Climatological lower thermosphere winds as seen by ground-based and space-based instruments

Abstract

Comparisons are made between climatological dynamic fields obtained from ground-based (GB) and space-based (SB) instruments with a view towards identifying SB/GB intercalibration issues for TIMED and other future aeronomy satellite missions. SB measurements are made from the High Resolution Doppler Imager (HRDI) instrument on the Upper Atmosphere Research Satellite (UARS). The GB data originate from meteor radars at Obninsk, (55° N, 37° E), Shigaraki (35° N, 136° E) and Jakarta (6° S, 107° E) and MF spaced-antenna radars at Hawaii (22° N, 160° W), Christmas I. (2° N, 158° W) and Adelaide (35° S, 138° E). We focus on monthly-mean prevailing, diurnal and semidiurnal wind components at 96km, averaged over the 1991-1999 period. We perform space-based (SB) analyses for 90° longitude sectors including the GB sites, as well as for the zonal mean. Taking the monthly prevailing zonal winds from these stations as a whole, on average, SB zonal winds exceed GB determinations by ~63%, whereas meridional winds are in much better agreement. The origin of this discrepancy remains unknown, and should receive high priority in initial GB/SB comparisons during the TIMED mission. We perform detailed comparisons between monthly climatologies from Jakarta and the geographically conjugate sites of Shigaraki and Adelaide, including some analyses of interannual variations. SB prevailing, diurnal and semidiurnal tides exceed those measured over Jakarta by factors, on the average, of the order of 2.0, 1.6, 1.3, respectively, for the eastward wind, although much variability exists. For the meridional component, SB/GB ratios for the diurnal and semidiurnal tide are about 1.6 and 1.7. Prevailing and tidal amplitudes at Adelaide are significantly lower than SB values, whereas similar net differences do not occur at the conjugate Northern Hemisphere location of Shigaraki. Adelaide diurnal phases lag SB phases by several hours, but excellent agreement between the two data sources exists for semidiurnal tidal phases throughout the year. These results are consistent with phase retardation effects in the MF radar technique that are thought to exist above about 90km. Prevailing and tidal amplitudes from Shigaraki track year-to-year variations in SB fields, whereas in the Southern Hemisphere poorer agreement exists. The above hemispheric differences are due in part to MF vs. meteor radar techniques, but zonal asymmetries and day-to-day variability, combined with inadequate sampling, may also be playing a role. Based on these results, some obvious recommendations emerge that are relevant to combined GB/SB studies as part of TIMED and other future aeronomy missions.