How individual clouds contrbute to the cloud overlap of fields of shallow cumuli

Abstract:

To represent the radiative impact of cloud fields in large scale models, an adequate parameterization of the cloud overlap is necessary. With the ever increasing resolution of weather and climate models, it becomes important to treat the cloud overlap parameterization seperately for different cloud regimes, and in the near future even consider the cloud overlap on a cloud-by-cloud basis. This study continues the work of Neggers et al (JGR, 2011) who used large-eddy simulations to find that the cloud overlap in warm cumulus cloud fields is much more inefficient than previously results for more generic cloud fields found. In this study, we attempt to create some physical understanding of the cloud overlap by looking at the cloud overlap of individual clouds, and comparing that to the entire cloud field. While the overlap ratio goes to zero for the cloud field, an asymptotic limit can be found when looking at individual clouds. More over, preliminary results suggest that a data collapse can be observed in the cloud overlap of individual clouds, while the overlap of the entire field changes significantly between different scenes in a field of slowly organizing cumulus clouds. The individual overlap can then be split out between contributions of the large-scale shear and of the fractal shape of the cloud. This work suggests that with a good knowledge of the turbulent state, the mean wind shear, and the cloud size distribution, a physical understanding of the cloud overlap can be found, leading to an improved basis for parameterizations.