A Model of Longwave Irradiance for Use with Surface Observations

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Abstract
Longwave irradiance is often poorly estimated in heat budget calculations for the sea surface. A model using the same hourly surface data as a successful shortwave irradiance parameterization (Lumb, 1964) is developed. Cloud layers in each hour's observation are assigned effective emittances. Cloud temperatures are calculated from temperature profiles constructed using heights and temperatures interpolated from synoptic charts. Contributions from the water vapor band are calculated using the temperature profile to the height of the lowest cloud base (up to 1000 m) and a linear profile of mixing ratio calculated from the surface value. The model compares to within 10 W m−2 over 2-week periods with measurements of longwave irradiance on two ships in the North Atlantic during three months in the summer season. Existing shortwave models and this model can be combined to give better estimates of the radiation balance at the sea surface.