Impact of imposed drying on deep convection in a cloud-resolving model

This study uses a cloud-resolving model under the weak temperature gradient (WTG) approximation to explore the response of deep convection to imposed drying. The drying, intended to mimic horizontal advection of dry air, is imposed as a linear relaxation of the humidity field towards zero within a specified layer. Radiative cooling and sea surface temperature are held constant and there is no mean vertical shear in the integrations. The statistically steady responses to drying in the lower, middle, and upper free troposphere are compared.

The rain rate decreases with drying when that drying is imposed in the lower or middle free troposphere, more strongly so for lower tropospheric drying. The decrease in rainfall is linearly proportional to the drying to a good approximation. Upper-tropospheric drying has almost no impact on the rain rate. For lower-tropospheric drying, the decrease in rain rate with drying can be well explained by assuming that both an appropriately defined normalized gross moist stability and surface turbulent fluxes remain constant as drying varies. For middle- and upper-tropospheric drying, the normalized gross moist stability decreases with drying, allowing the precipitation to decrease less rapidly or not at all. The variations in normalized gross moist stability in turn result from changes in the large-scale vertical motion profiles. These are top-heavy for weak or no drying, becoming somewhat less so as the strength of the drying increases. Over most of the range of drying strengths, the changes to the shape of the vertical motion profile are greatest for upper-tropospheric drying and smallest for lower-tropospheric drying. For very strong lower-tropospheric drying, however, the vertical motion profile becomes bottom-heavy and the normalized gross moist stability becomes negative.