BIRD MIGRATION AND BIAS OF WSR-88D WIND ESTIMATES

Migrating birds can greatly influence base velocity, velocity azimuth display (VAD), and VAD wind profile products of the WSR-88D. This is d ocumented by comparing estimates of wind velocity and direction from t hese products with corresponding radiosonde and pilot balloon data. Me an absolute differences between wind velocities estimated from base ve locity products and the corresponding radiosonde data were significant ly greater on days with bird migration than on days with no migration. Even low-density migrations can increase VAD wind velocities relative to winds measured simultaneously by balloon. Because birds usually mi grate with following winds, wind directions measured by VAD are less a ffected, but in some cases the difference can be as much as 128 degree s. Consequently, when winds aloft are nearly calm and variable in dire ction, the data in the VAD wind profile and base velocity products may pertain almost exclusively to migrating birds.