Observations and mechanism of GATE waterspouts.

Abstract

The GATE data base for days 261 and 186 is used for a combined observational and numerical investigation of interacting cumulus processes that may be important in the generation of waterspouts. The results suggest that the existence of cumulus-scale parent vortices is a necessary condition for the production of waterspouts, but not in itself sufficient. For generation of a visible funnel, the vortices must undergo intensification below cloud base to sea level during the short time span in which the convective updraft is active. A high-resolution version of Schlesinger's three- dimensional cumulus model with a Kessler-type precipitation scheme is used to analyze the organization of cumulus-scale vorticity on these two days, which had contrasting thermal stratification and cloud features. The observations from both days suggest that the waterspouts formed ahead of the wind shift, resulting from passage of a gust front, both in zones where it may be surmised that two gust fronts were approaching one another. A brief summary is given of results on tropical gust fronts to provide a basis for discussing their role in the generation of tropical waterspouts. Reasons for the rarity of GATE waterspouts are suggested, and a renewed observational program is proposed for the Florida Keys relating waterspouts to cloud interactions and boundary layer features.- from Authors Lab for Atmospheres, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA.