How wind shear affects trade-wind cumulus convection by modulating cold-pool dynamics

Abstract:

Motivated by the large uncertainties of shallow cumuli in climate projections and by observations of a close relationship of marine low cloud cover and surface wind speed, we investigate how wind shear affects trade-wind cumulus convection. Using DALES, we simulate shallow cumulus and congestus clouds typical of the subtropical trades, imposing varying amounts of zonal wind shear on increasingly large domains (up to 200 x 200 km²).

We find that backward shear (as opposed to no shear) effectively limits the vertical growth of clouds, leading to a shallower and moister trade-wind layer. This first-order effect of shear to limit convection can be explained by lower surface heat fluxes as a consequence of different surface wind speeds and by weakened updrafts due to a larger downward pressure perturbation force. On the other hand, forward shear on sufficiently large domains only slightly limits the clouds’ growth, allowing for somewhat deeper convection. This is attributable to larger moisture aggregations that are favoured by forward shear and that benefit cloud deepening. In this presentation, we test the idea that a favourable interaction of cold pools and forward shear helps explain the observed cloud response.