Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 10 8 2010 10.5194/acpd-10-18467-2010 http://www.atmos-chem-phys-discuss.net/10/18467/2010/ http://www.atmos-chem-phys-discuss.net/10/18467/2010/acpd-10-18467-2010.html http://www.atmos-chem-phys-discuss.net/10/18467/2010/acpd-10-18467-2010.pdf 18467 18505 2010-08-04 Simulation of low clouds in the Southeast Pacific by the NCEP GFS: sensitivity to vertical mixing R. Sun ruiyu.sun@noaa.gov S. Moorthi H. Xiao C.-R. Mechoso UCAR/Environ. Modeling Center/NCEP/NOAA, 5200 Auth Rd., Camp Springs, MD, 20746, USA Environment Modeling Center/NCEP/NOAA , 5200 Auth Rd., Camp Springs, MD, 20746, USA Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, California, USA 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. 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