Atmospheric hydrological cycle in the tropics in twentieth century coupled climate simulations
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Atmospheric Hydrological Cycle in the Tropics in Twentieth Century Coupled Climate Simulations. Hailan Wang and William Lau Laboratory for Atmospheres, NASA/GSFC. Climate Model Evaluation Project (CMEP). 30 th Climate Diagnostics and Prediction Workshop October 26, 2005. Motivation.

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Atmospheric hydrological cycle in the tropics in twentieth century coupled climate simulations

Atmospheric Hydrological Cycle in the Tropics in Twentieth Century Coupled Climate Simulations

Hailan Wang and William Lau

Laboratory for Atmospheres, NASA/GSFC

Climate Model Evaluation Project (CMEP)

30th Climate Diagnostics and Prediction Workshop

October 26, 2005


Motivation
Motivation

  • Identify and understand long-term change of tropical hydrological cycle in 20th Century climate simulations by Coupled GCMs

    • Precipitation

    • Clouds

  • Provide input for IPCC AR4 in 2007


  • Coupled GCMs

    • State-of-the-art

    • Fully coupled

      • Time signature differs

    • Driven by time-varying external climate forcings

      • No agreed-upon forcing functions

      • The diversity in external forcing in the CGCMs is regarded as a measure of forcing uncertainties


    NASA GISS_E

    http://www.giss.nasa.gov

    /research/modeling/


    Coupled GCMs (Cont’d)

    • 16 CGCMs analyzed

    • 1 run of each CGCM used

      • Monthly mean fields

      • 1900-1999

    • Linear trend (actual linear change over time period concerned)

      • Models: 1950-1999

      • Observations

        • 1979-1999 for GPCP precip

        • 1984-1999 for ISCCP clouds


    USA: 5; France: 2; Japan: 2; UK: 2;

    Australia: 1; Canada: 1; China:1; Germany: 1; Russia: 1


    Linear Change of Surface Temp

    Annual Mean 1950-1999

    HadCRU (#17); NCEP CAMS (#18);

    NOAA extended SST (#19)


    Linear Change of Precip

    Annual Mean 1979-1999

    GPCP (#17)


    Linear Trend of

    Surface Temp (1950-99) and Precip (1979-99)

    Obs

    16 AR4 Model EnsMean


    Distribution of GPCP Rain as a function of Rain Rate

    Annual Mean 1979-1999; Tropical Ocean

    Clim

    Light: <1mm/day

    Medium: 2-8mm/day

    Heavy: >9mm/day

    Trend

    Rain Rate


    Trend_Model*4

    GFDL CM2.0

    NASA GISS ER

    MIROC3.2 hires

    NCAR CCSM3

    UKMO HadCM3

    GPCP


    Linear Change of Total Cloud Cover

    Models (1950-1999) vs ISCCP/4 (1984-1999)

    ISCCP (#17)


    Clim and Linear Trend of 3-D Cloud in GFDL CM2.0

    [1000mb-10mb]

    [30S-30N]

    [0-360E; 30S-30N]


    Clim

    Linear Trend


    Linear Change over 1950-1999

    500mb

    Upward motion enhances

    OLR at TOA

    Chen et al (2002);

    Wielicki et al (2002)

    OLR reduces


    OLR

    Tropopause

    Surface Evaporation

    Cool Climate

    Ocean surface

    Tropical Ocean

    OLR

    Less OLR

    Tropopause

    More cold and bright high cloud at tropopause and lower stratosphere

    Less high cloud

    Intensified

    deep convection

    Strengthened

    updraft

    Less mid-to-low cloud

    Increased

    light rain

    Enhanced

    heavy rain

    Greatly reduced

    moderate rain

    Enhanced Surface Evaporation

    Warm Climate

    Ocean surface

    Warmer Tropical Ocean


    Conclusions
    Conclusions

    • CGCMs are reasonably consistent in depicting aspects of long term changes in the 20th Century climate and the tropical hydrological cycle:

      • Surface warming over tropical ocean and land

      • Increasing/decreasing precipitation over tropical ocean/land

      • Increasing heavy and light rain, but decreasing moderate rain

      • A reduction in total cloud cover in the tropics.

    • CGCMs significantly underestimate the magnitudes of the observations, by a factor of at least 4.

      • This likely leads to gross errors in model simulation of tropical radiative fluxes

    • Difference between CGCMs and observations

      • Time scale

      • CGCMs: certain processes may be missing

      • Obs: e.g. problems in retrieving high level thin clouds


    Future
    Future

    • Improvement in representing physical processes associated with clouds and their interaction with radiation in the CGCMs

    • Observational data

      • Quality

      • Long term


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