A numerical study on the contribution of melting layer composed by coated ice spheres to the upwelling brightness temperatures in TMI channels using VDISORT algorithm

To examine the influence of bright band on the retrieval of precipitation rate, the performance of melting layer composed by coated ice precipitable particles on the satellite-based measurement of polarized microwave brightness temperatures is discussed in this article by a vector discrete ordinate radiative transfer model. After comparing the simulated brightness temperatures in different TMI channels with and without the melting layer, we conclude that: (1) The melting layer composed by liquid-coated ice spheres weakens the upwelling microwave brightness temperatures because of the absorption/emission effect caused by the liquid coat. This effect is more conspicuous in middle and high frequency channels (19, 37 and 85 GHz) but, in 85 GHz channel, with the increase of rain rate, the multi-scattering can weaken its effect. (2) In a specific frequency, the horizontally polarized brightness temperature is more severely weakened by the melting layer than the vertically polarized. With the “cold” background (ocean surface, for example), this character is more conspicuous than that with a warm background. That is to say, the inner structure of a cloud system is easier to be detected under a cold background. Only in the 85 GHz frequency and when the rain rate is larger than 4 mm/h can we find that the vertically polarized brightness temperature is more severely weakened than the horizontally polarized one. (3) The melting layer with the assumption of coated ice spheres can change the difference of brightness temperatures between the vertically and horizontally polarized channels in the same frequency. In general, the value of such difference with the assumption of melting layer is larger than that without it. With a warm background, this value is negative and only in middle frequency (37 GHz), it is both stable and conspicuous.