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Land Surface Processes in Global Climate Models (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?. Model experiments. 57 years (1948-2004) of CLM offline simulation Resolution: T42 (~2.8º)
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Study water cycle trend using the NCAR Community Land Model (CLM3) Question: What is the trend of water cycle in the last 50 years?
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 Qian, Dai, Trenberth and Oleson (2006), J. Hydrometeor.
Simulated mean annual cycle for the world’s 10 largest rivers: validation using observation NCEP Adjusted NCEP unadjusted Observation Mississippi Amazon Yenisey Congo Parana Orinoco Changjiang Lena Bahmaputra Mekong
Simulated long-term mean streamflow for the world’s 200 largest rivers: Validation using observation
Simulated water-year annual mean streamflow for 12 rivers: validation against observation
Simulated long-term mean runoff: Validation against Fekete et al. data Qian, Dai, Trenberth and Oleson (2006), J. Hydrometeor.
Comparison of soil moisture: CLM3 vs measurements at Illinois, U.S.
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.
E participates in two balances at the land surface The evaporation trend in the Mississippi River basin Energy balance Water balance Fsw Flw LH SH P E Previous studies: Incomplete observations lead to conflicting estimates of E trend (1) Energy balance: Cloud is increasing, Fsw is decreasing, Pan evaporation (Epan) 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 Epan, so E is increasing Roderick and Farquhar (2002, Science): E is negatively correlated with Epan 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 G dS/dt R
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 Qian, Dai, and Trenberth (2007), J. Climate
Model sensitivity experiments on factors affecting E trend: dominated by precipitation change
Trend map of water budget Precip Runoff Evap Storage Qian, Dai, and Trenberth (2007), J. Climate
Trend map of energy budget SW LH LW SH
Summary: Water cycle trend of Mississippi River basin P E Flw LH SH Fsw dS/dt R G Energy budget Water budget Both energy balance and water balance support that evaporation is increasing, and the water cycle is intensified during the last 50 years!