening within the cold pool (gust front).

Highlights

Triggered by the World Climate Research Programme's grand challenge on clouds, circulation, and climate sensitivity (Bony et al, 2015), tremendous research efforts have been undertaken in recent years to study maritime shallow clouds, with an increasing interest in their organization
Under subcloud-layer forward shear (FS), where the wind strengthens with height, clouds move at greater speed than their roots and precipitation falls downwind away from emerging updrafts
We aim to address why cloud deepening may be inhibited more under backward shear (BS) than under FS and, in particular, if this is dependent on the presence or absence of cold pools as suggested in HNRS20

Summary

Introduction

Triggered by the World Climate Research Programme's grand challenge on clouds, circulation, and climate sensitivity (Bony et al, 2015), tremendous research efforts have been undertaken in recent years to study maritime shallow clouds, with an increasing interest in their organization. Under FS, convection appears to have a tendency to grow deeper, which seems related to this system's enhanced potential to aggregate column moisture on mesoscales Another noteworthy observation of HNRS20 is that wind anomalies within cold pools depend on the direction of the shear. As moist near-surface air is lifted to higher levels above the level of free convection, it can moisten the upper boundary layer, lower the troposphere, and trigger new convective events This forced uplift may be enhanced by the collision of two or more cold-pool fronts (e.g., Feng et al, 2015; Meyer & Haerter, 2020). We discuss and summarize our findings in the concluding section

Experimental Design

Cold-Pool Structure and Behavior

System Development Without Evaporation of Precipitation

Discussion and Conclusion