The Morphology of Simulated Trade‐Wind Convection and Cold Pools Under Wind Shear. Issue 20 (13th October 2021)

• Record Type: Journal Article
• Authors: Helfer, K. C.; Nuijens, L.
• Is Part Of: Journal of geophysical research. Volume 126:Issue 20(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-10-13
• Subjects: shallow convection -- wind shear -- congestus -- trade wind -- large‐eddy‐simulation -- precipitation; Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1029/2021JD035148
• Languages: English
• ISSNs: 2169-897X

Abstract

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). Plain Language Summary: The most common type of clouds in Earth's trade wind regions are precipitating cumulus clouds with tops up to 4 km height. The precipitation from such clouds is frequent and intense enough to cause so‐called cold pools: cold and dry air that spreads out laterally near the surface in a circular fashion triggering new clouds in arc‐like patterns. We used a high‐resolution atmospheric model to investigate how the morphology of such clouds and the associated cold pools is affected by vertical changes in the wind speed (shear). When the wind speed at the surface and at cloud base is the same, clouds remain above their "roots, " and downward‐moving air associated with rain falls into those cloud roots, which hinders the consequent deepening of these clouds. When the wind speed increases from the surface to cloud base (which it often does), clouds move away from their roots, which separates the location of updrafts and downdrafts, allowing for the development of deeper clouds. This type of shear also promotes the formation of new clouds further away at the edge of cold pools. Key Points: Without cold pools, new convection is triggered upwind of existing cells, and with cold pools, new convection is triggered downwind In the absence of (or with only weak) subcloud‐layer shear, rain falls into emerging updrafts, hindering the convective development Uplift at the cold‐pool gust fronts is strongest under subcloud‐layer forward shear (increasing wind speed with height)