Influence of Three Dimensional Thermal Radiation on Cloud Droplet Growth

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Solar and thermal radiation drive weather and climate and strongly affect cloud formation. However, 3D radiation effects on clouds are still poorly understood and have not been investigated systematically. Due to the complexity and the computational costs of accurate 3D radiative transport, radiation is often neglected in today's cloud resolving models or, at best, treated using a plane-parallel 1D approximation which neglects horizontal radiation transport. A better understanding of the physical processes taking place in clouds will improve cloud parameterizations in numerical weather prediction and climate models and thus improve weather and climate prediction. A fast method to calculate 3D thermal radiative transfer in cloud resolving models was developed by the proposer. This method allowed for the first time to study 3D radiation effects on cloud dynamics and microphysics in detail. First studies of the effects of 3D thermal radiation on cloud dynamics were made within the PhD thesis of the proposer. The next consequent step is to investigate the effect of 3D thermal radiation on cloud microphysics which is the aim of the proposed study. Radiative cooling at cloud sides and cloud top could speed up droplet growth significantly. The cloud side cooling is a 3D effect which cannot be accounted for with 1D radiative transfer approximations. It is possible that the radiative effect on droplet growth could close the existing gap in droplet growth theory: Diffusional droplet growth slows down considerably at about 10 micron droplet size. However, the following process of collision and coalescence only takes place when droplets have a minimum size of 20 micron. For the proposed study, a cloud particle size resolving microphysical model (bin microphysics model) is needed. The hosting group (at NOAA, Boulder) has developed such a model (TAU Cloud Microphysical Code) and has been working with it for many years. For the proposed study, the above mentioned 3D thermal radiative transfer solver will be implemented into a cloud resolving model including bin microphysics (e.g. System of Atmospheric Modeling (SAM), used by the hosting group). In a second step, simulations will be setup and performed to address the questions how 3D thermal radiation affects cloud droplet growth or the development of precipitation. Finally, the simulation results will be evaluated and the effects of 3D radiation in comparison to simulations with no-radiation or 1D-radiation have to be investigated.