The Surface Downward Longwave Radiation in the ECMWF Forecast System

Abstract

The surface downward longwave radiation, computed by the European Centre for Medium-Range Weather Forecasts (ECMWF) forecast system used for the ECMWF 40-yr reanalysis, is compared with surface radiation measurements for the April–May 1999 period, available as part of the Baseline Surface Radiation Network (BSRN), Surface Radiation (SURFRAD), and Atmospheric Radiation Measurement (ARM) programs. Emphasis is put on comparisons on a 1-h basis, as this allows discrepancies to be more easily linked to differences between model description and observations of temperature, humidity, and clouds. It is also possible to compare the model and observed temporal variability in the surface radiation fluxes. Comparisons are first carried out at locations for which the spectral model orography differs from the actual station height. Sensitivity of the model fluxes to various algorithms to correct for this discrepancy is explored. A simple interpolation–extrapolation scheme for pressure, temperature, and specific humidity allows the improvement of model calculations of the longwave surface fluxes in most cases. Intercomparisons of surface longwave radiation are presented for the various longwave radiation schemes operational since the 15-yr ECMWF Re-Analysis (ERA-15) was performed. The Rapid Radiation Transfer Model of Mlawer et al., now operational at ECMWF, is shown to correct for the major underestimation in clear-sky downward longwave radiation seen in ERA-15. Sensitivity calculations are also carried out to explore the role of the cloud optical properties, cloud effective particle size, and aerosols in the representation of the surface downward longwave radiation.