Simultaneous MST radar and radiosonde measurements at Gadanki (13.5°N, 79.2°E) 1. Causative mechanism and characteristics of radar backscatterers at VHF

Abstract

Simultaneous MST radar and radiosonde observations have been carried out from Gadanki (13.5°N, 79.2°E), a tropical station in India. The radar measurements are used to obtain winds, vertical shear of horizontal winds (S), reflectivity (η), aspect sensitivity and horizontal correlation length (ζ) of the radar backscatterers. The radiosonde measurements of pressure, temperature and humidity are utilized for the computations of Brunt Vaisala frequency (N) and potential refractive index gradient (M). These measurements are used to understand the characteristics of the radar backscatterers in terms of prevailing background atmospheric conditions. Observations show strong vertical shears of horizontal winds in the height range of 16–18 km, at the upper edge of the tropical easterly jet (TEJ) associated winds. Results show that these shears and N2 both contribute to enhance the echo power in oblique directions (χ = 10°). The oblique beam observed enhancement of echo power, due to enhanced wind shears, is noticed to be confined to a narrow height range and associated backscatterers are more or less isotropic. At the height where N2 (or MD2) contribution to oblique echo power is significant, the radar backscatterers appear to be relatively more anisotropic with the horizontal correlation length (ζ) of 16–20 m. Present observations thus bring out clearly that even at oblique beam with zenith angle χ = 10° contribution of enhanced N2 to radar reflectivity (η) from refractivity structure with correlation length of ∼10–20 m can be very significant especially at the height above 17.5 km. These results have to be necessarily taken into account while interpreting the observed radar reflectivity observations.