AGEOSTROPHIC DEVIATIONS AND WIND PROGNOSES

Abstract

This paper examines apparent ageostrophic deviations at 300 and 200 mb over the United States, their relationship to upper-air synoptic patterns, and the extent to which ageostrophic deviations affect wind forecasts made from geostrophic maps. For spot winds, ageostrophic RMS vector deviations are 19 kn at 300 mb and 20 kn at 200 mb. For the same set of data, geostrophic wind forecasts (36-hr) result in a 34-kn RMS vector error if verified by the geostrophic winds, and 38-kn RMS error if verified by the rawins. Prognostic errors of route winds decrease as the route length increases: for a route length 1900 n mi, the 36-hr geostrophic wind forecast gives RMS vector error 17 kn if verified by geostrophic wind and 19 kn if verified by the rawins. Instrumental uncertainties inherent in the present upper-wind reports prevent the use of the rawins as an absolute standard for verification. In all, it appears that a medium-period forecast of spot and route winds can be made with tolerable approximation from the geostrophic prognostic charts.