Intercalibration of HRDI and WINDII wind measurements

The High Resolution Doppler Imager (HRDI) and the Wind Imaging Interferometer (WINDII) instruments, which are both on the Upper Atmosphere Research Satellite, measure winds by sensing the Doppler shift in atmospheric emission features. Because the two observation sets are frequently nearly coincident in space and time, each provides a very e€ective validation test of the other. Discrepancies due to geophysical di€erences should be much smaller than for comparisons with other techniques (radars, rockets, etc.), and the very large sizes of the coincident data sets provide excellent statistics for the study. Issues that have been examined include relative systematic o€sets and the wind magnitudes obtained with the two systems. A signi®cant zero wind position di€erence of 6 m s is identi®ed for the zonal component, and it appears that this arises from an absolute perturbation in WINDII winds of )4 m s and in HRDI of +2 m s. Altitude o€sets appear to be relatively small, and do not exceed 1 km. In addition, no evidence is found for the existence of a systematic wind speed bias between HRDI and WINDII. However, considerable day-to-day variability is found in the quality of the agreement, and RMS di€erences are surprisingly large, typically in the range of 20±30 m s.