Numerical Simulation of Organized Convection. Part I: Model Description and Preliminary Comparisons with Squall Line Observations

A numerical model designed for the simulations of mesoscale flows perturbed by deep convective clouds is discussed. It is based on the time dependent coupling between a three-dimensional nonhydrostatic mesoscale model and a quasi-one-dimensional cloud model. The evolution and motion of individual convective cells are simulated by the cloud model since they cannot be explicitly resolved by the mesoscale model. This implies that in cloudy areas each model simulates the time evolution of the same variable at the same location and at the same time, under the influence of different processes (large-scale processes for the mesoscale model and microphysical processes for the cloud model). The comparison between the cloud and mesoscale rates of change leads to an evaluation of the coupling terms which transmit the cloud influences into the mesoscale model so that mesoscale fields are perturbed accordingly. For example, the nonhydrostatic pressure field reacts to the cloud development by building up an overpressure dome above the cloud top. On the other hand, the environmental conditions of each cell, including the vertical gradient of the nonhydrostatic pressure, represent the mesoscale influence on the cloud model. These conditions are deduced from the local values of the perturbed mesoscale fields at each cloud position. A particular simulation is analyzed. Analysis reveals that the main characteristics of the flow perturbed by the convective cells are similar to those of a squall line deduced from ground station measurement. The typical signatures of the gust front are simulated, even though the magnitude of the perturbations are not all well simulated. The convective transport of horizontal momentum appears to be of fundamental importance to the organization process.