Evaluating climate model simulations of tropical cloud

Abstract

The representation of tropical cloud and its radiative effects in the Hadley Centre climate model are evaluated using a combination of Earth observation data and meteorological reanalyses. It is shown that useful information regarding the model’s physical parametrizations can be obtained by considering cloud radiative effects and cloud types in terms of ‘dynamical regimes’, defined in terms of sea surface temperature and large-scale vertical motion. In addition to comparisons with observed top-of-atmosphere radiation budget parameters and total cloud amount, information is obtained through direct comparisons of International Satellite Cloud Climatology Project (ISCCP) cloud types, defined according to cloud top pressure and optical depth, with corresponding model diagnostics. An analysis of the atmosphereonly model, HadAM3, demonstrates how errors in the albedo and outgoing long-wave radiation can be related to the simulation of particular cloud types in the different dynamical regimes. Inconsistencies between the simulations of the various cloud types and the top-of-atmosphere radiation budget are also highlighted. A version of the model including several new cloud-related parametrizations is then examined. A more consistent comparison with the observed radiation budget and cloud amounts is obtained, although deficiencies in the simulation still remain. A parametrization for the radiative effects of convective anvils and the impact of a new boundary layer mixing scheme are examined in more detail. Finally, it is shown how the climate model’s ability to simulate the observed interannual variability of cloud in the equatorial Pacific follows directly from the analysis according to dynamical regimes.