Onset of Indian summer monsoon over Gadanki (13.5°N, 79.2°E):: Study using lower atmospheric wind profiler

The onset of Indian summer monsoon (ISM) over Gadanki (13.5 degrees N; 79.2 degrees E) is identified using variations in signal-to-noise ratio (SNR), wind speed, wind direction and vertical velocity at 1.5 km (similar to 850 hPa) using lower atmospheric wind profiler (LAWP). Strengthening of the low level wind speed attaining 8 ms(-1) with directional change from south-easterlies to south-westerlies defines the beginning of the monsoon. Enhancement in SNR few days before with noticeable magnitude of 5- 10 dB at the time of onset combined with clear-air vertical velocity reversal from downward to upward few days before onset and persisting during monsoon activity supplements the wind speed criteria in identifying onset. Hydrometeor velocity shows large downward values exceeding more than 1 ms(-1) indicating occurrence of rainfall. It is proposed that UHF radar at a location can be used to identify the onset of ISM based on wind speed without considering rainfall separately.