Closing the Global Water Cycle in Fully-Coupled Climate System Models: Terrestrial Hydrology and River Transport for the NCAR CSM Land Component

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Abstract ATM-9617980 ATM-9707953 ATM-Supplement UCAR CA Famiglietti, James Vorosmarty, Charles Schimel, David University of Texas University of National Center for New Hampshire Atmospheric Research Closing the Global Water Cycle in Fully-Coupled Climate System Models: Terrestrial Hydrology and River Transport for the NCAR CSM Land Component The global hydrological cycle plays a central role in the interactive functioning of the Earth's climate. However, the current representation of the hydrological cycle in GCMs is inadequate to properly characterize its important role and interactions within the climate system. A critical shortcoming is that the global water cycle is not closed so that the land and oceans remain uncoupled, i.e. river runoff from the continents is not added as an input to the oceans. The purpose of this research is to close the global hydrological cycle in the CSM under development at NCAR by incorporating horizontal transport of water across the continents (i.e river transport) into its land component. A grid-scale land surface parameterization will be coupled to algorithms for subgrid-scale runoff generation and an explicit representation of global river networks to accurately route river flow across the continents to the oceans. We refer to this coupled land parameterization - subgrid algorithm - river transport scheme as a Global Land Hydrology Model (GLHM). The board scope of this work includes the development and testing of a GLHM which comprehensively models the cycling of water over and through the continental and surfaces; coupling this model to the CSM to link the land and the oceans; and using the CSM to improve our predictive understanding of the role of the global hydrological cycle in the Earth's climate system. Specific near-term objectives are to: develop a prototype version of the GLHM from existing or simplified model components; test the ability of the prototype GLHM to reproduce observed continental hydroclimatology; and to perform initial coupled model experiments to determine the effect of closing the global water cycle on CSM simulations. Specific contributions of this work to the NCAR (or any other) CSM effort include: conservation of fresh water balance simulation due to water cycle closure; computation of physically realistic river hydrographs along continental margins for input into ocean, sea ice, and later, biogeochemical transport models; the ability to simulate subgrid soil moisture distributions and subgrid fractions of inundated floodplain, both of which are intimately linked to the generation of grid-scale water, energy, and biogeochemical fluxes; an improved ability to validate CSM performance since streamflow is the most observable and well documented of the land surface fluxes; and an improved framework for understanding the global water cycle and its complex interactions within the Earth's climate system. This is the first attempt at a coordinated effort to build a state- of-the-science GLHM that explicitly resolves the vertical and horizontal circulation of terrestrial water for the specific purpose of closing the global water cycle in CSMs.