GEOSTROPHIC AND GRADIENT DEPARTURES IN JET STREAMS

Abstract

Wind measurements made by aircraft of Project Jet Stream during forty-eight flights are compared with geostrophic and gradient winds (computed on upper-air charts) in order to determine geostrophic and gradient departures. Due to random errors in radiosonde data and to a lesser extent in the aircraft winds, individual departures are not reliable. Therefore, the jet stream area, as represented on a vertical cross-section perpendicular to the wind flow, is arbitrarily divided into nine sectors. Average departures are computed for each sector and for certain combinations of sectors. In cyclonic jet streams (i.e., in the vicinity of upper troughs), these calculations gave the following results: observed wind speeds were, on the average, 27.5 kn (or 18.4 per cent) less than geostrophic speeds but were in excellent agreement with gradient speeds. Although the average gradient departure was approximately zero, these departures tended to be negative (observed winds less than gradient winds) on the south side of the jet core and slightly positive on the north side. The standard deviation of this population of true geostrophic departures was 20 kn, while the standard deviation of true gradient departures was 6 kn. Gradient winds were therefore considerably superior to geostrophic winds as an approximation to observed winds in cyclonically curved jet streams. In straight jet streams, observed winds were 2.4 kn (2.8 per cent) sub-geostrophic (and sub-gradient) on the average; however, this departure should be considered as essentially zero. Sub-geostrophic flow was most pronounced in the layer immediately above the jet core. Sufficient data were not available for determining the departures in anticyclonic jet streams. The statistical significance and theoretical consequences of the departures are discussed.