GEOSTROPHIC AND GRADIENT DEPARTURES IN JET STREAMS

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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 ... 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 ...

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