An MST radar operating at 53 MHz with an average power aperture product of 7 × 108 W m2 has been established at Gadanki (13.5°N, 79.2°E), India. The radar development has been accomplished in two phases. In the first phase it was commissioned in ST mode using a partial system comprising one quarter (16 × 16)of the Yagi antenna array and 16 driver units of the transmitters providing an average power aperture product of 4.8 × 106 W m2. In this part we present the radar system description, including off‐line data processing, and some sample high‐resolution vector wind measurements made in ST mode operation.

In the present study the characteristics of rain integral parameters during tropical convective (C), transition (T), and stratiform (S) types of rain are studied with the help of Joss-Waldvogel disdrometer (JWD), L-band, and very-high-frequency wind profilers at Gadanki (13.58N, 79.208E). The classifications of three regimes are made with the help of an L-band wind profiler. For rain rate R , 10 mm h 21 larger drops are found in S type of rain relative to C and T rain, and for R $ 10 mm h 21 larger drops are found in convective rain. Empirical relations are developed for D m -R, D m -Z, N 0 * -R, Z-R, and Z/D m -R by fitting the power-law equations. Event to event, no systematic variation of the coefficients and exponents could be found for Z-R and Z/D m -R relations during the three types of rain. Overall, the C and S events are found to be number controlled, and T events are size controlled. During C type of rain, bigger mean raindrops are found during the presence of strong updrafts. During S type of rain, bigger mean raindrops are found to be associated with the higher mean thickness of the bright band and strong velocity gradient. For each of the developed empirical relations, the correlation coefficients are found in the order of T . C . S rain. During the three types of rain, correlations are found in the order of Z/D m -R . Z-R . D m -Z . D m -R. Significant improvement is observed in rain retrieval by using the Z/D m -R relation relative to the conventional Z-R relation. By utilizing the Z/D m -R relations, the root-mean-square error was reduced by 19%-46%.

[1] The Gadanki VHF radar observations of the upper E region field aligned irregularities are presented. These are the first observations of low-latitude upper E region irregularities that resemble the characteristics of intermediate layers observed over Arecibo. The most interesting aspect of these observations is their occurrence at altitudes as high as 160 km, which requires an interpretation in terms of their source mechanism. These irregularities were found to trigger about 21 LT first at higher altitude (140 -160 km) and propagate downward with time. The signal intensities are found to be lower by about 12 dB and the Doppler spectra narrower by more than a factor of two compared to that of the normal E region echoes. The Doppler velocities are found to be both upward and downward with values less than m s À1 .

[1] The present study examines the possible role of the ''freeze and dry effect,'' which is generally believed to occur when the tropopause temperature is lower than 191 K. So far, there is observational evidence to support the role of the ''freeze-dry mechanism'' over the western Pacific region, in particular, over Indonesia. The paper presents observations from four intensive radiosonde/GPS sonde campaigns during summer monsoon and postmonsoon seasons from a number of stations in the tropical Indian region. Observations provide clear evidence of the existence of tropopause temperature lower than 191 K. Analysis suggests that cold tropopause occurs over a wide area of 5°Â 5°in latitude and longitude and more often around local midnight hours. Cold tropopause temperature appears to occur more often over the ocean as compared to the coastal stations. The observations at the coastal stations show that on the days of low temperature, the tropopause is cooler by $8 K than the climatological temperature for the Indian equatorial region and $3.5 K from the mean of all observations. It is noted that on the days of observed cold tropopause, the lapse rate just below the tropopause is closer to the dry adiabatic lapse rate. This indicates a rapid cooling in a narrow height range. The present observations thus provide evidence that the freeze-dry mechanism could be operating over a wide area, which includes the Indian tropical region. The possible role of atmospheric convection and also of atmospheric waves in producing low tropopause temperatures is examined and discussed.

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 C2n is determined using SNR for 20° off‐zenith beams pointed in east, west, north, and south directions. Profiles of C2n 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 C2n is also computed using meteorological parameters and compared with radar C2n. The observed and model C2n profiles are found to agree within 5 dB. Radar C2n 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.