Impacts on the surface temperature variation

The positive feedback between the surface temperature and the TP stratus deck may help to explain the climate variation downstream of the plateau. It is conceivable that the cloud radiative feedback during the surface cooling period could be more robust than that in the warming period. When the surface cools, the increased static stability favors stratus cloud formation while it restrains deep convection and related cirrus cloud formation, which results in more intensified solar radiative cooling that dominates the cloud-induced greenhouse warming. However, when the surface warms, the induced unstable stratification might, in part, favor cumulus convective clouds that could weaken the positive cloud feedback.

The case study indicates more robust positive cloud feedback in the cooling period. During the warming period from 1986 to 1987, the surface temperature increased 0.78°C; the net cloud radiative forcing increased by 6 W-m"2 (the total cloud fraction decreased by 1%, and the stratus cloud amount decreased by 3.3%). Therefore, the cloud radiative feedback is only 7.7 W-m^K"1 if changes in cloud are entirely related to the changes in surface temperature. On the other hand, during the cooling period from 1987 to 1989, the surface temperature decreased 0.78°C, and the net cloud radiative forcing decreased by 13 W-m"2 (the total cloud fraction increased by 5.5% and stratus clouds increased by 6.2%). Thus, the cloud radiative feedback amounts to 16.7 W-m^K"1 if changes in cloud are entirely related to the changes in surface temperature. Based on this estimation, the positive cloud radiative feedback during the cooling period is obviously stronger than that during a warming period. The relatively weak cloud radiative feedback during the warming period could be due to the nonstratus cloud formation. During the warming period of 1986~87, 70% of the stratus cloud decrease is balanced by nonstatus clouds, while during the cooling period 1987~89, only 11% of stratus cloud increase is offset by non-stratus clouds. Thus the change of stratus clouds during the cooling period more effectively dominates the total cloudiness variation than that during a warming period.

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