Vertical resolution increase of the Japan Meteorological Agency Global Spectral Model

Japan Meteorological Agency (JMA) has a plan to raise the topmost level of Global Spectral Model (GSM) from 0.1hPa to 0.01hPa, which makes GSM to include whole stratosphere and place model lid on the mesopause. The number of total model layers will be increased from 60 to 100. Motivations of the increase of the number of model layers and the extension of the model lid are to improve the representation of atmospheric vertical structure and the atmospheric processes, to enable better use of satellite channels that have sensitivity for the middle atmosphere on data assimilation, and to improve accuracy of forecast in the both troposphere and stratosphere. In this presentation, specifications of the upgraded GSM and its performance are shown. Further developments activities regarding to development of non-orographic gravity wave drag scheme are also presented.

Various modifications are applied to current operational model described in JMA (2013). The changes contain an introduction of two-stream approximation for long wave radiation scheme, changing the application criteria of energy correction term in convective parameterization, an upgrade of monthly ozone climatology in the mesosphere, adding 2nd-order linear horizontal diffusion in the divergence equation and adjusting 4th-order linear diffusion as a sponge layer around the model top region, and modification of the Rayleigh friction. The forecast score statistics shows the overall improvements in the northern hemisphere and the stratosphere but slight degradation for some elements in the southern hemisphere and the tropics. Model forecast characteristics change significantly due to the modification of energy correction term in convective parameterization, especially in the tropics. In the stratosphere and mesosphere, improvements result from two-stream approximation long wave radiation scheme, an upgrade of ozone climatology, and modification of horizontal diffusion.

To achieve better forecast performance in the middle atmosphere, a non-orographic gravity wave drag scheme based on Scinocca (2003) is under development and testing for the new GSM. The revised model successfully reproduces QBO in the tropical stratosphere although its period is shorter and amplitude is weaker than observed. The model climatology of the middle atmosphere is also improved compared to the results with Rayleigh friction.