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

1. 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

2. 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

3. 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

4. NASA GISS_E http://www.giss.nasa.gov /research/modeling/

5. 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

6. USA: 5; France: 2; Japan: 2; UK: 2; Australia: 1; Canada: 1; China:1; Germany: 1; Russia: 1

7. Linear Change of Surface Temp Annual Mean 1950-1999 HadCRU (#17); NCEP CAMS (#18); NOAA extended SST (#19)

8. Linear Change of Precip Annual Mean 1979-1999 GPCP (#17)

9. Linear Trend of Surface Temp (1950-99) and Precip (1979-99) Obs 16 AR4 Model EnsMean

10. 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

11. Trend_Model*4 GFDL CM2.0 NASA GISS ER MIROC3.2 hires NCAR CCSM3 UKMO HadCM3 GPCP

12. Linear Change of Total Cloud Cover Models (1950-1999) vs ISCCP/4 (1984-1999) ISCCP (#17)

13. Clim and Linear Trend of 3-D Cloud in GFDL CM2.0 [1000mb-10mb] [30S-30N] [0-360E; 30S-30N]

14. Clim Linear Trend

15. Linear Change over 1950-1999 500mb Upward motion enhances OLR at TOA Chen et al (2002); Wielicki et al (2002) OLR reduces

16. 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

17. 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

18. Future • Improvement in representing physical processes associated with clouds and their interaction with radiation in the CGCMs • Observational data • Quality • Long term