Abstract
Abstract. This paper discusses aircraft observations and large-eddy simulation (LES) modeling of 15 May 2008, North Sea boundary-layer clouds from the EUCAARI-IMPACT field campaign. These clouds are advected from the northeast by the prevailing lower-tropospheric winds and featured stratocumulus-over-cumulus cloud formations. An almost-solid stratocumulus deck in the upper part of the relatively deep, weakly decoupled marine boundary layer overlays a field of small cumuli. The two cloud formations have distinct microphysical characteristics that are in general agreement with numerous past observations of strongly diluted shallow cumuli on one hand and solid marine stratocumulus on the other. Based on the available observations, a LES model setup is developed and applied in simulations using a novel LES model. The model features a double-moment warm-rain bulk microphysics scheme combined with a sophisticated subgrid-scale scheme allowing local prediction of the homogeneity of the subgrid-scale turbulent mixing. The homogeneity depends on the characteristic time scales for the droplet evaporation and for the turbulent homogenization. In the model, these scales are derived locally based on the subgrid-scale turbulent kinetic energy, spatial scale of cloudy filaments, mean cloud droplet radius, and humidity of the cloud-free air entrained into a cloud, all predicted by the LES model. The model reproduces contrasting macrophysical and microphysical characteristics of the cumulus and stratocumulus cloud layers. Simulated subgrid-scale turbulent mixing within the cumulus layer and near the stratocumulus top is on average quite inhomogeneous, but varies significantly depending on the local conditions.
Highlights
Diluted shallow cumuli on one hand and solid marine stra- the extremely inhomogeneous mixing takes place, the droplet tocumulus on the other
The model features a double-moment warm-rain geneity of mixing has been argued to depend on the relative bulk microphysics scheme combined with a sophisticated magnitude of the time scales for droplet evaporation and for subgrid-scale scheme allowing local prediction of the ho- turbulent homogenizatioHny(BdarkoerloagndyLaatnhadm, 1979; Baker mogeneity of the subgrid-scale turbulent mixing
Et al, 2007; 1A9n8d0r;eJjceznusekneatnadl.,BE2a0ka0err9,t;h1L9e8Sh9m;yBasnutnrenemett aaln.,d2B0r0e9n)g.uHieor, mogeneous mixing takes place Swhceinetnhecteursbulent homoge-. These scales are derived locally based on the nization time scale is much smaller than the droplet evaposubgrid-scale turbulent kinetic energy, spatial scale of cloudy ration time scale because all droplets are exposed to the filaments, mean cloud droplet radius, and humidity of the same conditions during evaporation
Summary
The IMPACT (Intensive Observation Period At Cabauw Tower) field campaign was a part of the EUCAARI (European Integrated Project on Aerosol Cloud Climate and Air Quality Interactions; Kulmala et al, 2011) project funded under the EU Framework Programme 6. These observations will guide the discussion of microphysical effects resulting from the turbulent entrainment in simulated cumulus and stratocumulus clouds. The data for the cumuli (Fig. 4) show a wide range of droplet concentrations and relatively small values of the cloud water mixing ratio. The delay of cloud water evaporation during turbulent mixing is facilitated by predicting two additional model variables, the characteristic scale (width) λ of cloud filaments and the fraction of gridbox volume occupied by the cloudy air β. Qc∗ is applied to the cloud water mixing ratio, and the droplet concentration is reduced depending on the homogeneity of the subgrid-scale mixing, that is, through the parameter α (see Appendix A).
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