Marine Stratus and Its Relationship to Regional and LargeScale Circulations: An Examination with the

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Marine Stratus and Its Relationship to Regional
and Large-Scale CirculationsAn 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)
Liquid Water
Water Vapor
Temperature

• Well mixed boundary layer of 1-1.5 km capped by
  inversion
• Stratus cloud layer of 500m atop the boundary
  layer

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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 CDAS1 Cloudiness
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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