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A Model of the Zenith Excess Attenuation Due to the Melting Layer

First, a model of melting layer of precipitation is proposed and the maximum depth of the melting layer is defined. The melting layer is assumed to be composed of spherical melting snow particles. Size distribution and average dielectric constant are used to characterize the melting snow particles. The maximum depth, the size distribution and the dielectric constant are found to be connected with the physical and meteorological parameters. The specific attenuation in the melting layer and the zenith excess attenuation which is defined as the difference between zenith attenuation in the melting layer and zenith attenuation of an equal-path length in rain are then computed in a frequency range of 1-100 GHz by using the Mie scattering theory. The k · R¿ (R in mm/h) relation has also been used to approach both the average specific attenuation of the melting layer and the rain specific attenuation. The zenith excess attenuation is finally formulated for four size distributions and rain rates below 12.5 mm/h by using the listed parameters k and ¿. Numerical results show that the zenith excess attenuation is at maximum around 25 GHz. Good agreements have also been seen between the computed result and the formulated result.