Coupling Atmospheric, Ecologic, and Hydrologic Processes in a Regional Climate Model

Land surface characteristics play a key role in partitioning energy received at the Earth's surface. Vegetation, through its transpiration and evaporation, modifies atmospheric and land surface hydrological processes. Both observational and modeling studies have shown that two-way atmosphere and biosphere interactions are very important components of atmospheric and ecosystem dynamics.

A coupled RAMS/CENTURY modeling system has been developed to study regional-scale two-way interactions between the atmosphere and biosphere. Both atmospheric forcings and ecological parameters (LAI, etc.) are prognostic variables in the linked system. The atmospheric and ecosystem models exchange information on a weekly time step. CENTURY receives as input: air temperature, precipitation, radiation, wind speed, and relative humidity simulated by RAMS. From CENTURY-produced outputs, variables including leaf area index, vegetation transmissivity, vegetation fractional coverage, displacement height, roughness length, rooting profile, and albedo can be computed and returned to RAMS. In this way, vegetation responds to weekly and seasonal atmospheric changes are simulated and fed back to the atmospheric/land-surface hydrology model.

The coupled model has been used to simulate the two-way interactive biosphere and atmosphere feedbacks from January 1 through December 31, for 1988, 1989, and 1993, which represent dry, average and wet years, respectively, focusing on the central United States. Validation is performed for the atmospheric portion of the model by comparing with over 3,800 meteorological-station observations over the entire domain, and for the ecological component by comparing to AVHRR remote-sensing NDVI data sets.

A series of sensitivity experiments has been conducted to highlight interactions and feedbacks between atmospheric and land surface processes. The coupled control run's atmospheric lateral boundary conditions have been perturbed to create both dry and wet springs. The model's ability to represent the interannual and seasonal variations in both climate and biomass has been examined. The results show that seasonal and interannual vegetation phenological variation strongly influences regional climate patterns through its control over land-surface water and energy exchange. The coupled model captures the key aspects of weekly, seasonal, and annual feedbacks between the atmosphere and ecological systems. In addition, it has demonstrated its usefulness as a research tool for studying complex interactions between the atmosphere, biosphere, and hydrosphere.