Gravity wave drag estimation from global analyses using variational data assimilation principles. II: A case study.

Abstract

An estimate of the three-dimensional gravity-wave drag field derived from middle-atmosphere analyses is presented. It is obtained using a novel technique based on variational data assimilation principles described in Part I. The assimilation is performed for a one-week period starting on 1 July 2002 using Met Office middle-atmosphere analyses as the observations as well as the initial condition. The rotational component of the gravity-wave drag is robustly estimated by the technique. The zonal-mean drag estimates reach maximum amplitudes of 50 m s−1day−1 at 0.4 hPa near the winter-hemisphere polar jet. In the middle stratosphere, drag estimates are much weaker with zonal-mean peak values of 5 m s−1day−1 and show transient small-scale features not obviously correlated with the local background flow. Vertical variations in the sign and pattern of the estimated drag suggest filtering of the gravity-wave spectrum by the background flow.