Role of a Critical Level in a Shear Flow with Diabatic Forcing
Abstract
The response of a twodimensional, stratified shear flow with a critical level to diabatic cooling, which represents the evaporative cooling of falling precipitation in the subcloud layer, is investigated using both a linear analytic theory and a nonlinear numerical model. The ambient wind has either a constant shear in the whole domain or a 3layer structure with a constant shear layer bounded by uniform wind layers. It is found that the wind shear can modify the condition at which the upstream propagation of the density current is just opposed by the ambient wind. When the wind shear and the basic wind are of opposite sign in the cooling region, a relatively weak ambient wind is enough to prevent the upstream propagation of the density current. Results from the nonlinear numerical simulations indicate that the response of the atmosphere to a steady cooling in a shear flow may be categorized as either a stationary cold pool or a density current, depending upon the strength of the effective cooling. In the linear, steadystate, threelayer theory, it is shown that almost all of the wave energy is absorbed near the critical level when the shear layer is stable. For a flow with an unstable layer bounded below and above by stable, uniform wind layer, the wave is overreflected when nu [{equiv } (1/4rm Ri)^{1/2}] has a value near 0.4. The transmission coefficient increases as the reflection coefficient does. Therefore, the wave can be overreflected and overtransmitted simultaneously when it travels through the critical level. From the nonlinear numerical simulations, it is found that the wave amplitude in the lower layer is dependent upon the depth of the lower layer. It is shown that in order for the wave to be over reflected and overtransmitted simultaneously, the stability above the shear layer should be increased. Finally, the dynamic and thermodynamic characteristics of squall line are investigated using a numerical model with moist convective processes resolved explicitly. Results indicated that the mesoscale circulation is strongly dependent upon the ambient wind for a given environmental sounding. (Abstract shortened with permission of author.).
 Publication:

Ph.D. Thesis
 Pub Date:
 1991
 Bibcode:
 1991PhDT.......120C
 Keywords:

 THERMAL FORCING;
 Physics: Atmospheric Science