Effects of baroclinicity on stratiform rain production and storm divergence in the subtropics

Divergence structures associated with the spectrum of precipitating systems in the subtropics and midlatitudes are not well documented. A previous study employing mesoscale model (MM5) simulations of storms in southeast Texas has already shown that different divergence profiles are produced from variations in environmental baroclinicity (i.e., tropical/barotropic, midlatitude/strongly baroclinic, and combination/weakly baroclinic) and storm types (e.g., leading line-trailing stratiform MCSs, horizontally extensive regions of non-convective stratiform rain, and scattered convection). The cases modeled indicated that weakly baroclinic systems had the largest magnitudes of mid-level convergence. This difference may be explained by variations in stratiform rain fractions and how stratiform rain is produced within different baroclinic environments. Weakly baroclinic systems appear to favor a combination of strong convection and large stratiform rain regions forming from the convection, whereas strongly baroclinic systems tend to be composed of non-convective stratiform rain. Scattered convection with less associated stratiform rain characteristic of barotropic environments display weak mid-level convergence, but strong low-level convergence.

This study uses well-modeled case studies of different storm types and forcings (e.g., cold fronts and upper level disturbances) to quantify the relative importance different baroclinic environments have on divergence profiles within convective and stratiform rain regions associated with storms in the subtropics. Divergence profiles averaged over a 100 x 100 nested grid with 3-km grid spacing are calculated from the model-derived wind fields for each storm. Rainfall totals and the horizontal and vertical structure of radar reflectivity output by MM5 are also analyzed and checked for consistency with observations. The cumulus and microphysics schemes (including graupel) generating the most realistic representation for each storm are utilized. Objectively determined stratiform and convective rain regions based on a separation algorithm by Steiner et al. (1995) are used to create divergence profiles for both rain types and determine stratiform rain fractions for each storm. Some attention may also be given to how the choice of microphysics and cumulus schemes affects MM5-derived stratiform rain fractions, reflectivity structures, and mean divergence profiles for cases that produce multiple good runs. This study attempts to provide a microphysical explanation for why varying degrees of baroclinicity generate different mean divergence structures in the subtropics. After showing this, future work will be able to determine the climatological dynamic response caused by these divergence profiles.