Development of a new radiation scheme for the global atmospheric NWP model

A radiation scheme plays an important role in global atmospheric models. As not only its accuracy but also its computing efficiency is important, JMA developed a more accurate and efficient radiation scheme for a global atmospheric model.

In this new radiation scheme, a longwave spectrum is divided into 9 bands and the radiation flux is calculated in each band separately. Considering a trade-off between accuracy and efficiency, gaseous transmittance is computed using three different approaches depending on the absorber and the spectral band. These are a pre-computed table look-up method and a k-distribution method for line absorption, and a parameterization for water vapor continuum absorption. Effects of absorption by trace gases such as CH₄, N₂O and CFCs are also introduced. In addition to above changes in the longwave scheme, the absorption coefficient parameterization is refined in the new shortwave scheme.

By taking account of the Doppler absorption effect properly, errors in longwave cooling around the stratopause are reduced. Errors in cooling of the troposphere are also reduced by improvement of transmittance calculation of line absorption and introduction of P-type absorption of water vapor continuum. Accuracy of downward longwave flux at the ground expressed by the global model with the new scheme is also improved. A difference between forecasts and data derived from satellite observations (SRB: the Surface Radiation Budget) is smaller than 10W/m² over the wide area.

As it was confirmed that medium-range forecast skill of the global atmospheric model improved with the new scheme, it has been in operation at JMA since December 2004.