The effect of the melting layer on the microwave emission of clouds over the ocean

A study was carried out in order to estimate the effect of melting particle s od simulated brightness temperatures at microwave frequencies between 10. 7 and 85.5 GHz for precipitation over the Ocean. The meteorological model f ramework is based on the assumption that the strongest radiometric effect i s due to the drastically increased permittivity of melting particles driven by the volume fraction of liquid water. Thus, effects caused by particle a ggregation and breakup are neglected. Different approaches for calculating the effective permittivity of mixed pa rticles are compared. The resulting extinction coefficients, single scatter ing albedos, and asymmetry parameters indicate a maximum effect when the pa rticle is composed of a water matrix with air-ice inclusions. In particular the extinction coefficient may vary by more than two orders of magnitude r ight below the freezing level dependent on frequency and the applied mixing formula. In the melting region also the strongest dependence of the optica l properties on the droplet spectrum is observed. Extreme local differences of 100% between the particle optical properties employing either a Marshal l-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 t o the strong dependence of the extinction coefficient on the implemented me lting 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 e mission excess seems to be overexpressed. The systematic effect of including the melting process in radiative transfe r calculations for the development of surface rain retrievals was also inve stigated. Over 550 model atmospheres were used to estimate the relative dev iation of surface rain-rate estimates using a set of operational rain retri eval algorithms. Neglecting the melting effect may lead to severe overestim ations of surface rain rates by up to 100% in stratiform conditions. Howeve r, if the melting layer is either weakly expressed or nonuniformly distribu ted in space, the relative overestimation is much lower. If the effective p ermittivity of melting particles is calculated using the weighted mixing ap proach of Meneghini and Liao, considerably less effect of melting particles on passive microwave emission is observed.