Comparison of surface radiative flux parameterizations: Part I: Longwave radiation

This paper presents a comparison of several longwave (LW) downwelling radiative flux parameterizations with hourly averaged pointwise surface-radiation observations made at Sodankylä, Finland, in 1997 and 1999. Both clear and cloudy nighttime conditions are considered. The comparisons covered a 2-m temperature range from +11°C all the way to −49°C. ear-sky comparisons included eight simple LW parameterizations, which mainly use screen-level input data, and four radiation schemes from numerical weather prediction (NWP) models: the former European Centre for Medium-Range Weather Forecast (ECMWF) scheme, the Deutscher Wetterdienst (DWD) scheme, the High Resolution Limited Area Model (HIRLAM) scheme, and the new ECMWF LW scheme (Rapid Radiative Transfer Model, RRTM). Atmospheric-sounding profiles were used as input for the NWP schemes. For the cases with clouds, three simple cloud-correction methods were tested. all LW schemes usually underestimated the downwelling clear-sky flux, particularly, in cold (surface inversion) conditions. Overall, the RRTM scheme performed best. The simple cloud-correction methods turned out to be useful in the LW region. Finally, some new simple parameterization formulas were developed using the present data.