The Evaluation of Winds from Geopotential Height Data in the Stratosphere

Abstract

Several methods of obtaining horizontal wind fields in the extratropical stratosphere from geopotential height data are evaluated and compared to geostrophic estimates, with focus on the poleward fluxes of momentum and heat and on the resulting Eliassen–Palm (EP) flux divergence estimates. Winds derived from a coupled iterative solution of the zonal and meridional momentum equations (“balance” winds) are proposed and tested, in addition to winds derived from linearizing these equations about the zonal mean flow (“linen” winds). Comparison of the different analysis methods are made for a general circulation model simulation of the Northern Hemisphere (NH) winter stratosphere, and for NH and Southern Hemisphere (SH) winter observational data. The balance and linear wind estimates of poleward momentum flux are similar and substantially smaller than geostrophic values in the high-latitude stratosphere; neglect of local curvature effects is the primary cause of the geostrophic overestimate. The relative errors are larger in the southern winter stratosphere due to the stronger polar night jet. Poleward beat flux estimates are not substantially changed. Use of the improved wind fluxes results in a sizable reduction in the EP flux divergence in the high-latitude stratosphere. Comparison with model winds suggests that the balance method is the superior analysis technique for evaluating local winds, particularly in the NH winter where local nonlinear effects can be important. Based on observed balance winds, estimates are made of the relative importance of rotational versus divergent motions in the winter stratosphere.