Thermal Heating Rates in 3D Cloud Fields and Potential Feedback on Cloud Formation

Solar and thermal radiation influence the development and microphysics of clouds. In the thermal spectral range, the main interactions are absorption and emission by clouds and molecules. This results in cloud side and cloud top cooling and in modest warming of the cloud bottom. Both effects are supposed to have an impact on cloud formation. To our knowledge, cloud side effects are neglected in all current cloud-resolving models where radiation is generally calculated with the independent column approximation. For the accurate calculation of these heating and cooling rates, fast 3-dimensional radiative transfer models are necessary. We developed a fast and accurate Monte Carlo Method, including a variance reduction technique, based on an optimized distribution of photon emission locations in the cloud for the calculation of thermal heating rates in a 3D environment. The method uses backward tracing of photons and combines two ways of calculating heating rates. The method has been implemented into MYSTIC (Monte Carlo code for the phYSically correct Tracing of photons In Cloudy atmospheres). While this model can provide “true” 3D heating and cooling rates, it is still computationally far too expensive to study the influence of radiation on clouds in a cloud-resolving model. Therefore, based on results from case studies with MYSTIC, a parametrization is developed which allows a fast and accurate calculation of thermal heating rates in a 3D world.

A simple example and an application on a real 3D cloud field will be shown, both calculated with the accurate Monte Carlo Method and the parametrization respectively. Additionally, the effects of the thermal heating rates as a possible feedback on cloud formation will be addressed.