EFFECTS OF VARIABLE WIND SHEAR ON THE MESOSCALE CIRCULATION FORCED BYSLAB-SYMMETRICAL DIABATIC HEATING

We examine the response of stably stratified airflow to a slab-symmetr ic diabatic forcing associated with condensation in long-lasting preci pitation bands. The steady-state linearized Boussinesq equations are u sed to model the diagnostic relationship between the vertical motion f ield, the heating source and the ambient flow. The basic-state flow is assumed to be horizontally uniform and non-rotating, but the static s tability and wind vary in the vertical Linear theory shows that the sp eed of the along-band wind component is unimportant for slab-symmetric heating since it cannot contribute towards the advection of buoyancy or vertical motion. For typical atmospheric stratification and a movin g heating source associated with a cloud band, the Taylor-Goldstein eq uation is solved numerically. The numerical results show that the cros s-band wind shear tilts the updraft core and broadens it. While the ma gnitude of the shear is increased, the circulation becomes stronger. T he details of the wind profile are also important in determining the i ntensity and structure of the circulation. When the wind profile indic ates a convex bulge (i.e. the low-level shear is weaker than the upper -level shear), the circulation becomes slightly weaker in comparison w ith the linear wind profile. Conversely, the circulation becomes stron ger when the wind profile has a concave shape. Increasing the concave bulge tends to enhance the circulation but not in a monotonic fashion. This non-monotonic relation between the vertical motion and the parab olic wind profile is interpreted in terms of kinetic energy changes of parcels that interchange their altitudes.