Effects of sea surface temperature and large-scale dynamics on the thermodynamic equilibrium state and convection over the tropical western Pacific

Abstract

The effects of sea surface temperature (SST) variations and large-scale dynamics on the cloud feedback and water vapor feedback are quantified using a fine-scale numerical model, or cloud-resolving model, in which the cloud-scale dynamics is explicitly treated instead of being parameterized as is necessary in a general circulation model. The SST variation has large impacts on the water vapor feedback and small impacts on the cloud feedback, radiation budget, and surface energy budget under a given large-scale dynamic state. As the SST gets warmer (increasing 2°), the warm and moist equilibrium state of temperature and water vapor mixing ratio is obtained; the upper tropospheric relative humidity is enhanced; and the cloud amount and convective intensity are slightly reduced. The cooling (about 10 W m-2) at the surface due to the increase of surface evaporation is almost compensated by the warming at the surface due to the increase of surface shortwave flux, which results in a small increase of net surface heat flux. However, the change of large-scale dynamics has large effects on the cloud feedback, radiation budget, and surface energy budget and small effects on the water vapor feedback under a constant SST. An increased large-scale forcing slightly affects the equilibrium states of temperature and water vapor mixing ratio; the relative humidity is decreased above 10 km and increased below; and the cloud amount and convective intensity are enhanced. Both the variations of SST and large-scale dynamics are positively correlated with the surface precipitation. Copyright 1999 by the American Geophysical Union.