A case study of symmetric instability in the presence of a small-scale cyclone

Abstract

The case study presented here examines the role of conditional symmetric instability (CSI) in the formation of a meso(x scale (100 km wide) wintertime rainband. The rainband of interest occurred on January 19, 1986, during the first observing period of the GALE (Genesis of Atlantic Lows Experiment) project. The rainband developed in a region of large-scale southerly flow 500 km ahead of a synoptic-scale cold front and was associated with a small-scale cyclogenesis event along the Eastern Coast of the U. S, Doppler data from the CP-4 radar at Cape Hatteras were obtained for a twelve hour period encompassing the rainband to investigate the temporal evolution of the band. The main objective of this study is to further understanding of mesoscale flow associated with slantwise convection. Also noteworthy in this case study are along-band variations of dry inflow and embedded convection. A synthetic dual-Doppler technique was applied to the Doppler data from the Cape Hatteras CP-4 and has allowed the determination of mesoscale flow characteristics which are associated with slantwise convection in the absence of frontal forcing. In particular the evolution of slantwise updrafts and downdrafts which produce a roll-like structure is presented. It was found that the slope of the mid and upper-level slantwise flow was very near that of temperature and absolute consistent with present CSI the surfaces of equivalent potential temperature and absolute momentum. This observation is consistent with present CSI theory. Also consistent with CSI theory was the persistence of moist slantwise neutrality on the time scale of hours.