Passive microwave signatures of the melting layer in stratiform clouds

A study was carried out in order to estimate the effect of melting particles on brightness temperatures at microwave frequencies between 10.7 and 85.5 GHz. A meteorological model framework was set up to determine the melting stage of various frozen particles below the freezing layer. Different approaches for calculating the effective permittivity of mixed particles were compared. The resulting extinction coefficients, single scattering albedos, and asymmetry parameters indicate a maximum effect when a water matrix with air/ice inclusions is assumed. Extreme local differences of 20-25% between the optical parameters employing either a Marshall-Palmer or a Gamma-type dropsize distribution occur. When radiative transfer calculations are carried out, maximum deviations of >20 K at low frequencies are observed. Neglecting the melting effect may lead to severe overestimations of surface rainrates by up to 100% in stratiform conditions.