Evaluation of Statistical Distributions for the Parametrization of Subgrid Boundary-Layer Clouds |
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Authors: | Emilie Perraud Fleur Couvreux Sylvie Malardel Christine Lac Valéry Masson Odile Thouron |
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Institution: | (1) Center for Climate System Research, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8568, Japan;(2) National Institute for Environmental Studies, Tsukuba, Japan;(3) Frontier Research Center for Global Change, JAMSTEC, Yokohama, Japan |
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Abstract: | In numerical weather prediction and climate models, planetary boundary-layer (PBL) clouds are linked to subgrid-scale processes
such as shallow convection. A comprehensive statistical analysis of large-eddy simulations (LES), obtained for warm PBL cloud
cases, is carried out in order to characterize the distributions of the horizontal subgrid cloud variability. The production
of subgrid clouds is mainly associated with the variability of the total water content. Nevertheless, in the case of PBL clouds,
the temperature variability cannot be completely discarded and the saturation deficit, which summarizes both temperature and
total water fluctuations, provides a better representation of the cloud variability than the total water content. The probability
density functions (PDFs) of LES saturation deficit generally have the shape of a main asymmetric bell-shaped curve with a
more or less distinct secondary maximum specific to each type of PBL clouds. Unimodal theoretical PDFs, even those with a
flexible skewness, are not sufficient to correctly fit the LES distributions, especially the long tail that appears for cumulus
clouds. They do not provide a unified approach for all cloud types. The cloud fraction and the mean cloud water content, diagnosed
from these unimodal PDFs, are largely underestimated. The use of a double Gaussian distribution allows correction of these
errors on cloud fields and provides a better estimation of the cloud-base and cloud-top heights. Eventually, insights for
the design of a subgrid statistical cloud scheme are provided, in particular a new formulation for the weight of the two Gaussian
distributions and for the standard deviation of the convective distribution. |
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