Path-average drop size distribution estimation from dual-wavelength measurements of attenuation

Microwave path-average measurements of rainfall attenuation at 25.35 GHz (A_25H), 38.025 GHz (A_38H), using the upgraded 2.3-km NASA TRMM microwave link, are used to estimate parameters (Λ and N₀-μ fixed) of a path-average gamma drop size distribution N¯(D). This dual-wavelength technique requires the computation of the forward scattering amplitude f(D) for oblate raindrops to solve for the drop size distribution parameters numerically. The resulting N¯(D) is then used to compute the corresponding instantaneous rain rate, R¯, which is compared to rainfall rate estimates based on empirical power law relations, and to path-average rainfall rate measurements from a network of optical and tipping bucket raingauges located beneath the link path. Rain rate estimates using link path-average polarization differential phase-shift measurements (Φ_DP) at 8.45 GHz are also implemented