Combined cloud-microwave radiative transfer modeling of stratiform rainfall

The simulation of explicit particle spectra during cloud evolution by a two -dimensional spectral cloud model was used to investigate the response of m icrowave radiative transfer to particle spectra development with special fo cus on the radiative effects of melting particles below the freezing level. For this purpose, 1) a particle-melting model was implemented with increas ed vertical resolution: 2) several models of the dielectric permittivity fo r melting particles were compared; 3) the dependence on size-density distri butions was evaluated; and 4) the influence on the results by the replaceme nt of explicit by parameterized particle spectra was tested. Radiative transfer simulations over ocean background at frequencies between 10.7 and 85.5 GHz showed a considerable increase in brightness temperature s (T-n) once melting particles were included. The amounts were strongly dep endent on the implemented permittivity model, the number concentrations of large frozen particles right above the freezing level, and the local cloud conditions. Assuming a random mixture of air, ice, and meltwater in the pan icle, T(n)s increased by up to 30 K (at 37.0 GHz) in the stratiform cloud p ortion For nadir view. If the meltwater was taken to reside at the particle boundaries, unrealistic T-n changes were produced at ail frequencies. This led to the conclusion that for large tenuous snowflakes the random-mixture model seems most appropriate, while for small and dense particles a nonuni form water distribution may be realistic. The net melting effect on simulat ed T(n)s, however, depended strongly on attenuation by supercooled liquid w ater above the freezing level, which generally suppressed the signal at 85. 5 GHz. Over land background, changes in T-n due to melting particles remain ed below 8 K, which would be difficult to identify compared to variations i n surface emission and cloud profile heterogeneity. Replacement of the explicit particle spectra for rain, snow, and graupel by parameterized spectra there, in exponential form with a fixed intercept) p roduced reductions of the melting signature by up to 40% over ocean. It was found that exponential size distribution formulas tended to underestimate number concentrations of large particles and overestimated those of small p articles at those cloud levels where sufficient particle sedimentation lead s to collection, aggregation, and evaporation, respectively, Consequently, the strongest differences between explicit and parameterized spectra occurr ed right, above the freezing level for snow and graupel, and close to the s urface for rain. Radiometrically, this resulted in an underestimation of sc attering above the freezing level and an underestimation of emission by mel ting particles below the freezing level as well as by rain toward the surfa ce. In the stratiform region, the net effect was a reduction of the melting signature: however, T-n's were still up to 15 K higher than from the no-me lting case for the random-mixture permittivity model.