Aircraft Measurements of Ageostrophic Winds

A winter storm struck Colorado March 07-09 1992. The storm intensified in southeast Colorado from 16Z/08-22Z/08 as the exit region of a 300-mb Polar Jet (PJ) propagated into the Texas/Oklahoma Panhandles. Ageostrophic winds and secondary circulations associated with the PJ provided kinematic forcing for cyclogenesis. The Wyoming King Air measured ageostrophic winds in the exit region along the 319.5 K isentropic surface between 20Z/08-22Z/08. Ageostrophic winds are derived from (1) differences between geostrophic and total winds and (2) Lagrangian vector decelerations. The first technique requires aircraft-derived Montgomery stream function (M), which assumes knowledge of geostrophic wind fields derived in part from knowledge of overflown terrain. It is demonstrated that aircraft -derived M and geostrophic winds are sensitive to the depiction of topography. However, a physically-consistent geostrophic wind field results when the roughest underlying terrain is removed from analysis, and topography is manually digitized from topographic maps. Because the flight occurred in a region of cyclogenesis, isallohypsic corrections to M are necessary. These were derived from nearby radiosonde observations. It is demonstrated that resulting ageostrophic winds are extremely sensitive to the magnitude of isallohypsic corrections. It is shown that halving or doubling the corrections changes resulting ageostrophic wind speeds by several hundred percent. The second technique requires Lagrangian vector deceleration measurements. It is assumed that the motion of a drifting pointer approximates the motion of an air parcel. Ageostrophic winds derived from decelerations are reasonably consistent with ageostrophic winds derived from differences between geostrophic and ageostrophic winds. However, error analyses show that ageostrophic wind speeds can be measured to within several ms^{-1} with the Lagrangian technique, which potentially exceeds the accuracy of ageostrophic winds derived from differences between geostrophic and total winds. This accuracy is achieved without knowledge of topography or isallohypsic corrections. Lagrangian ageostrophic winds compared favorably with results from a mesoscale model. This result is significant because ageostrophic winds are proportional to Lagrangian vector deceleration and imbalances between fundamental Coriolis and pressure gradient forces. Therefore, by measuring the vector deceleration, the fundamental forcing mechanism of a major winter storm was also measured.