Rainfall Morphology in Florida Convergence Zones: A Numerical Study

Abstract

Central Florida is the ideal test laboratory for studying convergence zone?induced convection. The region regularly experiences sea-breeze fronts and rainfall-induced outflow boundaries. The focus of this study is convection associated with the commonly occurring convergence zone established by the interaction of the sea-breeze front and an outflow boundary. Previous studies have investigated mechanisms primarily affecting storm initiation by such convergence zones. Few have focused on rainfall morphology, yet these storms contribute a significant amount of precipitation to the annual rainfall budget. Low-level convergence and midtropospheric moisture have been shown to be correlated with rainfall amounts in Florida. Using 2D and 3D numerical simulations, the roles of low-level convergence and midtropospheric moisture in rainfall evolution are examined. The results indicate that area- and time-averaged, vertical moisture flux (VMF) at the sea-breeze front?outflow convergence zone is directly and linearly proportional to initial condensation rates. A similar relationship exists between VMF and initial rainfall. The VMF, which encompasses depth and magnitude of convergence, is better correlated to initial rainfall production than surface moisture convergence. This extends early observational studies that linked rainfall in Florida to surface moisture convergence. The amount and distribution of midtropospheric moisture affects how much rainfall associated with secondary cells develop. Rainfall amount and efficiency varied significantly over an observable range of relative humidities in the 850?500-mb layer even though rainfall evolution was similar during the initial or ?first cell? period. Rainfall variability was attributed to drier midtropospheric environments inhibiting secondary cell development through entrainment effects. Observationally, a 850?500-mb moisture structure exhibits wider variability than lower-level moisture, which is virtually always present in Florida. A likely consequence of the variability in 850?500-mb moisture is a stronger statistical correlation to rainfall as noted in previous observational studies. The VMF at convergence zones is critical in determining rainfall in the initial stage of development but plays a decreasing role in rainfall evolution as the system matures. The midtropospheric moisture (e.g., environment) plays an increasing role in rainfall evolution as the system matures. This suggests the need to improve measurements of depth and magnitude of convergence and midtropospheric moisture distribution. It also highlights that the influence of the environment needs to be better represented in convective parameterizations of larger-scale models to account for entrainment effects.