Rainfall morphology in semi-tropical convergence zones

Using 2D and 3D numerical simulations, the scientific goal of this research was to examine the coupling of low-level convergence at a typical Florida convergence zone with mid-tropospheric moisture to understand their role in rainfall production and efficiency. Convergence zone-storms make a significant contribution to the annual rainfall budget of Florida, which is a primary TRMM ground validation site.

The results indicated that time-averaged, vertical moisture flux (VMF) at the sea breeze front/outflow convergence zone is directly and linearly proportional to early condensation rates. The proportionality between VMF and condensation rate establishes a similar relationship between VMF and rainfall rates and accumulations in the first convective cell. Results also indicated that vertical moisture flux, which encompasses depth and magnitude of convergence, is better correlated to surface rainfall than surface moisture convergence. This extends early observational studies which linked rainfall in Florida to magnitude and areal extent of surface moisture convergence.

The amount and distribution of mid-tropospheric moisture determines how rainfall associated with secondary convective 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 first cell period. Rainfall variability was attributed to drier (moister) mid-tropospheric environments inhibiting (promoting) secondary cell formation through entrainment effects. Recent studies of oceanic convective systems (e.g. low cloud base, strong shear, tilted updrafts, and moving systems) suggested that moisture up to 3.0 km primarily affected convective development. This study revealed that moisture variations in the 1.5-3.0 km (850-700 mb) or 3.0-5.5 km (700-500 mb) layer produced similar results in terms of rainfall production and efficiency. It is theorized that the weakly-sheared environment, quasi-stationarity, and erect updrafts in Florida convergence zone storms explain why mid-tropospheric entrainment effects may be equally significant to lower level moisture. Observationally, 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 moisture is a stronger statistical correlation to rainfall, which previous studies have noted.

The study unifies two schools of observation (low-level convergence and mid-tropospheric moisture) concerning rainfall production in small Florida storms. Vertical moisture flux forcing plays a decreasing role in rainfall formation at convergence zones as the system matures, while the mid-tropospheric moisture (e.g. environment) plays an increasing role through entrainment effects. This suggests the need to improve measurements of magnitude/depth of convergence and mid-tropospheric moisture distribution. It also highlights the need for better parameterization of entrainment and vertical moisture distribution in larger-scale models.