Low Altitude Wind Shear Detection with Doppler Radar

Abstract

The feasibility of using the next generation weather radar (NEXRAD) system to detect low-altitude wind shear near airports is investigated. We compare surface-measured horizontal shear with that observed aloft with Doppler radar to determine how the radar-estimated shear above the surface relates to the surface-measured shear. For five Oklahoma gust fronts, the Doppler radar estimate of shear (at heights between 50?600 m) averaged 1.6 times the shear measured at the surface. For none of 43 comparisons was the surface radial velocity difference across the gust front stronger than the radial velocity difference measured by Doppler radar aloft When the five gust fronts passed an instrumented tower a vertical profile through the lowest 440 m of the gust front could be determined. In all cases the wind speed and wind shear increased in the lowest 90 m of the atmosphere. In one case, the 90 m height had the peak wind shear, in all other case the peak wind shear was at a much higher altitude. The Federal Aviation Administration requires that NEXRAD radar coverage have a lowest scan of 60 m above the surface in the airport area (within 20 km of the airport), the strongest shears in the five gust fronts investigated in this study were at the 90 m or higher levels of the tower. Due to surface friction. it is expected that wind speeds and shears in downbursts will also be stronger aloft than at the surface; however, further study is necessary. It is suggested that a combination of Doppler radar data and information gleaned from a Low-Level Wind Shear Alert System (LLWSAS) would allow more accurate wind shear estimates in the terminal area of airports than would be possible with either system by itself.