Should General Circulation Model Radiation Schemes Include Rain?

Accurate simulation of the earth's radiation budget is crucial both for weather forecasting and climate modelling. Yet, large radiation errors persist in most general circulation models (GCMs). The effect of rain on radiative fluxes and heating rates is a missing process that exists in all GCMs. Yet, to our knowledge, the magnitude of the resulting radiative bias remains unquantified. Moreover, many GCMs suffer from large radiation biases in heavily precipitating regions, which calls into question whether neglecting the radiative effect of rain could be a source of errors. This study aims to provide a first estimate of how rain affects the atmospheric radiation budget at a range of temporal and spatial scales. This is a necessary first step towards determining whether GCM radiation schemes should include rain.

We estimate the rain radiative effect here by performing radiative transfer calculations with and without rain. We perform calculations using the SOCRATES (Suite Of Community Radiative Transfer codes based on Edwards-Slingo) radiative transfer scheme. Input atmospheric profiles are taken from two weeks (one week during boreal winter and the other during boreal summer) of a Goddard multiscale modelling framework (MMF) simulation.

Based on these calculations, we shall quantify and explain how rain affects the transfer of radiation through the atmosphere and thus radiative heating rates and both surface and top of atmosphere fluxes. To conclude, we shall discuss whether parametrization of rain radiative effects is necessary in GCMs.