Aircraft Measurements of Ageostrophic Winds

Abstract

The Wyoming King Air flew along the 319.5-K isentropic surface and measured ageostrophic winds in the exit region of a polar jet on 8 March 1992. Ageostrophic winds were derived from 1) differences between geostrophic and observed winds and 2) Lagrangian vector acceleration. The first technique required aircraft-derived Montgomery streamfunctions M and geostrophic winds, which were sensitive to the depiction of overflown terrain. Physically consistent geostrophic winds and M were obtained from manually digitized topographic maps with rough underlying terrain removed from the analysis. The flight was conducted during a cyclogenesis event, so that aircraft-measured M required corrections for unsteady (is- allohypsic) effects. Resulting ageostrophic winds were sensitive to the corrections; halving or doubling the corrections vary ageostrophic wind speeds by as much as 400%. The second technique required Lagrangian vector accelerators, which assume that a drifting pointer approximates a moving air parcel. Error analysis shows that the Lagrangian pointer technique can measure ageostrophic wind speeds to within several meters per second. Depending on the ageostrophic wind speed, this represents 10%?50% error, which exceeds the accuracy of ageostrophic winds derived from the flat method. This result is significant because it shows that Lagrangian pointer techniques can measure ageostrophic winds without knowledge of overflown terrain and corrections for isallohypsic effects.