A Numerical Study of Stratified Airflow over Mesoscale Heat Sources with Application to Carolina Coastal Frontogenesis

Abstract

This paper presents the results from a numerical investigation of the responses of stratified airflow to prescribed near-surface mesoscale axisymmetric (circular) and elongated (elliptical) heat sources under uniform basic wind conditions using a simple three-dimensional model. Model results indicate that the structure of the response depends on the Froude number (Fr = V/NH) and the Rossby number (Ro = V/fL) associated with an axisymmetric thermal forcing, where H and L are the vertical and horizontal scales of the heat source, V the wind speed, N the Brunt-Väisälä frequency, and f the Coriolis parameter. However, the response to an elongated thermal forcing depends not only on the Froude and Rossby numbers, but also on the ratio of the alongstream scale (Ls) to the cross-stream scale (Ln) of the heat source, which are determined by the horizontal shape of the heat source and the direction of the basic flow. When Ls/Ln is less than 1, that is, the heat source is elongated in the cross-stream direction, the response resembles that of two-dimensional solutions. In this case, the heat-induced vertical motion is in phase with the heating field when Fr < 1 and is out of phase with the heating field when Fr > 1. Meanwhile, vertically propagating inertia-gravity waves are induced. However, when Ls/Ln > 1, that is, the heat source is elongated in the alongstream direction, the response appears quite different. In this case, the heat-induced vertical motion is primarily upward but confined to the lower atmosphere. The phase relationship between the vertical motion and the heating cannot be determined by the Froude number. The horizontal distribution of surface sensible heat fluxes observed off the Carolina coast often appears to be elongated along the Gulf Stream front. These surface heat sources can induce coastal frontogenesis. In order to explain the effect of surface sensible heat fluxes on Carolina coastal frontogenesis, the shape factor of the heat source and the basic wind direction need to be taken into account.