Impacts of Atmospheric Stabilization Due to Globalwarming on High Cloud Area Changes through Turbulentmixing

Since the high clouds have large impacts on the Earth’s radiative energy budget and the formation of the environmental fields for the convective activity, the better understanding of the high cloud changes in response to global warming is vital for climate projection science. It has been recognized that the turbulent effects influence the cloud dynamics. However, the relationship between the change of turbulence and high clouds associated with global warming is not well investigated. The present study examined this relationship based on Radiative-convective equilibrium (RCE) experiments with a global cloud resolving model.

We conducted simulations with RCE configurations using the Nonhydrostatic Icosahedral Atmospheric Model. Turbulent closure was calculated using the level-2 of a modified version of Mellor-Yamada-Nakanishi-Niino scheme, and we used a two-moment bulk cloud microphysics scheme. The experiments are conducted with a horizontal resolution of 28 km with the earth-size sphere. Sea surface temperatures (SSTs) are fixed at 300K and 304K.

We found that the turbulent diffusivity was weakened with the increase of SST in the convective cores and the top of high clouds. The analyses of the static stability, which is a controlling factor of the turbulent diffusivity, revealed that this weakening of the turbulent diffusivity was caused by the reduction of the frequency of the static instability due to the increase of environmental static stability. The results of sensitivity simulations in which the environmental values of the static stability used for the evaluation of the turbulent diffusivity were modified showed that the stronger static stability can reduce the high clouds. These suggest that the increase of the static stability associated with global warming can lead to the reduction of high clouds through the weakening of the turbulent mixing in the cloud layers.