A Simulation of the Development of Successive Cells Along a Cold Outflow Boundary

Abstract

A three-dimensional numerical simulation is presented in which five new cells (clouds) develop in succession over a 4 h period. The cells that develop have common characteristics including a lifetime of ?40 min. They form at 30 min intervals in a convergent region along the cold outflow boundary that is established by previous cells. The propagation of the cloud system is several meters per second faster than individual cell movement due to the discrete development of new cells on the right flank of previous ones. Analysis of the simulation indicates that the most pronounced changes near the surface are due to downdraft development. In contrast, there is only a weak signature of new updraft development near the surface in the convergence field. Each new updraft develops along the cold outflow boundary which moves slowly away from the previous cell. The air participating in the generation of a new cell appears to have originated 1?1.5 km below cloud base (2.6 km). As the cell grows and rain begins falling out of it, the updraft loses its roots in the subcloud layer and a new cell begins to form. The sounding used to initialize the model was taken on a day that successive development occurred. Comparison of the observed and modeled cell behavior indicates some broad similarities, but also many differences. For example, new cell development was observed every 15 min. An explanation for this difference and the wide range of frequencies for new cell formation observed in other storms will require further study.