The Effect of the Melting Layer on the Microwave Emission of Clouds over the Ocean

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Abstract
A study was carried out in order to estimate the effect of melting particles on simulated brightness temperatures at microwave frequencies between 10.7 and 85.5 GHz for precipitation over the ocean. The meteorological model framework is based on the assumption that the strongest radiometric effect is due to the drastically increased permittivity of melting particles driven by the volume fraction of liquid water. Thus, effects caused by particle aggregation and breakup are neglected. Different approaches for calculating the effective permittivity of mixed particles are compared. The resulting extinction coefficients, single scattering albedos, and asymmetry parameters indicate a maximum effect when the particle is composed of a water matrix with air–ice inclusions. In particular the extinction coefficient may vary by more than two orders of magnitude right below the freezing level dependent on frequency and the applied mixing formula. In the melting region also the strongest dependence of the optical properties on the droplet spectrum is observed. Extreme local differences of 100% between the particle optical properties employing either a Marshall–Palmer or gamma-type drop size distribution occur. When radiative transfer calculations are carried out, average deviations of 20–30 K at low frequencies (10.7 and 19.35 GHz) are observed, mainly due to the strong dependence of the extinction coefficient on the implemented melting process. However, this effect is driven by the applied mixing formula rather than the drop size distribution; that is, for particles composed of a water matrix and air–ice inclusions independently of melting stage the emission excess seems to be overexpressed. The systematic effect of including the melting process in radiative transfer calculations for the development of surface rain retrievals was also investigated. Over 550 model atmospheres were used to estimate the relative deviation of surface rain-rate estimates using a set of operational rain retrieval algorithms. Neglecting the melting effect may lead to severe overestimations of surface rain rates by up to 100% in stratiform conditions. However, if the melting layer is either weakly expressed or nonuniformly distributed in space, the relative overestimation is much lower. If the effective permittivity of melting particles is calculated using the weighted mixing approach of Meneghini and Liao, considerably less effect of melting particles on passive microwave emission is observed.

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