Interactions between Cloud Microphysics and Cumulus Convection in a General Circulation Model

In the Colorado State University general circulation model, cumulus detrainment of cloud water and cloud ice has been, up to now, the only direct coupling between convective and large-scale condensation processes. This one-way interaction from the convective to the large-scale environment parameterizes, in a highly simplified manner, the growth of anvils spreading horizontally at the tops of narrow cumulus updrafts. The reverse interaction from the large-scale to the convective updrafts, through which large-scale cloud water and cloud ice can affect microphysical processes occurring in individual convective updrafts, is missing. In addition, the effects of compensating subsidence on cloud water and cloud ice are not taken into account. A new parameterization of convection, called ‘‘EAUCUP,’’ has been developed, in which large-scale water vapor, cloud water, and cloud ice are allowed to enter the sides of the convective updrafts and can be lifted to the tops of the clouds. As the various water species are lifted, cloud microphysical processes take place, removing excess cloud water and cloud ice in the form of rain and snow. The partitioning of condensed vapor between cloud water and cloud ice, and between rain and snow, is based on temperature. The effects of compensating subsidence on the large-scale water vapor, cloud water, and cloud ice are computed separately. Convective rain is assumed to fall instantaneously to the surface. Three treatments of the convective snow are tested: 1) assuming that all snow is detrained at the tops of convective updrafts, 2) assuming that all snow falls outside of the updrafts and may evaporate, and 3) assuming that snow falls entirely inside the updrafts and melts to form rain. Including entrainment of large-scale cloud water and cloud ice inside the updrafts, large-scale compensating subsidence unifies the parameterizations of large-scale cloud microphysics and convection, but have a lesser impact than the treatment of convective snow on the simulated climate. Differences between the three alternate treatments of convective snow are discussed. Emphasis is on the change in the convective, large-scale, and radiative tendencies of temperature, and change in the convective and large-scale tendencies of water vapor, cloud water, cloud ice, and snow. Below the stratiform anvils, the change in latent heating due to the change in both convective and large-scale heatings contributes a major part to the differences in diabatic heating among the three simulations. Above the stratiform anvils, differences in the diabatic heating between the three simulations result primarily because of differences in the longwave radiative cooling. In particular, detraining convective snow at the tops of convective updrafts yields a strong increase in the longwave radiative cooling associated with increased upper-tropospheric cloudiness. The simulated climate is wetter and colder when convective snow is detrained at the tops of the updrafts than when it is detrained on the sides of the updrafts or when it falls entirely inside the updrafts. This result highlights the importance of the treatment of the ice phase and associated precipitation in the convective cloud models used in cumulus parameterizations.