Air recycling in boundary layer clouds and its effect on cloud microstructure

We investigate cloud residence times of air parcels based on LES simulation of stratocumulus boundary layer clouds. The model used in the study is the CIMMS LES explicit (size-resolving) microphysical model (Khairoutdinov and Kogan 1999); the experiment was initialized with data obtained during ASTEX flight A209 on 12-13 June 1992. The results show that cloud residence times are significantly larger than the cloud eddy turnover time. For all cloud levels about 70% of air parcels reside in clouds for more than one cloud eddy turnover time, about 50% have been cycling in the cloud for 2-5 times. The results demonstrate significant inhomogeneity of air parcels residence time; as a result old and new air parcels frequently coexist in close proximity on a variety of scales. The implications of recycling for cloud microstructure formation are analyzed and discussed. It is hypothesized that, for instance, the drop spectral broadening may be the result of mixing of parcels with different histories, i.e., representing drop size distributions at different stages of their evolution. In this sense we distinguish between fine-scale turbulence that affects local parameters in an air parcel and large-scale turbulence that affects air parcel history.