The three-dimensional structure of precipitating shallow cumuli. Part one: The kinematics

The detailed 3D structure of shallow cumulus clouds is analyzed using data from large eddy simulations. The vertical and horizontal cross sections of the flow fields show that, for both non-precipitating and precipitating cumulus clouds, the in-cloud circulation can be partitioned into two layers: a convergence layer in the lower part of the clouds and a divergence layer in the upper part of the clouds. Generally the air converges in the center of the lower part (1.0-1.3. km) of the clouds and rises within the center. Then the air diverges in the upper part (from 1.3. km to the cloud top) of the clouds and sinks through the edges. This is consistent with previous findings that toroidal circulation exists in shallow cumulus clouds. A thin convergence layer can also be found just above the cloud top due to the downward motion that is induced by the evaporative cooling of cloud drops around the cloud top. Despite the similar in-cloud structure for both non-precipitating and precipitating clouds, precipitation can decrease the intensity of convergence in the lower layer of the clouds. Results also indicate that precipitation has a significant effect on the circulations below the clouds. For non-precipitating cumuli, there is only weak convergence below the clouds. However, for precipitating cumuli, the layer close to the surface in precipitating regions is dominated with divergence that is due to the precipitation-induced downdrafts.