The Role of Surface Divergence and Vorticity in the Life Cycle of Convective Rainfall. Part I: Observation and Analysis

Abstract

The role of the surface velocity fields in the formation, maintenance and decay of convective storms is examined using approximately 90 days of measurements in a densely instrumented network (660 km2) in south Florida. The results show statistically strong cause and effect relationships between surface convergence and onset of rain, storm intensity and duration. Short-term prediction of the onset of rain and the amount of rain produced proves possible. The surface fields of divergence provide an estimate of storm mass and moisture transports. The size of the surface area of convergence, by governing the supply of moisture, plays a controlling role in storm intensity. Large storms are efficient (72%), in terms of moisture supplied to rain produced, compared to smaller storms (37%). Within the confines of the experiment network, weak storms are in near mass balance, while inflow greatly exceeds outflow in the intense storm. The near mass balance of the weak storm suggests cloud-to-subcloud layer interaction as a further control in storm intensity.