A. R. Jain

Indian MST radar 1. System description and sample vector wind measurements in ST mode

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.

First observations of equatorial spread F from Indian MST radar

The newly established MST radar at Gadanki (13.5°N, 79.2°E) was operated on a few occasions in ionospheric field-aligned backscatter mode to investigate the small-scale (∼ 3 m) irregularities associated with the equatorial spread F. Observations are presented of sample Doppler spectra and height-time variations of signal intensity and weighted mean Doppler velocity. The Doppler spectra are mostly of the composite type similar to that observed at Jicamarca and are quite variable both in height and time. The Doppler velocities of the 3-m irregularities are found to be consistent in general with the slopes observed of the scattering structures in the height-time-intensity plots. The peak signal intensities are found to be as high as 40 dB above the mean noise level, and the peak Doppler velocities are around 70 m s−1.

Radar observations of 2.8 m equatorial spread-F irregularities

Abstract VHF radar observations made of two intense equatorial spread-F events at Gadanki (geographic 13.5 °N, 79.2 °E; geomagnetic 6.3 °N) are presented in the form of Doppler spectra and height-time variations of signal intensity, Doppler velocity and spectral width. The Doppler spectra are found to be discrete and narrow during the early phase of instability evolution and fairly broad at a later stage of evolution, involving rising plasma bubbles and turbulent irregularities. The most intense backscattered signals are associated with the plume structures with peak power values at 42 dB above the noise level. The Doppler velocities, presented only for the bottomside in view of the restricted Doppler window, are found to be in accord with the slopes observed of the scattering structures in the height-time-intensity plots. The drifts are predominantly downward with a peak value of about 70 m s−1 which is significantly above that of the background plasma. The downdrafting structures in both the events are found to descend all the way to the E region, an unusual feature not seen in the RTI plots observed over the magnetic equator. The spectral widths are of the order of 10–20 m s−1 during the initial phase and exceed 100 m s−1 at a later stage of the spread-F evolution.

Tropical precipitation studies using VHF/L-band windprofilers and disdrometer over Gadanki, India

We have been working on rainfall observation project at tropical site in India, in order to study tropical storm structure and raindrop size distribution (DSD) characteristics for improving the current PR rainfall retrieval algorithm, and for making the ground validation of TRMM PR observation. At Gadanki (53-MHz VHF MST radar, an L-band lower atmospheric wind profiler (LAWP), a disdrometer and an optical rain gauge (ORG) are set up to obtain more knowledge on vertical properties of DSD and rain structure during monsoon season. Measurements of drop size distribution (DSD) with disdrometer have been providing the information to study basic DSD characteristics in tropical India. We have found a clear seasonal dependence in Reflectivity (Z) - Rainfall (R) relations (i.e. DSD characteristics) in India. Our results indicate that there are about 3-times differences peak-to-peak in estimates of rain rate using a single Z-R relation. It seems that this type of seasonal dependence should be taken into account to improve the accuracy of the PR algorithm. Drop size distribution characteristics were retrieved in moderate/heavy precipitation using VHF wind profiler. The retrieved drop size parameters were compared to corresponding disdrometer data and found that there is reasonably good agreement between the measurements, lending credence to the profiler retrievals of DSD parameters. Preliminary study on the ground validation of TRMM PR shows fairly good agreement between the disdrometer and TRMM precipitation radar measurements.

Campaign mode observation of tropical convection using ground-based radar systems

Tropical convection plays an important role in enhancing rainfall and also creates uncertainty in the model-based predictions of weather in tropics due to the latent heat released into the troposphere. Ground-based radar systems are important tools available for the effective characterization of convective events. Availability of different radar systems ideally suited to study tropical convection in an area popularly known as the Golden triangle for weather observations in southern part of India, led to the organization of an interagency program for a campaign mode of observations using the different radar systems and associated rain gauges etc. to observe tropical convection during the period Oct-Dec, on a few days when the north east monsoon was prevalent over the east coast of peninsular India. The Golden triangle consists of Sriharikota Island where the Space Launch Complex of ISRO is located at the Satish Dhavan Space Center, the Cyclone Detection Radar site of IMD at Chennai and the National MST Radar Facility (NMRF) at Gadanki in close proximity to the temple city of Tirupati. An indigenously developed S band Doppler weather radar is commissioned at Sriharikota Island on the east coast of India in December 2002, as an interagency program between India Meteorological Department and Indian Space Research Organization. A siphoning type fast response rain gauge and a tipping bucket rain gauge are located within 10 km from the radar. Another S band DWR is located in Chennai, India by IMD. These radars have the capability to measure precipitation and Doppler velocity and provide in real time the 3 base products viz., reflectivity, velocity, and spectral width of the hydrometeors within radar sample volume with good accuracy up to 250 km. From these base products, other meteorological products like rainfall rate, rainfall accumulation, Cappi, echotop etc. are derived. Indian MST radar, a VHF profiler (at 53 MHz) normally used for estimating the winds and turbulence and an L band lower atmospheric wind profiler operating at 1357.5 MHz for estimating the winds, turbulence, and precipitating weather systems in tropical latitudes are operating at NMRF since 1990. Apart from these radar systems, a disdrometer and an optical rain gauge are also located at this facility as collocated instrumentations for the measurement of rainfall rate and rainfall accumulation. All these three locations are geographically located within 80-100 km from each other and form a triangle. These instrumentation systems provide an excellent ground-based network for the characterization of tropical convection. The paper describes the campaign details including the detailed characteristics of the radar systems used and provide intercomparison of the data obtained as the convective systems transited over the terrain which is essentially coastal for Chennai and SHAR and mountainous for Gadanki.