Observations and Mechanisms 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. On day 261, the soundings near but outside the cloud cluster had a relatively deeper cloud layer with weaker conditional instability and vertical wind shear at low levels. In the numerical simulation of the congestus, downdraft under-runs updraft early, so that the strong vortex pair at midcloud levels do not extend to the surface, where the vorticity remains weak even though a wide range of wind profiles was tried in the model. On day 186, the soundings near the waterspouts showed a more unstable subcloud and lower cloud layer capped by a stable dry layer restricting cumulus growth below 4 km. Four wind profiles were used in this case to initiate the numerical model; two with strong low-level shear resulted in strong parent vortices with their maximum intensity at the surface. These vortices strengthened in the convergence between side-by-side updraft and downdraft, which both extended to the surface, a rare configuration for GATE but characteristic of many midwestern tornadic developments. 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, and it is shown that 1) they can produce a favorable environment for the parent vortices and 2) they can cause additional vortex intensification. Order-of-magnitude vorticity calculations suggest that small-scale, low-level convergence may have been sufficient, particularly on day 186 when the waterspout signatures were observed at intersecting convergent features to intensify the parent vortex in as little as 5?10 min. On day 261, additional model experiments simulating pregust front conditions, namely low-level destabilization and increased shear, show stronger parent vortices at low levels. 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.