On the influence of different radiation parametrizations on model‐generated radiation fields

Radiation fields for 10‐day forecasts with the operational ECMWF model using three different radiation schemes are presented. Simultaneous measurements of the radiation budget at the top of the atmosphere derived from TIROS‐N AVHRR data are compared with the computed budgets. Initial values of the radiation fields computed by the three schemes are compared and large intrinsic differences are found. Discrepancies in the heating rate profiles are located in the stratosphere and in the tropical lower layers. Land‐sea contrasts in the maps of outgoing longwave fluxes, related to differences in cloudiness, vary from one scheme to another, and are usually smaller than those derived from satellite measurements. Over the 10‐day period of integration, all three schemes lead to similar changes in the cloud fields, with a decrease over the ITCZ and an increase at high latitudes. This effect can be related to deficiencies in the hydrological cycle of the global model. Despite relatively large differences between the initial values of the radiation fields, compensating mechanisms are such that the computed fields, averaged over the 10 days, show smaller deviations from the observed fields.