Initiating Moist Convection in an Inhomogeneous Layer by Uniform Ascent

Abstract

Abstract Using aircraft data from the recent Dominica Experiment (DOMEX) project in Dominica, the authors evaluate a modified version of Woodcock’s theory of moist convective initiation. Upstream of Dominica, anticorrelated fluctuations in temperature and specific humidity are found in the subcloud layer related to ambient trade wind convection. The associated variances in virtual temperature and air density are surprisingly small due to buoyancy adjustment. When this air is quickly lifted by terrain, the moist patches, having a lower lifting condensation level, become “seeds” for convection. The authors model this process by uniformly lifting an observed layer of air moist adiabatically from 300 to 1300 m. The resulting variations of buoyancy within the layer are converted to vertical accelerations accounting for strong “added mass” effects that include an estimate of layer depth. These estimated fluctuations in vertical acceleration agree with aircraft measurements of updraft speed and length scale over the terrain of Dominica. The authors speculate on the breadth of applicability of this mechanism of convective initiation.