A melting-layer model for passive/active microwave remote sensing applications. Part II: Simulation of TRMM observations

The one-dimensional, steady-state melting-layer model developed in Part I o f this study is used to calculate both the microphysical and radiative prop erties of melting precipitation, based upon the computed concentrations of snow and graupel just above the freezing level at applicable horizontal gri d points of three-dimensional cloud-resolving model simulations. The modifi ed 3D distributions of precipitation properties serve as input to radiative transfer calculations of upwelling radiances and radar extinction/reflecti vities at the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager ( TMI) and precipitation radar (PR) frequencies, respectively. At the resolut ion of the cloud-resolving model grids (similar to1 km), upwelling radiance s generally increase if mixed-phase precipitation is included in the model atmosphere. The magnitude of the increase depends upon the optical thicknes s of the cloud and precipitation, as well as the scattering characteristics of the mixed-phase particles and ice-phase precipitation aloft. Over the s et of cloud-resolving model simulations utilized in this study, maximum rad iance increases of 43, 28, 18, and 10 K are simulated at 10.65, 19.35, 37.0 , and 85.5 GHz, respectively. The impact of melting on TMI-measured radianc es is determined not only by the physics of the melting particles but also by the horizontal extent of the melting precipitation, given that the lower -frequency channels have footprints that extend over tens of kilometers. At TMI resolution, the maximum radiance increases are 16, 15, 12, and 9 K at the same frequencies. Simulated PR extinction and reflectivities in the mel ting layer can increase dramatically if mixed-phase precipitation is includ ed, a result consistent with previous studies. Maximum increases of 0.46 (s imilar to2 dB) in extinction optical depth and 5 dB in reflectivity are sim ulated based upon the set of cloud-resolving model simulations.