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 documented by comparing estimates of wind velocity and direction from these products with corresponding radiosonde and pilot balloon data. Mean absolute differences between wind velocities estimated from base velocity products and the corresponding radiosonde data were significantly 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 migrate with following winds, wind directions measured by VAD are less affected, but in some cases the difference can be as much as 128 degrees. Consequently, when winds aloft are nearly calm and variable in direction, the data in the VAD wind profile and base velocity products may pertain almost exclusively to migrating birds.