1. Land Surface Processes in Global Climate Models (2)

2. Study water cycle trend using the NCAR Community Land Model (CLM3) Question: What is the trend of water cycle in the last 50 years?

3. Model experiments  57 years (1948-2004) of CLM offline simulation  Resolution: T42 (~2.8º)  The forcing dataset: 3-hrly and T62 (~1.875°) Precipitation, surface air temperature, downward solar radiation, specific humidity, wind speed and air pressure Combined the intramonthly variations from the NCEP/NCAR 6-hrly reanalysis with monthly time series from station records (T, P, CLD)  The validation datasets include streamflow from 921 rivers around the world, continental freshwater discharge, surface runoff and soil moisture  57 years (1948-2004) of CLM offline simulation  Resolution: T42 (~2.8º)  The forcing dataset: 3-hrly and T62 (~1.875°) Precipitation, surface air temperature, downward solar radiation, specific humidity, wind speed and air pressure Combined the intramonthly variations from the NCEP/NCAR 6-hrly reanalysis with monthly time series from station records (T, P, CLD)  The validation datasets include streamflow from 921 rivers around the world, continental freshwater discharge, surface runoff and soil moisture Qian, Dai, Trenberth and Oleson (2006), J. Hydrometeor.

4. Simulated mean annual cycle for the world’s 10 largest rivers: validation using observation Observation NCEP unadjusted NCEP Adjusted Amazon Congo Orinoco Changjiang Bahmaputra Mississippi Yenisey Parana Lena Mekong

5. Simulated long-term mean streamflow for the world’s 200 largest rivers: Validation using observation

6. Simulated water-year annual mean streamflow for 12 rivers: validation against observation

7. Simulated long-term mean runoff: Validation against Fekete et al. data Qian, Dai, Trenberth and Oleson (2006), J. Hydrometeor.

8. Comparison of long-term mean freshwater discharge into the global oceans

9. Comparison of soil moisture: CLM3 vs measurements at Illinois, U.S.

10. Summary of global simulation using CLM3  The CLM3 reproduces many aspects of the long-term mean, annual cycle, interannual and decadal variations, and trends of streamflow for many large rivers (e.g., the Orinoco, Changjiang, Mississippi, etc.), although substantial biases exist.  The simulated long-term-mean freshwater discharge into the global and individual oceans is comparable to 921 river-based observational estimates.  Observed soil moisture variations over Illinois and parts of Eurasia are generally simulated well, with the dominant influence coming from precipitation.  It is also shown that unrealistically low intensity and high frequency of precipitation in original NCEP reanalysis result in too much evaporation and too little runoff, which leads to lower than observed river flows. This problem can be reduced by adjusting the precipitation rates using observed precipitation-frequency maps.  The results suggest that the CLM3 simulations are useful for climate change analysis.  The CLM3 reproduces many aspects of the long-term mean, annual cycle, interannual and decadal variations, and trends of streamflow for many large rivers (e.g., the Orinoco, Changjiang, Mississippi, etc.), although substantial biases exist.  The simulated long-term-mean freshwater discharge into the global and individual oceans is comparable to 921 river-based observational estimates.  Observed soil moisture variations over Illinois and parts of Eurasia are generally simulated well, with the dominant influence coming from precipitation.  It is also shown that unrealistically low intensity and high frequency of precipitation in original NCEP reanalysis result in too much evaporation and too little runoff, which leads to lower than observed river flows. This problem can be reduced by adjusting the precipitation rates using observed precipitation-frequency maps.  The results suggest that the CLM3 simulations are useful for climate change analysis.

11. The evaporation trend in the Mississippi River basin Previous studies: Incomplete observations lead to conflicting estimates of E trend (1) Energy balance: Cloud is increasing, Fsw is decreasing, Pan evaporation (E pan ) is decreasing Peterson et al. (1995, Nature): E is decreasing Brutsaert & Parlange (1998, Nature) and Golubev et al. (2001, GRL): E is negatively correlated with E pan, so E is increasing Roderick and Farquhar (2002, Science): E is negatively correlated with E pan only under specific conditions, not in general. So E is decreasing (2) Water balance: P is increasing, R is increasing, P-R is increasing Milly & Dunne (2001, GRL); Walter et al. (2004, J. Hydrometeor.): E is increasing Previous studies: Incomplete observations lead to conflicting estimates of E trend (1) Energy balance: Cloud is increasing, Fsw is decreasing, Pan evaporation (E pan ) is decreasing Peterson et al. (1995, Nature): E is decreasing Brutsaert & Parlange (1998, Nature) and Golubev et al. (2001, GRL): E is negatively correlated with E pan, so E is increasing Roderick and Farquhar (2002, Science): E is negatively correlated with E pan only under specific conditions, not in general. So E is decreasing (2) Water balance: P is increasing, R is increasing, P-R is increasing Milly & Dunne (2001, GRL); Walter et al. (2004, J. Hydrometeor.): E is increasing Water balance Energy balance F sw LHF lw SH E participates in two balances at the land surface dS/dt P E R G

12. Questions: Has ET increased or decreased? Can we satisfy BOTH the heat and water budgets? Our study: Complete evaluation of these two budgets using observation-constrained global land model simulations Has ET increased or decreased? Can we satisfy BOTH the heat and water budgets? Our study: Complete evaluation of these two budgets using observation-constrained global land model simulations Qian, Dai, and Trenberth (2007), J. Climate

13. Basin-averaged energy budget

14. Basin-averaged water budget

15. Model sensitivity experiments on factors affecting E trend: dominated by precipitation change

16. Precip Runoff Evap Storage Trend map of water budget Qian, Dai, and Trenberth (2007), J. Climate

17. SWLH LWSH Trend map of energy budget

18. F sw LHF lw SH dS/dt P E R G Both energy balance and water balance support that evaporation is increasing, and the water cycle is intensified during the last 50 years! Summary: Water cycle trend of Mississippi River basin Energy budgetWater budget