1. Introduction
The modeling of microwave radiative transfer in clouds containing precipitation requires the treatment of problems like size distributions, particle shape, and particle
inhomogeneity when used for sensitivity studies, comparison with aircraft or satellite radiometric measurements, and retrieval algorithm development. There are numerous
studies where hydrometeor distributions from cloud models are used to investigate the sensitivity of top-of-the-atmosphere microwave radiances to microphysical cloud proper-
ties because these models provide consistent hydrometeor fields in three dimensions and
Ž .
over cloud evolution e.g. Mugnai and Smith, 1988; Panegrossi et al., 1998 . The treatment of hydrometeors and size distributions in the radiative transfer simula-
tions depends on the available output from the cloud models. Most of them calculate bulk microphysical properties such as liquid water contents assuming fixed size distribu-
tions. This, however, does not allow the direct treatment of the above mentioned problems. However, there are examples where ‘a posteriori’ tuning was applied to the
model output fields because the simulations could not satisfactorily explain the observa-
Ž tions when the cloud model inherent parameterizations were used Panegrossi et al.,
. 1998; Schols et al., 1999 .
Melting hydrometeors have recently been rediscovered as a possible source of uncertainty when neglected in microwave radiative transfer simulations. Sensitivity
studies have identified them as a source of extra emission at frequencies between 10 and Ž
. 90 GHz Bauer et al., 1999, 2000; Olson et al., 1999 . The results seem to justify the
proposal of a general and flexible treatment of the melting layer to be added to previously modeled hydrometeor fields because cloud models include melting processes
but do not output melting particle species. They also do not account for the vertical resolution required for the calculation of their integrated optical properties.
This study presents a comparably simple approach for the inclusion of melting hydrometeors in cloud model simulation fields avoiding the cost of upgrading the cloud
model and rerunning the simulations. Therefore, it allows the generation of improved fields for the generation of cloud–microwave radiance data simulations. A serious
constraint is the possible violation of cloud model physics consistency because the modifications employ stationary assumptions.
Ž . The final results are generated in the form of: 1 a simple recipe to retrieve the
available melting particle mass from existing profiles to be used for estimating the bulk Ž .
radiative properties of the layer and 2 tables of hydrometeor optical properties for all main species and both homogeneous and melting conditions. Section 2 presents the
set-up of particle size spectra according to most cloud models and the adjustment of profiles for the inclusion of melting particles. In Section 3, the calculation of microwave
optical properties is briefly presented, including examples of an application to cloud
Ž model simulations with the Goddard Cumulus Ensemble model GCE, Tao and Simp-
´
. Ž
son, 1993 and the Meso Echelle–Non Hydrostatique model Meso-NH, Lafore et al.,
´
. 1998 . Finally, a comparison of these simulations with measurements from the Tropical
Ž .
Ž .
Rainfall Measuring Mission TRMM Microwave Imager TMI and the Precipitation Ž
. Radar PR aiming at the isolation of characteristic melting layer signatures is carried
out, followed by a discussion of the results, in Section 4.
2. Hydrometeors
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