CLM 3 . 5 Documentation

The circulation of water through the Earth system is of critical importance to life on Earth. The hydrological cycle is also intimately linked to the energy cycle and to biogeochemical processes including the carbon cycle. Simulating the various processes that interact to form the hydrological cycle is a daunting task for climate models. In particular, over land, interactions between precipitation and the vegetation/soil system determine the partitioning of water into various storage reservoirs and the subsequent release of water vapor to the atmosphere. Successful simulation of these interactions by the land surface component of a climate model requires detailed representation of processes such as interception, throughfall, canopy drip, snow accumulation and ablation, infiltration, surface and sub-surface runoff, soil moisture, and the partitioning of evapotranspiration between canopy evaporation, transpiration, and soil evaporation. Depending on the capabilities of the model, the water cycle components may interact with and affect the simulation of biogeochemical processes such as the carbon and nitrogen cycle, dust and trace gas emissions, water and carbon isotopes, and vegetation dynamics. The Community Land Model version 3 (CLM3) is a computer model that represents land surface processes within the context of global climate simulation (Oleson et al. 2004). Dickinson et al. (2006) described the climate statistics of CLM3 when coupled to the Community Climate System Model (CCSM3) (Collins et al. 2006). Hack et al. (2006) provided an analysis of selected features of the land hydrological cycle. Bonan and Levis (2006) evaluated global plant biogeography and net primary production from CLM3 when coupled to a dynamic global vegetation model (DGVM). Lawrence et al. (2007) examined the impact of changes in CLM3 hydrological parameterizations on partitioning of evapotranspiration (ET) and its effect on the 1 timescales of ET response to precipitation events, interseasonal soil moisture storage, soil moisture memory, and land-atmosphere coupling. Although the simulation of land surface climate by CLM3 is in many ways adequate (Dickinson et al. 2006), many of the more unsatisfactory aspects of the simulated climate described in these studies can be traced directly to a deficient simulation of the hydrological cycle. A poor simulation of the hydrological cycle in the Amazon basin is indicative of the hydrologic deficiencies in CLM3. Here, the simulated present-day climate is biased warm and dry with lower runoff than observed (Dickinson et al. 2006). In part this is due to insufficient precipitation from the atmospheric model but is exacerbated by unrealistic partitioning …