Impact of Cloud Microphysics on Convective–Radiative Quasi Equilibrium Revealed by Cloud-Resolving Convection Parameterization

Wojciech W. Grabowski National Center for Atmospheric Research,* Boulder, Colorado

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Abstract

This paper investigates how cloud microphysics impact global convective–radiative quasi equilibrium on a constant-SST aquaplanet. A novel approach, the cloud-resolving convection parameterization (CRCP), also known as “superparameterization,” resolves both global dynamics and cloud-scale dynamics, as well as small-scale coupling among convective, radiative, and surface processes, within a single computational framework. As a result, the effects of cloud microphysics on the tropical large-scale dynamics and climate can be investigated with greater confidence than traditional approaches allow. Idealized simulations highlight extreme effects of the microphysical parameterizations to expose the impacts.

The results suggest that cloud microphysics impact quasi-equilibrium temperature and moisture profiles substantially, but the relative humidity is only weakly affected. Small cloud and precipitation particles result in a climate that is warmer and moister. This is explained by the impact of cloud microphysics on the coupling between convection and surface exchange during the approach to quasi equilibrium. With interactive radiation, this effect is supplemented by gradual evolution of mean cloud fraction profiles and the mean radiative cooling. The mean quasi-equilibrium radiative cooling is weaker in the simulation featuring small cloud and precipitation particles. Cloud–radiation interactions, explicitly treated in CRCP, play a significant role in setting up the quasi-equilibrium cloudiness.

The simulations further support the conjecture that the main impact of cloud microphysics in the Tropics is on the net energy budget at the ocean surface. The net energy flux into the ocean in quasi equilibrium is considerably smaller in simulations having small cloud and precipitation particles.

Corresponding author address: Dr. Wojciech W. Grabowski, NCAR, P.O. Box 3000, Boulder, CO 80307-3000. Email: grabow@ncar.ucar.edu

Abstract

This paper investigates how cloud microphysics impact global convective–radiative quasi equilibrium on a constant-SST aquaplanet. A novel approach, the cloud-resolving convection parameterization (CRCP), also known as “superparameterization,” resolves both global dynamics and cloud-scale dynamics, as well as small-scale coupling among convective, radiative, and surface processes, within a single computational framework. As a result, the effects of cloud microphysics on the tropical large-scale dynamics and climate can be investigated with greater confidence than traditional approaches allow. Idealized simulations highlight extreme effects of the microphysical parameterizations to expose the impacts.

The results suggest that cloud microphysics impact quasi-equilibrium temperature and moisture profiles substantially, but the relative humidity is only weakly affected. Small cloud and precipitation particles result in a climate that is warmer and moister. This is explained by the impact of cloud microphysics on the coupling between convection and surface exchange during the approach to quasi equilibrium. With interactive radiation, this effect is supplemented by gradual evolution of mean cloud fraction profiles and the mean radiative cooling. The mean quasi-equilibrium radiative cooling is weaker in the simulation featuring small cloud and precipitation particles. Cloud–radiation interactions, explicitly treated in CRCP, play a significant role in setting up the quasi-equilibrium cloudiness.

The simulations further support the conjecture that the main impact of cloud microphysics in the Tropics is on the net energy budget at the ocean surface. The net energy flux into the ocean in quasi equilibrium is considerably smaller in simulations having small cloud and precipitation particles.

Corresponding author address: Dr. Wojciech W. Grabowski, NCAR, P.O. Box 3000, Boulder, CO 80307-3000. Email: grabow@ncar.ucar.edu

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