Instability Bursts Associated with Extratropical Cyclone Systems (ECSs) and aForecast Index of 3-12 Hour Heavy Precipitation

摘要

Instability Bursts (IBs) are one of the primary mechanisms for producing heavyprecipitation from Extratropical Cyclone Systems (ECSs). ECSs represent the mesoscale features within synoptic scale weather systems that range in size from subsynoptic scale waves and vortices to large synoptic scale baroclinic leafs, comma heads and cloud bands. IBs are depicted in satellite imagery as convective subsynoptic or mesoscale cloud patterns that form or develop rapidly and produce localized heavy precipitation (rain or snow) as the air mass quickly destabilizes. IBs are best detected by using a combination of satellite imagery and instability analyses derived from surface and upper air data and numerical model forecasts. In the satellite imagery, IBs are identified as developing subsynoptic scale wave patterns, baroclinic leaf-type patterns of convective cloud areas (or bands) embedded within the ECS cloud pattern. Often these cloud patterns are growing and becoming colder. In the surface and upper air data (including numerical model forecasts), IBs are associated with (1) the maximum advection of unstable air, or (2) an upper level disturbance (or jet streak) passing over an unstable mass. Examples of IBs and heavy precipitation are presented in this study. (Author) (KR)