Numerical Experiments on the Influence of the Mesoscale Circulation on the Cumulus Scale

Abstract

The EML mesoscale model developed by Pielke (1974) was used to simulate the sea breeze circulation over South Florida with its modification of the thermal and moisture structure of the synoptic air mass over South Florida. The numerical experiment was performed for a case study day (16 May 1968) during which extensive cloud observations were performed. To examine the response of the cumulus scale, the one-dimensional time-dependent cumulus model developed by Cotton (1975) was initiated with the theoretical soundings predicted by the mesoscale model, along with cloud scales and cloud areal coverage observed on the case study day. The mesoscale model results demonstrated that the sea breeze over South Florida alters the synoptic environment by 1) substantially perturbing the vertical thermodynamic profile, 2) increasing the depth of the planetary boundary layer, 3) inducing larger surface fluxes of momentum, heat, and moisture, 4) changing the vertical shear of the horizontal wind in lower levels of the atmosphere, and 5) developing intense, horizontal convergence regions of heat, moisture and momentum, and cloud material. The cumulus-scale model responded by developing a significantly deeper, longer lifetime, precipitating cloud under the forcing of the perturbed sounding. The cloud-scale model consistently underpredicted cloud top height or overpredicted rainfall. This behavior may he attributed to the inadequacy of the nonlinear eddy viscosity model of eddy transport, to the inability of the one-dimensional cumulus cloud model to incorporate vertical wind shear and strong boundary layer fluxes of heat, momentum and moisture induced by the mesoscale circulation, and or to the fact that the mesoscale model without a convective parameterization scheme predicted soundings which might be too dry and too stable aloft.