Impact of a new radiation package, McRad, in the ECMWF Integrated Forecasting System
Jean-Jacques Morcrette, Howard W. Barker, Jason S. Cole, Michael J. Iacono, and Robert Pincus
Monthly Weather Review, Vol. 163, pages 4773-4798. doi:10.1175/2008MWR2363.1.

Abstract

A new radiation package (McRad) has become operational with cycle 32R2 of the Integrated Forecasting System (IFS) of the European Centre for Medium-Range Weather Forecasts (ECMWF).McRad includes an improved description of the land surface albedo fromMODIS observations, theMonte-Carlo Independent Column Approximation treatment of the radiation transfer in clouds, and the RRTM short-wave scheme.

The impact of McRad on year-long simulations at T159L91 and higher-resolution ten-day forecasts is then documented. McRad is shown to benefit the representation of most parameters over both short and longer time-scales, relative to the previous operational version of the RT schemes. At all resolutions, McRad improves the representation of the cloud-radiation interactions, particularly in the tropical regions, with improved temperature and wind objective scores through a reduction of some systematic errors in the position of tropical convection, due to change in the overall distribution of diabatic heating over the vertical, inducing a geographical redistribution of the centres of convection. While smaller, the improvement is also seen in the r.m.s. error of geopotential in the Northern and Southern hemispheres and over Europe.

Given the importance of the cloudiness in modulating the radiative fluxes, the sensitivity of the model to cloud overlap assumption (COA) is also addressed, with emphasis on the flexibility inherent to this new RT approach when dealing with COA.

The sensitivity of the forecasts to the space interpolation required to deal efficiently with the high computational cost of the RT parametrization is also revisited. A reduction of the radiation grid for the EPS (Ensemble Prediction System) is shown to be of little impact on the scores while reducing the computational cost of the radiation computations.

McRad is also shown to decrease the cold bias in ocean surface temperature in climate integrations with a coupled ocean system.

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