Numerical modeling of a line of towering cumulus on day 226 of GATE

A three-dimensional numerical model with warm rain bulk cloud physics is used to investigate the shallow convection observed on day 226 of GATE. This convection had cloud tops at 3.0 km, cloud bases at 0.4 km, and similar to 0.1 cm of rain at the surface. The simulated convection shows a strong sensitivity to the criterion for the onset of autoconversion of cloud water into rain water. The strongest convection occurs for the case in which no rain water forms. This case, however, does not conform to the observed convection, lacking the downdraft below cloud base and the observed strong surface outflow. The primary simulation produces a finger of convection propagating to the northeast, perpendicular to the northwest-southeast orientation of the larger scale line of convection. The orientation and propagation speed of the calculated convection are in excellent agreement with observed radar data. This simulation also has a well-defined leading edge and strong surface outflow as observed. In poorer agreement, the cloud base was too high and the rainfall at the surface was less than observed. Present calculations indicate that the boundary layer air is flowing through the line from southwest to northeast below cloud base. The primary moisture source for the cloud is the upper half of the subcloud layer, with nearly horizontal flow entering the cloud.