ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-10-12261-2010 Simulation of low clouds in the Southeast Pacific by the NCEP GFS: sensitivity to vertical mixing Sun R. 1 Moorthi S. 2 Xiao H. 3 Mechoso C. R. 3 UCAR/Environmental Modeling Center/NCEP/NOAA, Rm 209, 5200 Auth Rd., Camp Springs, MD, 20746, USA Environmental Modeling Center/NCEP/NOAA, Rm 209, 5200 Auth Rd., Camp Springs, MD, 20746, USA Department of Atmospheric and Oceanic Sciences University of California, Los Angeles, Los Angeles, California, USA 23 12 2010 10 24 12261 12272 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/10/12261/2010/acp-10-12261-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/12261/2010/acp-10-12261-2010.pdf

The NCEP Global Forecast System (GFS) model has an important systematic error shared by many other models: stratocumuli are missed over the subtropical eastern oceans. It is shown that this error can be alleviated in the GFS by introducing a consideration of the low-level inversion and making two modifications in the model's representation of vertical mixing. The modifications consist of (a) the elimination of background vertical diffusion above the inversion and (b) the incorporation of a stability parameter based on the cloud-top entrainment instability (CTEI) criterion, which limits the strength of shallow convective mixing across the inversion. A control simulation and three experiments are performed in order to examine both the individual and combined effects of modifications on the generation of the stratocumulus clouds. Individually, both modifications result in enhanced cloudiness in the Southeast Pacific (SEP) region, although the cloudiness is still low compared to the ISCCP climatology. If the modifications are applied together, however, the total cloudiness produced in the southeast Pacific has realistic values. This nonlinearity arises as the effects of both modifications reinforce each other in reducing the leakage of moisture across the inversion. Increased moisture trapped below the inversion than in the control run without modifications leads to an increase in cloud amount and cloud-top radiative cooling. Then a positive feedback due to enhanced turbulent mixing in the planetary boundary layer by cloud-top radiative cooling leads to and maintains the stratocumulus cover. Although the amount of total cloudiness obtained with both modifications has realistic values, the relative contributions of low, middle, and high layers tend to differ from the observations. These results demonstrate that it is possible to simulate realistic marine boundary clouds in large-scale models by implementing direct and physically based improvements in the model parameterizations.

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