HRDI daytime observations of mesospheric thermal inversion layers and their relationship to wind shear

The formation of mesospheric thermal inversion layers is investigated in High Resolution Doppler Imager (HRDI) daytime temperature measurements within the altitude range of 65-105 km. For this analysis, monthly average zonal-mean temperature profiles are computed within 10-degree latitude and 1-hour local time bins. The slow orbital precession means that the data falling within each monthly latitude/local time bin usually comes from only 3-5 consecutive days within the month. Large-amplitude thermal inversion layers (>10 K) are robust features of the HRDI data, occurring in 20-50% of the monthly mean profiles at nearly all latitudes and seasons (mid- to high-latitude cold summer mesopause conditions being the one exception). Under equinox conditions near the equator, the inversion layers are observed to propagate downward at a rate of about 1 km/hr, closely matching the tidal descent rate, but only during those seasons when the migrating tides are particularly strong. Otherwise, the inversion layer is observed at a constant altitude independent of local time, with the local maximum in temperature near 87.5 km. This paper will examine the geographic distribution and horizontal scale of the inversion layer structures which result in the observed zonal mean profiles, as well as the seasonal variations in the frequency of occurrence of these inversion features. Correlations between vertical shears in the HRDI daytime zonal and meridional wind fields and the thermal inversion layers are explored to test the hypothesis that dissipation in unstable shear layers is the heating mechanism driving the formation of these thermal inversion layers.