Meteorology: Understanding the Atmosphere            Ackerman and Knox



The "enhanced-v" occurs in satellite IR images and is directly related to the overshooting top, but is slightly more complex. Its appearance in an infrared imagery resembles a V-shape or boomerang-shaped area of cold brightness temperatures, and is bordered by an area of warm brightness temperatures downwind. The origin of this signature is thought to lie in the circulation forced by the overshooting top. As air approaches from downstream, it flows around the overshooting top as if a solid obstacle (like water in a stream flowing around a boulder, for example). Downwind of the overshooting top, the air subsides and warms, producing the warm wake characteristically observed in tandem with the enhanced-v.

Due to their often rapid development and potential for causing loss of life, severe thunderstorms present a constant challenge to the forecaster. Methods of detecting the precursors of severe weather are vital. The enhanced-v indicates an intensifying storm and is often seen in infrared imagery before the onset of severe weather. Detection of the enhanced-v can be a useful tool for identifying severe thunderstorms. It has been found that the median lead time (time from the initial identification of the feature on satellite imagery to the first reports of severe weather on the ground) is about 30 minutes.

Enhanced-VIR images from overpasses of the NOAA-14 and NOAA-15 polar orbiting satellites are shown below. The higher resolution of the AVHRR IR detectors (1km, versus 4km on GOES) reveals greater detail in the structure of the cloud top temperature field. An "enhanced-v" signature became apparent on the GOES-8 IR imagery around 22:40 UTC, but the signature is more obvious on the 22:58 UTC NOAA-14 image (below, left). The storm was producing large hail (1.00 and 1.50 inches in diameter) around the time of the NOAA-14 image. In addition, the AVHRR cloud top temperatures in the overshooting top regions were significantly colder (5 to 13 C) than those measured by GOES-8. The IR image from the 01:37 UTC overpass of NOAA-15 (below, right) was about 11 minutes after the 101 mph wind gust at the Grand Forks NWS office, and about 19 minutes before the 79 mph gust at Crookston (CKN) -- cloud tops as cold as -81 C (light purple enhancement) were detected.

NOAA-14 IR image - Click to enlarge

NOAA-14 10.8 um IR channel

(22:58 UTC on 08 August)

NOAA-15 IR image - Click to enlarge

NOAA-15 10.8 um IR channel

( 01:37 UTC on 09 August )

Practice identifyin the enhanced-v in a satellite image.