Indian MST radar 2. First scientific results in ST mode

Abstract

Clear air radars operating in the VHF range provide excellent information on middle atmospheric structure and dynamics with fine height and time resolutions. One such radar is installed at Gadanki, a tropical station in India. Experiments were carried out using the ST mode of this newly established MST radar to study the atmospheric wind field, characteristics of atmospheric stable layers, and clear air turbulence over this tropical station. Atmospheric stable layers are observed at various heights in the troposphere and lower stratosphere. Multilayer structures are observed near the tropopause and in the lower stratosphere. The range-time-intensity (RTI) maps for the zenith beam show that once these structures are formed, they are seen to last for more than an hour, indicating their large horizontal extent. The model computations of radar signal-to-noise ratio (SNR) for zenith beam, using simultaneous 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 profiles being the lower height resolution of the radiosonde measurements. The refractivity turbulence structure constant C²_n is determined using SNR for 20° off-zenith beams pointed in east, west, north, and south directions. Profiles of C²_n 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 radar, is the same. The parameter C²_n is also computed using meteorological parameters and compared with radar C²_n. The observed and model C²_n profiles are found to agree within 5 dB. Radar C²_n profiles are found to show large diurnal and day-to-day variability. The results of an experiment conducted to determine the effect of transmitted pulse length on the received signal spectral width show that the wind shear effect is important for oblique beams and for longer pulse lengths, where as the beam-broadening effect is important for both oblique and vertical beams for all pulse lengths. Various turbulence parameters are determined using the observed spectral width after correcting for these effects.