CLIMATOLOGICAL MEAN WIND OBSERVATIONS FROM THE UARS HIGH-RESOLUTION DOPPLER IMAGER AND WIND IMAGING INTERFEROMETER - COMPARISON WITH CURRENT REFERENCE MODELS

Direct wind measurements from the high-resolution Doppler imager (HRDI ) and wind imaging interferometer (WINDII) instruments onboard the Upp er Atmosphere Research Satellite (UARS) represent a substantial increa se in the number of available wind observations in the mesosphere-to l ower-thermosphere region. These unique data sets provide an opportunit y to evaluate current climatological reference models in the 80-to 120 -km region which are based on limited empirical data coverage availabl e during the pre-UARS time period. Below 80 km the general climatologi cal features have been fairly well established by previous observation al databases. The reference models therefore provide a means for valid ating the climatological mean UARS observations in the lowe-mesosphere , especially at midlatitudes. We have used 2-3 years of UARS HRDI and WINDII observations to construct a monthly and diurnal mean zonally av eraged climatology of zonal and meridional wind for the 50-to 120-km r egion. We use this to compare with the current CIRA-86 and HWM-93 refe rence models. Comparisons are limited to regions where the tidal oscil lations can be adequately removed from the UARS observations using sim ple binning and averaging methods. At 50-80 km the reference models, w hich are based on gradient winds derived from temperatures, compare we ll with the HRDI data in reproducing the overall seasonal variations, including the magnitudes and positions of the winter westerly and summ er easterly jets at midlatitudes. At 95 km, HRDI and WINDII show year- round easterlies at low latitudes and indicate a strong westerly jet d uring summer and weaker, more variable flow during winter at midlatitu des. The CIRA-86 gradient winds at 95 km show some qualitative agreeme nt with the UARS data at midlatitudes, especially in simulating the su mmertime jet, However, the comparison is poor at low latitudes. We att ribute this to the lack of available temperature measurements in this region from which the gradient winds are derived, The HWM-93 model, wh ich is based mainly on medium frequency and meteor radar winds at 95 k m, shows qualitatively similar seasonal variations in both zonal and m eridional wind compared with the UARS data in this region. However, th e HWM model is consistently smaller by a factor of 2-3 in wind magnitu de relative to the UARS measurements at 95 km.