Vertical Motions in Convective Clouds Over Darwin, Australia

Vertical motions are essential to the parameterization of convection in large-scale model grids. However, in tropical cloud systems vertical motions are difficult to obtain, particularly in areas of active convection. This study uses three months of profiler data from Darwin, Australia to directly compare vertical velocity and spectrum width profiles with reflectivity at a height of 1 km (a proxy for near-surface rain) for shallow, mid-level, and deep convective clouds. Additionally, vertical velocities for all convective cloud types were compared to echo-top heights of varying reflectivity thresholds in order to better understand the dynamics of convective clouds in relation to their vertical structure.

In shallow convective clouds (tops < 4 km), there are three distinct regimes: a weak updraft-downdraft couplet at the weakest reflectivities (0-15 dBz), a robust updraft at moderate reflectivities (20-35 dBz), and strong downdrafts at large reflectivities (>40 dBz). These regimes potentially represent different stages in the convective cloud life cycle with strong updrafts and moderate near-surface reflectivity occurring in the growing phase of the cloud and strong downdrafts and large reflectivity occurring in the mature phase. The weak updraft-downdraft couplet and small near-surface reflectivities suggest the dissipating stage of the cloud. Mid-level convective clouds (tops between 4 and 8 km) also show three distinct regimes: moderate updrafts at the lowest reflectivities (possible growing phase), a weak updraft-downdraft couplet at moderate reflectivities (possible dissipating stage), and the strongest up-and downdrafts at large reflectivities (or the mature phase). Deep convective clouds (tops >8 km) show strong updrafts above 4 km across all reflectivity values with the strongest downdrafts occurring at the largest reflectivities. While maximum updrafts varied in height and occurred at different reflectivities between cloud types, mean downdraft depth never exceeded 3 km and downdrafts were always strongest at reflectivities > 40 dBz, which may allow better characterization of cold pool properties associated with convection. Throughout all convective cloud types, spectrum width appears to have the highest values at lower heights than where the strongest up-and downdrafts occur while also exhibiting a maximum value core around the transition height from up to downdrafts. In addition, maximum vertical motions occur at or just beneath the 30-dBz echo-top height in all convective clouds, especially in the mixed phase region, which suggests potential links to electrification.