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Observed Screen (Air) and GCM Surface/Screen Temperatures: Implications for Outgoing Longwave Fluxes at the Surface

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  • 1 CSIRO Division of Atmospheric Research, Aspendale, Victoria, Australia
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Abstract

There is direct evidence that excess net radiation calculated in general circulation models at continental surfaces [of about 11–17 W m−2 (20%–27%) on an annual basis is not only due to overestimates in annual incoming shortwave fluxes [of 9–18 W m−2 (6%–9%)], but also to underestimates in outgoing longwave fluxes. The bias in the outgoing longwave flux is deduced from a comparison of screen-air temperature observations, available as a global climatology of mean monthly values, and model-calculated surface and screen-air temperatures. An underestimate in the screen temperature computed in general circulation models over continents, of about 3 K on an annual basis, implies an underestimate in the outgoing longwave flux, averaged in six models under study, of 11–15 W m−2 (3%–4%). For a set of 22 inland stations studied previously, the residual bias on an annual basis (the residual is the net radiation minus incoming shortwave plus outgoing longwave) varies between 18 and −23 W m−2 for the models considered. Additional biases in one or both of the reflected shortwave and incoming longwave components cannot be ruled out.

Abstract

There is direct evidence that excess net radiation calculated in general circulation models at continental surfaces [of about 11–17 W m−2 (20%–27%) on an annual basis is not only due to overestimates in annual incoming shortwave fluxes [of 9–18 W m−2 (6%–9%)], but also to underestimates in outgoing longwave fluxes. The bias in the outgoing longwave flux is deduced from a comparison of screen-air temperature observations, available as a global climatology of mean monthly values, and model-calculated surface and screen-air temperatures. An underestimate in the screen temperature computed in general circulation models over continents, of about 3 K on an annual basis, implies an underestimate in the outgoing longwave flux, averaged in six models under study, of 11–15 W m−2 (3%–4%). For a set of 22 inland stations studied previously, the residual bias on an annual basis (the residual is the net radiation minus incoming shortwave plus outgoing longwave) varies between 18 and −23 W m−2 for the models considered. Additional biases in one or both of the reflected shortwave and incoming longwave components cannot be ruled out.

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