The broadening of mesoscale convective cells (MMC) during cold-air outbreak (CAOB) situations is studied using large-eddy simulation (LES). The study is focused on the question about which physical processes are responsible for the large aspect ratios of MMC observed during CAOB situations. Indications are given by earlier numerical simulations of CAOBs, that adiabatic heat sources are causing the cell broadening and are thus responsible for these large aspect ratios. Nevertheless, due to insufficient computer resources, the principle question of cell broadening remains from these studies, because they either used a too small model domain or a too coarse horizontal grid resolution (grid spacing of 1 km).

Using the parallelized LES model PALM we have been able for the first time to carry out simulations for a large area combined with a fine grid resolution avoiding the shortenings of the earlier studies. We performed two principal runs using an initialization according to a situation observed during the ARKTIS1991 experiment. For all runs the model domain covers an area of 70 x 70 km**2 in the horizontal and 5 km (resulting in 700 x 700 x 80 grid points). The simulations were performed on 256 processor elements (PE) of a CRAY-T3E covering a period of 12.5 h. Each PE required 115 CPU-h in total. Run1 includes the whole water cycle, whereas run2 takes the same initial parameters but the water cycle was switched off in order to study the influence of adiabatic heat sources on cell formation and broadening.

During Run1 the formation of MMC is identified. A clear visual signal of MMC can be detected in the horizontal cross sections of the thermodynamic variables (e.g., liquid water content), whereas the dynamic field variables are characterized by spatially randomly-distributed up- and downdrafts so that the organized cells can hardly be detected from them by eye. Here, each MMC is an organized conglomeration of many up- and downdrafts - a phenomenon which could not be observed in earlier studies due to their much coarser grid resolution. Nevertheless, a clear peak at the wavelength of the cells is seen in the spectra of all variables. In good agreement with previous studies the dominating aspect ratio increases to about 10. In contrast to run1 during run2 no formation of MMC could be identified. The flow structure and the dominating aspect ratios during run2 agree with the ones detected in cloudless convective boundary layers. Within the whole period of the simulation the aspect ratios remain between 3 and 3.5.

Since in run2 the formation of MMc fails to appear, our study gives strong evidence that diabatic heat sources are responsible for the existence and broadening of MMC, avoiding the shortcomings of earlier investigations. A more detailed explanation for the cell broadening mechanism will also be given.