JETLET FORMATION FROM DIABATIC FORCING WITH APPLICATIONS TO THE 1994 PALM SUNDAY TORNADO OUTBREAK

The three-dimensional responses of simple stably stratified barotropic and baroclinic flows to prescribed diabatic forcing are investigated using a dry, hydrostatic, primitive equation numerical model (the Nort h Carolina Stale University Geophysical Fluid Dynamics Model). A time- dependent diabatic forcing is utilized to isolate the effects of laten t heat release in a midlatitude convective system. Examination of the mass-momentum adjustments to the diabatic forcing is performed with a focus on the development of an isolated midlevel wind maximum. The res ults of both cases suggest the formation of a midlevel wind maximum in the form of a perturbation meso-P-scale cyclone, which later propagat es downstream as the heating is decreased. The scale of the perturbati on cyclone remains at a sub-Rossby radius of deformation length scale. Therefore, the mass perturbations adjust to the wind perturbations as the mesocyclone propagates downstream. Transverse vertical circulatio ns, which favor ascent on the right flank of the wind maximum, appear to be attributed to compensatory gravity wave motions, initially trigg ered by the thermal forcing, which laterally disperses as the heating is reduced. The simple model simulations are used to explain more comp lex results from a previous mesoscale modeling study (the Mesoscale At mospheric Simulation System, MASS), in which it was hypothesized that an upstream mesoscale convective complex triggered a midlevel jetlet t hrough geostrophic adjustment of the wind to the latent heat source. T he MASS simulated jetlet attained a transverse vertical circulation th at favored ascent on the right flank of the midlevel jetlet. The jetle t and accompanying transverse vertical circulations later propagated d ownstream aiding in the formation of the 27-28 March 1994 tornadic env ironment in Alabama and Georgia.