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Error Characteristics Caused by Radiation of Global NWP Model

Error Characteristics Caused by Radiation of Global NWP Model

Shigeki MURAI and Hiroto KITAGAWA

Numerical Prediction Division, Japan Meteorological Agency
1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, JAPAN

It is well known that NWP models have much uncertainty in their schemes related to cloud-radiation interaction. To know this uncertainty, influence on forecast performance by changing parameters of a radiation scheme is studied using a global atmospheric model of Japan Meteorological Agency (JMA), as a part of the RR2002 project funded by MEXT.

In order to save a calculation cost, our model calculates longwave radiation every 3 hours and shortwave hourly. On the other hand, cloud cover and water content are prognostically determined by the cloud scheme and are used in radiation calculations. This may lead to inconsistency between clouds given for the radiation scheme and those predicted in the cloud scheme. Influence of the inconsistency was investigated on radiation flux, cloud field and air temperature. The impacts on their diurnal cycle were also evaluated.

Our model has specific errors in radiation fields at the surface and the top of atmosphere. The errors may partly come from inadequate treatment of clouds in the radation calculations. Cloud overlap assumption is one of important parameters to simulate cloud radiative forcing in a model. The relevance of some overlap assumptions are evaluated for models with various spatial resolution focusing on the radiation flux at the surface and TOA.