The morphology of simulated trade-wind convection and cold pools under
wind shear
Abstract
A growing body of literature investigates convective organisation, 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 for cold-pool
development, and another set inhibits evaporation of precipitation and
thus cold-pool formation. Without (or with only weak) subcloud-layer
shear, conditions are unfavourable 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, 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. Forward shear 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).