Numerical Simulation of Low-Level Downdraft Initiation within Precipitating Cumulonimbi: Some Preliminary Results

Abstract

Two three-dimensional cloud model simulations are examined and compared in order to define some of the characteristics of the low-level downdraft initiation process within deep precipitating convection. The initial environment of each case displayed similar temperature profiles but different moisture profiles. In one case, relatively dry subcloud layers were capped by relatively moist middle levels, while the opposite moisture stratification existed for the second case. Although both simulations displayed peak low-level downdraft speeds of ?12 m s?1, downdraft spatial and temporal behavior showed significant differences. These differences can be related to dissimilarities in the environment of each case. In the dry subcloud case (the microburst case), peak downdraft speeds occurred near the 0.8 km level shortly after precipitation arrived at low levels. Low-level downdraft developed very rapidly in this case. In the other moist subcloud case, the low-level downdraft developed less rapidly and exhibited a peak magnitude significantly higher at 1.8 km. In both case the downdraft initiation process occurred within the downshear flank. Downdrafts were forced primarily over the lowest 2 km (below the melting level), where melting and evaporation of precipitation generated negative buoyancy. The results demonstrate that low-level downdraft characteristics are closely controlled by arrival of precipitation at low levels.