Abstract
Abstract. We use a 1-D cloud model with explicit microphysics and a binned representation of the aerosol size distribution to investigate the influence of entrainment of cloud condensation nuclei (CCN) on the microphysical development of warm cumulus clouds. For a more realistic representation of cloud drop spectral width, the model separates droplets that grow on aerosol that is initially present in the cloud from droplets growing on entrained aerosol. Model results are compared with observations of trade wind cumulus microphysics from the Rain in Cumulus over the Ocean experiment (RICO, 2004–2005). The results indicate that CCN are entrained throughout the entire cloud depth, and inside the cloud part of these may be activated. Compared to a simulation where entrainment of ambient CCN is neglected this leads to higher cloud droplet number concentrations (CDNC) and a continuous presence of droplets in the range smaller than ~5 μm that is consistent with the observations. Cloud dynamics are sensitive to the entrainment parameter as well as to the applied initial vertical velocity, as expressed by the liquid water content and cloud top height. However, simulated cloud drop spectra remain relatively unaffected for the specific conditions during RICO.
Highlights
Anthropogenic emissions of primary aerosol particles and aerosol precursors have increased atmospheric aerosol concentrations substantially since preindustrial times (e.g., Charlson et al, 1992; Solomon et al, 2007)
A model study of cloud microphysics requires a realistic representation of both cloud drop number concentration (CDNC) and the drop size distribution
We remark that the observed horizontal variability of relative humidity (RH) ranges up to ten percent, indicating a significant spatial variability of the atmospheric conditions under which the cumulus clouds are formed
Summary
The cloud drop number concentration (CDNC) is a crucial parameter for both effects (Lohmann and Feichter, 2005), because it is associated with the average droplet size. A model study of cloud microphysics requires a realistic representation of both CDNC and the drop size distribution. The 3-D Model with binmicrophysics presented by Leroy et al (2007, 2009) uses a time resolution in the order of second(s) Such relatively low time resolution may result in inaccurate simulation of microphysical processes, such as condensation and droplet activation, since their time scales are in the order of 0.1 s. It is not our aim to study the process of mixing in cumuli itself, but the influence of entrainment on cloud dynamical and microphysical parameters, especially the droplet spectra. We use a one-dimensional (1-D) cloud column model that contains a binned representation of aerosol It simulates aerosol activation, condensational growth and evaporation based on Kohler microphysics.
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