Marine Stratus and Its Relationship to Regional and Large-Scale Circulations: An Examination with the NCEP CFS Simulations P. Xie 1), W. Wang 1), W. Higgins 1), and P.A. Arkin 2) 1) NOAA Climate Prediction Center 2) ESSIC, Univ. of Maryland

Objectives:  To examine the CFS model bias associated with the insufficiently simulated marine stratus clouds over SE Pacific and SE Atlantic;  To investigate the atmospheric circulation involving the formation and variations of the marine stratus clouds;

Two Sets of CFS Simulations Examined  CFS AMIP Simulations [GFS] The atmospheric component of the CFS model (GFS03) is forced by observed oceanic condition; 1979 – 2003;  CFS CMIP Simulations [CFS] The GFS03 atmospheric model is fully coupled with an OGCM (MOM3) ; 4 sets of CMIP simulations for a 32-year period each

Annual Mean Precipitation  Large-scale precipitation patterns reproduced reasonably well;  Differences exist in the magnitude of precipitation and in the latitudinal position of the ITCZ;

Annual Cycle of the Atlantic ITCZ  GFS simulates annual cycle pretty well, with excessive precipitation;  The Atlantic ITCZ located too south during DJF and MAM in the CFS simulation;

Annual Cycle of E. Pacific ITCZ  Relatively good reproduction of precipitation fields by GFS;  ITCZ too strong and located a little bit too south during DJF and MAM in the CFS;

Annual Mean SST / Surf. Wind  Warm SST bias in the CFS over the southeast Atlantic and southeast Pacific;  Surface wind bias over the regions of warm SST bias.

Possible Causes for the Warm SST Bias over the Regions 1.Insufficient vertical transportation  oceanic observations (will not check this time) 2.Enhanced heat transport through advection  surface wind speed 3.Reduced evaporation  Surface wind speed 4.Excessive incoming solar radiation  cloudiness

Annual Mean Total Cloud (%)  In general, CFS cloud amount is smaller than that of observations almost everywhere;  Insufficient amount of clouds simulated by the CFS over the regions with warm SST bias;

Cloudiness in other NCEP Products  Only CDAS1 reproduced the cloudiness (mostly low clouds) reasonably well;  CDAS2, GFS and CFS failed to generate cloud amounts over the regions;

Annual Mean SW Radiation  Excessive incoming solar radiation over both the SE Atlantic and SE Pacific;  Differences of over 50W/m 2 over SE Pacific;

Comparison with EPIC Buoy Data at [20 o S,85 o W]  Close agreements between buoy and satellite observations;  Warm SST bias of ~2 o C and positive SW radiation of ~50W/m 2 in CFS compared to the EPIC buoy observations;

Cloud Picture over SE Pacific [from fig.3 of Bretherton et al. (2004)] Scattered stratus clouds over the regions;

Vertical Profiles of Boundary Layer [from fig.10 of Bretherton et al. (2004)] Well mixed boundary layer of km capped by inversion; Stratus cloud layer of ~500m atop the boundary layer; Liquid WaterWater VaporTemperature

Diurnal Cycle in ISCCP cloudiness  24-hour mean cloudiness (top) shows a bi-polar structure over the dry zone and nearby continent, suggesting a diurnal cycle cell caused by sea breeze;  3-hourly ISCCP cloud data (bottom) presents distinct diurnal cycles of different phases over the dry zone and the land area;

Diurnal Cycle in CDAS1 Cloudiness

Diurnal Cycle in circulation from CDAS1  24-hourly mean removed to examine the diurnal cycle;  Cells of regional circulations involving land-sea contrasts; 06Z

Summary  Overall good performance of CFS model in reproducing large-scale precipitation patterns;  Merdional shifts of ITCZ over Eastern Pacific and Atlantic sectors;  The displacements of the ITCZ closely related to the warm SST bias in the SE Atlantic and SE Pacific stratus deck regions;  The warm SST bias caused largely by insufficiently simulated stratus clouds; and  Strong diurnal cycle in the stratus clouds generated by regional circulation caused by land-sea contrasts between the oceanic regions and their adjacent continents.

Annual Mean Surface Wind  Small differences in surface wind speed between the CFS model and observations (QuikScatter) Over the SE Atlantic and SE Pacific;