Effect of Vertical Downdraft on Rain Attenuation Modeling Over an Earth-Space Path

The impact of vertical downdraft on rain attenuation estimation from rain rate measurements has been studied. The rain rate (
R
) and specific attenuation (γ) are derived from rain drop size distributions (DSD) by considering the widely accepted Gunn–Kinzer (G-K) relationship between drop size and drop fall velocity that is valid for stagnant air. In the real atmosphere, drops are carried with the vertical wind draft, and the actual fall velocity of the raindrops deviates from the G–K formulation. The ITU-R estimated rain attenuation values, based on DSDs, which are mostly valid for temperate regions, can deviate from actual rain attenuation measurements in tropical regions, especially at higher rain rates when downdraft is prominent. The present study uses disdrometer measurements of DSD, optical rain gauge measurements of rain rates, and Ku-band rain attenuation data over an earth-space path at a tropical location Kolkata (22°34′ N, 88°29′ E) to show that vertical downdrafts impact the rain attenuation estimation from rain rate measurements. At the present location, an overall representative vertical downdraft of 2 m/s needs to be considered to match the experimental attenuation measurements with the estimated values using the modified γ–
R
relationship in the ITU-R model.