|Title||A reduced radiation grid for the ECMWF Integrated Forecasting System|
|Year of Publication||2007|
|Authors||Morcrette, J-J, Mozdzynski, G, Leutbecher, M|
|Secondary Title||Technical Memorandum|
A specific interface between the radiation transfer calculations and the rest of the ECMWF model was introduced in 2003, potentially providing substantial economy in computer time by reducing the spatial resolution at which radiation transfer is evaluated, without incurring some of the de?ciencies produced by the sampling strategy previously used in the ECMWF model. The introduction of a new radiation package (McRad) in June 2007 has led to a more extensive use of this interface, and its impact is discussed in this note. For a given model resolution, the impact of a lower resolution radiation grid on the model behaviour is studied here, in the context of 10-day forecasts at high resolution (TL799L91), of medium resolution forecasts (TL399L62) used in the Ensemble Prediction System (EPS), and of low resolution simulations (TL159L91) as used for model development and seasonal forecasts with an interactive mixed-layer ocean. Results for the high resolution forecasts are compared in terms of objective scores and of the quality of "surface" parameters (total cloud cover, two-metre temperature and specific humidity, and 10-m wind) usually verified in a meteorological context. For the medium resolution forecasts, the impact of the radiation grid is studied in terms of the potential increase in the efficiency of the EPS system without deteriorating the probabilistic skill. Impact of changes in the radiation grid resolution on the low resolution versions of model is discussed in terms of cloud-radiation interactions and ocean surface temperature.