INDIAN MST RADAR .2. FIRST SCIENTIFIC RESULTS IN ST-MODE

Clear air radars operating in the VHF range provide excellent informat ion on middle atmospheric structure and dynamics with fine height and time resolutions. One such radar is installed at Gadanki, a tropical s tation in India. Experiments were carried out using the ST mode of thi s newly established MST radar to study the atmospheric wind field, cha racteristics of atmospheric stable layers, and clear air turbulence ov er this tropical station. Atmospheric stable layers are observed at va rious heights in the troposphere and lower stratosphere. Multilayer st ructures are observed near the tropopause and in the lower stratospher e. The range-time-intensity (RTI) maps for the zenith beam show that o nce these structures are formed, they are seen to last for more than a n hour, indicating their large horizontal extent. The model computatio ns of radar signal-to-noise ratio (SNR) for zenith beam, using simulta neous radiosonde observations taken at Madras, show a gross agreement with the observed SNR. However, the model SNR profiles do not show the fine structure observed by the radar, the limitation of the model pro files being the lower height resolution of the radiosonde measurements . The refractivity turbulence structure constant C-n(2) is determined using SNR for 20 degrees off-zenith beams pointed in east, west, north , and south directions. Profiles of C-n(2) for the four oblique beams are found to agree within 10 dB, indicating that the intensity of the turbulence for the same range bin, within the volume scanned by the ra dar, is the same. The parameter C-n(2) is also computed using meteorol ogical parameters and compared with radar C-2n. The observed and model C-n(2) profiles are found to agree within 5 dB. Radar C-n(2) profiles are found to show large diurnal and day-to-day variability. The resul ts of an experiment conducted to determine the effect of transmitted p ulse length on the received signal spectral width show that the wind s hear effect is important for oblique beams and for longer pulse length s, where as the beam-broadening effect is important for both oblique a nd vertical beams for all pulse lengths. Various turbulence parameters are determined using the observed spectral width after correcting for these effects.