A Further Study of Cumulus Interactions and Mergers: Three-Dimensional Simulations with Trajectory Analyses

Abstract

A total of nine three-dimensional experiments are made to study cloud interaction and merging under the influence of different imposed conditions. Large-scale lifting forcing, environmental wind shear and cloud microphysical processes are the three parameters to be varied. The basic design of the study is to generate several convective clouds randomly inside the model domain and, then, to observe and analyze the interactions and merging between the simulated clouds. The locations as well as the intensities of simulated clouds while they interact with each other are not predetermined. A two-dimensional version of the model has been used to investigate the effects upon merging produced by varying large-scale conditions with a GATE dataset. In this study, we continue studying the cloud interactions and merging problems through using a fully three-dimensional model and the same dataset. Ten merged systems involved precipitating clouds are identified in this numerical study. Eight mergers involve two previously separated clouds; seven of them generally lie along a line parallel to the initial environmental wind shear vector (called parallel cells). Only one merger lies along a line rather perpendicular to the wind shear vector prior to the merging (called perpendicular cells). A significant difference between the parallel and the perpendicular cells is that the latter cells are usually situated closer to each other prior to merging than the former cells. The distance between the perpendicular cells prior to merging is usually about 5 to 6 km. The distance between the parallel cells prior to merging can be 10 km or more. The remaining two merged systems involve three clouds and they are a combination of parallel and perpendicular cells. The merging mechanism associated with three cloud merging cases is studied through examining the temperature, pressure and wind fields prior to, during and following the merging of clouds. The first case involves a pair of precipitating clouds with differential propagation speeds. Both clouds propagate along the direction of the vertical wind shear. The second case is a perpendicular cell and the third case involves three clouds. A cloud bridge, which consists of a few low-level clouds which develop and connect the merging clouds prior to or during the merging process, occurs in all three cases. Trajectory analyses indicate that the high rising air parcels at the bridge area are strongly affected by either one or two interacting cold outflows. This specific study suggests that the primary initiating mechanism for the occurrence of a precipitating cloud merger is the cloud downdrafts and their associated cold outflows.