Mean winds observed with Indian MST radar over tropical mesosphere and comparison with various techniques

Temporal variation of mean winds between the 65 to 85 km height region from the data collected over the course of approximately four years (1995-99), using the Indian MST radar located at Gadanki (13.5 degrees N, 79.2 degrees E), India is presented in this paper. Mesospheric mean winds and their sea sonal variation in the horizontal and vertical components are presented in detail. Westward flows during each of the equinoxes and eastward flows duri ng the solstices are observed in the zonal component. The features of the s emi-annual oscillation (SAO) and the quasi-biennial oscillation (QBO) in th e zonal component are noted. In the meridional component, contours reveal a northward motion during the winter and a southward motion during the summe r. Large inter-annual variability is found in the vertical component with m agnitudes of the order of +/-2 ms(-1). The MST observed winds are also comp ared with the winds observed by the MF radar located at Tirunelveli (8.7 de grees N, 77.8 degrees E), India, the High Resolution Doppler Imager (HRDI) onboard the Upper Atmospheric Research Satellite (UARS), and with the CIRA- 86 model. A very good agreement is found between both the ground-based inst ruments (MST radar and MF radar) in the zonal component and there are few d iscrepancies in the meridional component. UARS/HRDI observed winds usually have larger magnitudes than the ground-based mean winds. Comparison of the MST derived winds with the CIRA-86 model in the zonal component shows that during the spring equinox and the summer, the winds agree fairly well, but there are a lot of discrepancies in the other seasons and the observed wind s with the MST radar are less in magnitude, though the direction is same. T he strengths and limitations in estimating reliable mesospheric mean winds using the MST radar are also discussed.