Atmos. Chem. Phys. Discuss., 10, 30971-30998, 2010
© Author(s) 2010. This work is distributed
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Vertical profiles of droplet effective radius in shallow convective clouds
S. Zhang1, H. Xue1, and G. Feingold2
1Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing, China
2NOAA Earth System Research Laboratory, Boulder, CO, USA

Abstract. Vertical profiles of droplet effective radius (re) in shallow convective clouds are investigated using results from large-eddy simulations (LES) for clean (aerosol mixing ratio Na=25 mg−1), intermediate (Na=100 mg−1), and polluted (Na=2000 mg−1) conditions. Cloud-top re for cloud populations comprising clouds with different heights and at different stages of their development are used to construct a vertical profile of re. For the polluted and intermediate cases where precipitation is negligible, the constructed re profiles represent the in-cloud re profiles fairly well with a low bias (about 10%). For the clean, drizzling case the in-cloud re can be very large and highly variable and profiling based on cloud-top re is less useful. The differences in re profiles between clean and polluted conditions derived in this manner are however, distinct. The subadiabatic characteristics of the simulated cumulus clouds are investigated to reveal the effect of mixing on re and its evolution. Results indicate that as polluted and moderately polluted clouds develop into their decaying stage, the subadiabatic fraction fad becomes smaller, representing a higher degree of mixing, and re becomes smaller (~10%) and more variable. However, for the clean case, smaller fad corresponds to larger re (and larger re variability), reflecting the additional influence of droplet collision-coalescence and sedimentation on re. Profiles of the vertically inhomogeneous clouds as simulated from the LES and those of the vertically homogeneous clouds are used as input to a radiative transfer model to study the effect of cloud vertical inhomogeneity on shortwave radiative forcing. For clouds that have the same liquid water path (LWP), re of a vertically homogeneous cloud must be about 76–90% of the cloud-top re of the vertically inhomogeneous cloud in order for the two clouds to have the same shortwave radiative forcing.

Citation: Zhang, S., Xue, H., and Feingold, G.: Vertical profiles of droplet effective radius in shallow convective clouds, Atmos. Chem. Phys. Discuss., 10, 30971-30998, doi:10.5194/acpd-10-30971-2010, 2010.
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