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Strong diurnal cycle in the stratus clouds generated by regional circulation caused by land-sea contrasts between the oceanic regions and their adjacent continents. ...

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Marine Stratus and Its Relationship to Regional and Large-Scale Circulations:An Examination with the NCEP CFS SimulationsP. Xie1), W. Wang1), W. Higgins1), and P.A. Arkin2)1) NOAA Climate Prediction Center2) ESSIC, Univ. of Maryland


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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;


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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


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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;


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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;


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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;


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Annual MeanSST / 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.


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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


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Annual MeanTotal 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;


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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;


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Annual MeanSW Radiation

  • Excessive incoming solar radiation over both the SE Atlantic and SE Pacific;

  • Differences of over 50W/m2 over SE Pacific;


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Comparison with EPIC Buoy Data at [20oS,85oW]

  • Close agreements between buoy and satellite observations;

  • Warm SST bias of ~2oC and positive SW radiation of ~50W/m2 in CFS compared to the EPIC buoy observations;


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Cloud Picture over SE Pacific [from fig.3 of Bretherton et al. (2004)]

  • Scattered stratus clouds over the regions;


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Vertical Profiles of Boundary Layer [from fig.10 of Bretherton et al. (2004)]

  • Well mixed boundary layer of 1-1.5 km capped by inversion;

  • Stratus cloud layer of ~500m atop the boundary layer;

Liquid Water

Water Vapor

Temperature


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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;



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Diurnal Cycle in circulation from CDAS1

06Z

  • 24-hourly mean removed to examine the diurnal cycle;

  • Cells of regional circulations involving land-sea contrasts;


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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.


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Annual MeanSurface Wind

  • Small differences in surface wind speed between the CFS model and observations (QuikScatter) Over the SE Atlantic and SE Pacific;


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