A study of the deviations of wind speeds and directions from geostrophic values

Abstract

AbstractFrom a knowledge of the three‐dimensional wind field as a function of space and time, the total wind acceleration and the trajectory curvature can be computed, permitting the evaluation of the geostrophic wind vector without recourse to analysed isobars or height contours.A study of 618 cases at 10,000 ft for a 3‐day November sequence, using observed winds over the North American continent, revealed that the mean absolute angle between the reported wind and the geostrophic wind directions was approximately 15°. Large deviation angles were associated with low geostrophic wind speeds, and thus indirectly with low latitudes. The corresponding mean cross‐isobar wind component was slightly greater than 5 mi hr−1. The mean absolute deviation between wind and geostrophic wind speeds was of the order of 7 mi hr−1, the corresponding mean relative deviation being just under 30 per cent. Large relative deviations were associated chiefly with low geostrophic wind speeds, hence indirectly with low latitudes.The mean total wind acceleration slightly exceeded 40 mi hr−1 day−1. The mean trajectory curvature corresponded to a radius of curvature of 175 mi. It was found that the mean components of these terms associated, formally, with horizontal and vertical advection were virtually identical in magnitude, illustrating the importance of the vertical velocity contribution to wind accelerations and curvatures. The mean local tendency component was numerically smaller than the two advective terms in the care of accelerations, but numerically larger in the case of trajectory curvatures.