Convergence zone studies from the High Plains suggest that wind shear, low-level horizontal vorticity balance, and moisture supply play key roles in storm development. However, it is not clear from the literature whether these factors or different factors (e.g. convergence depth, moisture depth, convergence magnitude) are more important in a moist, semi-tropical environment (e.g. central Florida). Central Florida can be quite different than the High Plains environment in terms of boundary layer structure, shear profile, and thermodynamic stability. Recent evidence from Florida suggests that under “favorable” thermodynamic and kinematic conditions for development, some convergence zones initiated rainfall-producing convection while others did not. These facts suggest that semi-tropical convergence zones (STCZ) need further study.
There has been little work validating or extending theories deduced from High Plains convergence zone studies in a moist, semi-tropical environment. Although understanding of semi-tropical rainfall is critical to NASA’s Tropical Rainfall Measuring Mission (TRMM), studies on the relationships between rainfall morphology and STCZs are rare. The purpose of this study is to identify and quantify how rainfall development (e.g. onset, amount, distribution) at Florida convergence zones is affected by characteristics of the convergence field, 3D moisture distribution, and wind flow. We focus on the very common, yet infrequently studied, convergence zone between a sea breeze front and outflow boundary. We hypothesize that given marginal instability and a weak shear profile (typical of Florida) rainfall morphology at an STCZ is directly proportional to the upward water vapor flux (WVF). WVF accounts for depth and magnitude of convergence as well as the amount of water vapor converging into the column.
Numerical experiments are designed to identify the sensitivity of rainfall development at a theoretical convergence zone to the magnitude and depth of convergence, wind flow, and moisture distribution (vertical and horizontal). Preliminary 2D results suggest that variance in convergence magnitude or depth (within observed variability) can delay (or accelerate) rainfall development by up to 20 minutes. Peak rainfall and total-domain surface rainfall amounts vary by factors of 0-10 times. Moisture experiments suggest that moist air behind convergence zones is also advected into rain-forming clouds not just air forced upward at the STCZ. Kingsmill and Wakimoto (1995) pondered this issue in their study of bores near sea breeze fronts. We confirm thatmoist mid-level air can yield up to 30 percent more rainfall suggesting that deep moisture is critical in offseting entrainment and/or as a moisture supply in Florida storms. Results from wind variability experiments suggest that low-level wind shear (Rotunno et al., 1988), though important, does not affect rainfall production in Florida storms as much as moisture convergence characteristics. We also found that stronger 700 mb flow can produce up to 10 times more total-domain surface rainfall. This is similar to recent observations of Wilson and Megenhardt (1997). We plan to validate 2D results with 3D configurations, compare results with High Plains studies, and theorize on the relative importance of these factors in rainfall morphology at STCZs using the WVF model.