Measurements of stratospheric winds by the high resolution Doppler imager

The high resolution Doppler imager (HRDI) on the Upper Atmosphere Research Satellite (UARS) determines winds in the stratosphere (10-40 km) by measuring the Doppler shift of the rotational lines in the O-2 atmospheric B and gamma bands. These lines are observed as absorption features in scattered sunlight. The Doppler shifts are found by fitting the observed high-resolution spectrum to a single-scattering model. The model includes effects due to scattering from clouds and the ground. Results are compared to radiosonde measurements and the standard deviation between the two measurement systems is found to be between 8 m/s and 12 m/s. This standard deviation includes the errors in both measurement systems as well as geophysical noise due to the different sampling times, sampling locations, and sampling volumes. Monthly averages of HRDI stratospheric winds near the equator are examined from December 1991 to April 1995. These averages reveal the three-dimensional structure of the quasi-biennial oscillation (QBO) and its evolution through one full cycle. In particular, we note that the QBO extends over a wide latitude and altitude range and that the easterly and westerly descent rates are similar above 25 km. HRDI measurements show that there is strong meridional shear in the zonal winds above 35 km during the solstices, indicating that inertial instability may play a role in the dynamics of the upper stratosphere. The HRDI wind measurements also show that there is a significant annual cycle in the tropics and that there is substantial interannual variability in the semiannual oscillation in the upper stratosphere.