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 Upper Atmosphere Research Satellite (UARS) represent a substantial increase in the number of available wind observations in the mesosphere-to lower-thermosphere region. These unique data sets provide an opportunity to evaluate current climatological reference models in the 80-to 120-km region which are based on limited empirical data coverage available during the pre-UARS time period. Below 80 km the general climatological features have been fairly well established by previous observational databases. The reference models therefore provide a means for validating 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 averaged climatology of zonal and meridional wind for the 50-to 120-km region. We use this to compare with the current CIRA-86 and HWM-93 reference models. Comparisons are limited to regions where the tidal oscillations can be adequately removed from the UARS observations using simple binning and averaging methods. At 50-80 km the reference models, which are based on gradient winds derived from temperatures, compare well with the HRDI data in reproducing the overall seasonal variations, including the magnitudes and positions of the winter westerly and summer easterly jets at midlatitudes. At 95 km, HRDI and WINDII show year-round easterlies at low latitudes and indicate a strong westerly jet during summer and weaker, more variable flow during winter at midlatitudes. The CIRA-86 gradient winds at 95 km show some qualitative agreement with the UARS data at midlatitudes, especially in simulating the summertime jet, However, the comparison is poor at low latitudes. We attribute this to the lack of available temperature measurements in this region from which the gradient winds are derived, The HWM-93 model, which is based mainly on medium frequency and meteor radar winds at 95 km, shows qualitatively similar seasonal variations in both zonal and meridional wind compared with the UARS data in this region. However, the HWM model is consistently smaller by a factor of 2-3 in wind magnitude relative to the UARS measurements at 95 km.