The formation of a multicell thunderstorm behind a sea-breeze front

Idealized three-dimensional numerical simulations are used to investigate the influence of sea breezes on the evolution of multicell thunderstorms in the Tropics. The study is motivated by a desire to understand a particular type of severe storm system that occurs occasionally over the ‘Top End’ region of northern Australia, but the results should have wider applicability. The calculations are carried out using Bryan’s cloud model. The simulations reproduce the main features of the evolution of an observed storm system. New cells develop on the gust front of the initial updraught, behind the sea-breeze front, and subsequently merge to form a multicell thunderstorm. The propagation speed and direction, the orientation, and length of the line of updraught cells are all similar to those observed. The sea breezes in the model play an important role in the evolution of the storm. In general, the low-level convergence at the gust front produced by the initial cell needs to be strong enough and to persist for a sufficiently long time to allow new cells to develop near this front. A strong cold pool occurs if the initial updraught is sufficiently tilted so that the downdraught does not fall into it, but supplies cold air close to the gust front. A generalized form of the Rotunno–Klemp–Weisman criterion was used to test whether the new updraughts developed at locations where the cold pool circulation was largely opposed by that of the environmental shear. The comparison of the shear components normal to the gust front showed that new cells develop even though this criterion is not fulfilled, suggesting that the criterion is not applicable to the more complex flow configurations studied here. Copyright c