Diurnal variation in diffusion scheme on a global atmospheric model

The influence of the diffusion scheme on the simulated diurnal variations and mean structure of atmosphere in a GCM were examined using the MYNN scheme developed by Nakanishi and Niino [2009], in which it is rearranged the closure constants for the stability functions, and is considered the effects of buoyancy the turbulent mixing length based on the LES dataset. The improved performance of the MYNN scheme mainly relies on the new formulation of the mixing length that realistically increases with decreasing the stability. A new mixing length shows evident diurnal variation in PBL and vertically decreases near the top of the boundary layer, affecting the convection penetrating into the overlying stable layer. As a result, it makes the vertical mixing to well respond to the surface diurnal cycle and helps to well transfer the heat, moisture and momentum from the surface to atmosphere.

The diurnal behaviors in PBL scheme affect the large-scale mean structure and improve their biases. And the result of the precipitation shows an improved horizontal distribution of tropical rainfall, especially over the western Pacific and Indian Ocean. The change of mean precipitation is primarily related with the convective precipitation and comes from the change of diurnal amplitude by affecting the maximum precipitation. Although the diurnal phase of precipitation is not changed significantly, the diurnal variation in diffusion scheme results in the reduced diurnal amplitude of the precipitation, which is closer to the observation. The change of the CAPE due to the PBL scheme affects the convective cumulus scheme and such response is clearly shown in the large-scale divergence fields.