The Interactive Role of Clouds and Radiative Transfer in Maintaining Atmospheric Circulation.

Radiative flux divergence is one of the principal physical forces for atmospheric circulation. It affects the temperature, winds, cloud evolution and structure, and the water budget on all time and space scales. In this study, numerical models are used to study the effects of radiative transfers on a variety of atmospheric circulation systems on time scales from several hours to several days, and on space scales of about 10 kilometers to 10 thousand kilometers. Shortwave and longwave radiative transfer are added to a mesoscale model which includes detailed microphysics and turbulence closure schemes. This model is used to study three cases where radiative transfer plays a role in the structure of the atmosphere. The first is stratocumulus clouds that form in the wake of thunderstorms. Longwave radiative flux divergence at the top of the boundary layer maintains the cloud layer by cooling at cloud top, enhancing the inversion strength. Eventually, shortwave energy reaching the surface creates sufficient buoyancy to overcome the longwave forcing of the cloud layer, and the cloud dissipates. The same longwave forcing is responsible for the formation of boundary layer clouds in the second case studied, the coastal front. In that case, the presence of clouds also affects the horizontal heating of the atmosphere, as clouds effectively trap longwave energy below clouds, but also reflect shortwave energy, so that surface heating over land is less. The differences in radiative forcing changes the relative intensity of the thermal circulations that form over the Gulf Stream and at the coastline, with further feedback to the clouds, winds, temperature, and atmospheric water vapor. Finally, the third case was an examination of the effects of radiative transfer on low level cyclogenesis. Again, the longwave flux divergence at the top of the clouds maintained a cloud layer against the destructive force of mixing form below and entrainment from above. Once clouds form, horizontal thermal gradients lead to increased vorticity generation and a more intense circulation. A stronger circulation feedback to the cloud formation processes and water budget results. Finally, radiative transfer was added to a numerical weather prediction model to study the Indian summer monsoon. The length and time scales in this study were much longer, covering a 180 degree longitude by 90 degree latitude domain for 10 day forecasts. The conclusions however were remarkably similar to those from the smaller model. Radiative transfer maintains the strength and structure of the components of the monsoon circulation system with feedback to cloud and precipitation processes.