Rainfall morphology in Florida convergence zones: A numerical study

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 studi es have investigated mechanisms primarily affecting storm initiation by suc h convergence zones. Pew have focused on rainfall morphology. yet these sto rms 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 numeric al simulations, the roles of low-level convergence and midtropospheric mois ture 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 linea rly proportional to initial condensation rates. A similar relationship exis ts between VMF and initial rainfall. The VME which encompasses depth and ma gnitude of convergence. is better correlated to initial rainfall production than surface moisture convergence. This extends early observational studie s that linked rainfall in Florida to surface moisture convergence. The amou nt and distribution of midtropospheric moisture affects how much rainfall a ssociated with secondary cells develop. Rainfall amount and efficiency vari ed 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 midtr opospheric environments inhibiting secondary cell development through entra inment effects. Observationally. a 850-500-mb moisture structure exhibits w ider variability than lower-level moisture, which is virtually always prese nt in Florida. A likely consequence of the variability in 850-500-mb moistu re is a stronger statistical correlation to rainfall as noted in previous o bservational studies. The VMF at convergence zones is critical in determining rainfall in the ini tial stage of development but plays a decreasing role in rainfall evolution as the system matures. The midtropospheric moisture (e.g., environment) pl ays an increasing role in rainfall evolution as the system matures. This su ggests the need to improve measurements of depth and magnitude of convergen ce and midtropospheric moisture distribution. It also highlights that the i nfluence of the environment needs to be better represented in convertive pa rameterizations of larger-scale models to account for entrainment effects.