Thermal Radiative Impact on Shallow Cumulus Clouds—Dynamical and Microphysical Insights

Radiation is a key driver for the development of clouds. Solar radiation heats the Earth’s surface and causes updrafts to rise, thus initiating cloud formation. When a cloud is formed, absorption and emission of thermal radiation by the cloud itself cause heating and cooling rates of several hundred K/d at the interface between cloud and cloudless sky. The magnitude of the cooling rates, compared to the commonly known clear sky cooling of 1-2 K/d, can alter cloud dynamics and microphysics and thus cloud development or lifetime.

We showed in a recent study that thermal radiation can change the development of a shallow cumulus cloud field. The application of radiative heating and cooling rates, either in 1D approximation, 3D solution, or even horizontally averaged has significant impact on the development of the morphology of a cloud field and on vertical velocities. While averaged thermal radiation causes a quasi-uniform distribution of the clouds in a modeling domain, 1D and 3D local heating and cooling rates lead to organization of the cloud field with different strength, causing cell-like structures and aggregation of clouds. Furthermore, it was shown that thermal radiation has the potential to lead to an earlier onset of rain formation, which again could change the dynamics and organization of the cloud field.

Following up on the above mentioned results, we take a step further by applying the thermal radiative heating and cooling rates not only to the dynamics of the LES system, but also coupling it directly to the microphysics. This allows us to investigate the thermal radiative effect on individual cloud droplet growth and the possible effect on rain formation.

In this talk we will demonstrate a) the impact of 1D and 3D thermal heating and cooling rates in an LES simulation of a non-precipitating shallow cumulus cloud field on the dynamics of the system and b) the impact of 1D and 3D thermal radiation on cloud droplet growth and rain formation.