Thunderstorm initiation, organization, and lifetime associated with Florida boundary layer convergence lines

The initiation, organization, and longevity of thunderstorms associated with boundary layer convergence lines in the Cape Canaveral, Florida, vicinity are examined using data from the Convection and Precipitation/Electrification (CaPE) experiment. The project was conducted during July and August 1991 under low vertical wind shear situations. This observational study is based on Doppler radar, mesonet, balloon sounding, and satellite data. The primary convergence lines studied were the east coast sea-breeze front (ECSBF) and a frequently occurring gust front from the west termed the west coast front (WCF) that originates with storms initiated by the west coast sea-breeze front. Significantly fewer storms were associated with the ECSBF in comparison to the WCF. This was because the convergence with the ECSBF was shallower and weaker and the updrafts were shallow and tilted. The environmental winds were generally westerly near the top of the ECSBF and at storm steering level. As a result, the low-level vertical wind shear directed normal to the ECSBF was small and did not produce a horizontal vorticity balancing that produced by the ECSBF cold pool. Rotunno et al. have shown with modeling experiments that this vorticity imbalance results in shallow, weak updrafts similar to those observed here. The westerly flow also causes the clouds and ECSBF to move in opposite directions causing the storms to rapidly move away from the convergence line. Storm merger, organization, and lifetime were greatly enhanced when the clouds were moving at a velocity similar to that of the convergence line. These results are similar to the modeling studies of Moncrieff and Miller and Weisman and Klemp that suggested storm organization and lifetime depend on the relationship of cloud motion to the convergence line motion. Florida forecasters can anticipate the amount of convective activity with convergence lines from different directions by estimating boundary relative cloud motions and then using guidance information that relates that value to storm initiation, organization, and longevity. The 2-4-km layer average wind can be used with very high confidence to estimate cloud motion.