A quantitative analysis of the enhanced-V signature in relation to severe weather

Enhanced longwave InfraRed (IR) satellite imagery of deep convection sometimes display a cloud-top V-shaped feature, where a relatively warm equivalent blackbody temperature (BT) region of the storm top is enclosed by a V-shaped region of colder BT (Negri 1982; McCann 1983; Heymsfield et al. 1983a, 1983b; Fujita 1982). This feature is known as the enhanced-V signature. Enhanced-V features and severe weather are related. The presence of these V-features signifies strong tropospheric shear and intense updrafts, both of which are also essential for severe thunderstorms (Heymsfield and Blackmer 1988).

Early enhanced-V studies (McCann 1983) used eight-kilometer spatial resolution and 30-minute temporal resolution Geostationary Operational Environmental Satellite (GOES) IR imagery. In contrast, the spatial resolution of current satellite imagery is one-kilometer for Polar Orbiting Environmental Satellite (POES) IR imagery. This improved spatial resolution in this study is used to detect and investigate quantitative parameters of the enhanced-V feature. This study describes and analyzes quantitative parameters of the enhanced-V feature, and explores categorizations, from analysis of scatter diagrams, of the parameters of the enhanced-V cases and the upper level winds based on four severe weather analysis categories. It provides a list of enhanced-V types and a Forecasting Rules of Thumb procedure for enhanced-V features.

The False Alarm Ratio for using enhanced-V features to detect severe weather was 32 percent (averaged) for the 2003 and 2004 enhanced-V seasons. The False Alarm Ratio for detecting tornadoes with the Weather Surveillance Radar 88 Doppler (WSR-88D) is much higher at 76 percent (Simmons and Sutter, 2005), which shows an advantage of using enhanced-V features from satellite imagery for severe weather detection in addition to radar-based algorithms. Warning decision making for severe weather should incorporate satellite imagery in the future, which has not occurred in many past studies (Andra et al., 2002). Enhanced-Vs should not be used as a stand-alone feature only. While radar is arguably the primary warning decision tool for severe convection, there are times when radar is either unavailable (outages) or ambiguous (for example, beam blockage). Satellite data should certainly be used to add confidence to the radar signatures (with their high false alarm ratios), especially for severe storms whose distance from the radar is great (or increasing).