Nonlinear Effects on Convectively Forced Two-Dimensional Mesoscale Flows

Abstract

ne of the important problems in mesoscale atmospheric dynamics is how the atmosphere responds to convective heating or cooling. Here, the authors examine nonlinear effects on convectively forced mesoscale flows in the context of the nonlinear response of a stably stratified flow to elevated steady heating in two dimensions using a nondimensional numerical model. Results of two-dimensional numerical experiments in a uniform flow show that even without vertical wind shear, a separation of an upwind cellular updraft from the steady heating-induced main updraft occurs in a highly nonlinear flow system. This separation occurs as the compensating cellular downdraft associated with a secondary maximum in the main updraft develops. As the nonlinearity of the flow system increases, the upwind cellular updraft is separated earlier and becomes stronger. Smaller viscous terms result in the separation of more cellular updrafts, which become stronger and move farther away from the main updraft region. In particular, in an inviscid flow, cellular updrafts are periodically separated from the main updraft, and the first cellular updraft and downdraft have intensities comparable to the intensity of the main updraft. In a viscid flow with a constant vertical wind shear up to a certain height, the propagating cellular updraft and downdraft are produced when the nonlinearity is large, as in a uniform flow. Stronger vertical wind shear leads to the earlier formation of the cellular updraft and its stronger intensity, faster propagating speed, and longer lifetime. Results of numerical experiments with squall line?type forcing imply that the highly nonlinear state is necessary for the development of cellular updrafts.