Simulating and quantifying land-surface biogeochemical, hydrological, and biogeophysical processes using the Community Land Model version 4

Shi, Mingjie
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Carbon and nitrogen cycles, the energy cycle, and the hydrological cycle interact with each other; all are crucial to atmosphere–land studies. Carbon and nitrogen cycle from the atmosphere to vegetation communities and soil micro-organisms through their transformation in inorganic and organic pools. Ecosystem equilibrium, which is usually disturbed by extreme events (e.g., fires or drought), depends on the speeds of carbon and nitrogen uptake and decomposition. Terrestrial biogeochemistry models typically require hundreds to thousands of years for carbon and nitrogen in various pools to reach steady-state solutions, which are generally a function of soil temperature and soil water. Hydrological processes such as the root transpiration/water removal and the cold-region infiltration with the soil ice freeze/thaw status involved affect soil water content and soil temperature, and regulate carbon- and nitrogen-stock variations. Last but not least, mineral dust, a type of atmospheric aerosol, alters surface radiation/energy balance, and may act as cloud condensation nuclei to modify precipitation rates and eventually the hydrological cycle. Therefore, we were motivated to investigate these processes in different ecosystems. Specifically, this research aims to 1) to elucidate the carbon- and nitrogen-pool adjustment processes in different ecosystems, 2) to evaluate how the root transpiration process affects ecosystem carbon exchange patterns in Amazonia, 3) to analyze the influence of soil impermeability, which is affected by the landscape freeze/thaw status in cold regions, on hydrological cycles at high latitudes, and 4) to explore the effects of surface vegetation distribution and model resolution on surface dust emissions. The Community Land Model version 4 (CLM4) was used in this study. We did numerical experiments in three environments: forest and grassland ecosystems, river basins in cold regions, and the Arabian Peninsula. Our main scientific findings are: 1) the adjustment time of the biogeochemistry components in CLM4 is longer for boreal forests than for other ecosystems, 2) with more water is lifted from deep soil, Amazonia ecosystems start to take up carbon during dry seasons, 3) the timing of boreal spring runoff simulations is improved by reducing the impermeable area underneath the snowpack, and 4) model-simulated dust emissions increase with model resolution as a result of the heterogeneities of vegetation cover and wind speed.