Microphysical aspects of tropical rainfall during Bright Band events at mid and high-altitude regions over Southern Western Ghats, India

Abstract

This study aims to explore the microphysical processes involved in stratiform precipitation, with in-situ observations carried out during Indian summer monsoon season of 2017, at two locations one each at the mid-altitude (Braemore; 400 m above MSL) and high-altitude (HACPO; Rajamallay; 1820 m above MSL) respectively, on the windward side of the Western Ghats (WG) mountains in peninsular India. The bright band (BB) events at both locations are identified using the observation from micro rain radar (MRR). The percentage of BB events are 27% higher and precipitation associated with BB events have higher consistency at high-altitude region. About 60% of observed BB peak height is found at 4.6 km in mid-altitude, whereas in high-altitude it varies from 4.4 to 4.6 km with percentage occurrences of 20% and 42% respectively. The BB bottom height variations in the range of 4.4 to 5.4 km contribute surface rain at mid-altitude in 0.1 < R ≤ 1 mmhr−1 rain class, however in high-altitude, all (0.1 ≤ R < 1 mmhr−1; 1 ≤ R < 2 mmhr−1; 2 ≤ R < 5 mmhr−1; 5 ≤ R < 10 mmhr−1) rain classes are influenced by bottom height variations. The drop size distribution (DSD) at the surface in mid-altitude shows a bimodal behaviour (dominant raindrops, 0.8 and 1.8 mm) in 0.1 ≤ R < 1 mmhr−1 rain category, while at high-altitude monomodal DSDs are observed irrespective of rain categories. The vertical profile of DSDs during BB events show that concave downward shape of DSDs are more prominent at mid-altitude which becomes concave upward at near surface levels that signifies collision and coalescence processes. The altitudinal variation of shape parameter indicates concave downward DSD in high-altitude throughout the layer and also less variation in slope (Λ) and intercept (N0) values represents the collision, coalescence and breakup processes. The coefficient and exponent values in radar reflectivity-rain rate (Z-R) relations such as Z = 474.7R1.04 at mid-altitude and Z = 340.7R1.06 at high-altitude indicates the microphysical differences in rainfall at different elevations over WG. The results explain that internal dynamics between the shallow stable cloud layer and the melting layer present in the high-altitude creates seeder-feeder effects which accelerate the growth of small and moderate size raindrops that determines the rain DSD and enhance surface rain rates over WG.