A New Reactive Transport Solver Integrated in CLM4 and CLM4.5 Allows More Mechanistic Treatment of Processes and Evaluation of Uncertainties
Recent developments in land biogeochemistry models include integrating more and more chemical species, yet we lack a generic model structure to consistently model these species and the range of biogeochemical processes in which they participate. To fill this infrastructure gap in the community earth system model (CESM), we developed the CLM4-BeTR model, a biogeochemical transport and reaction component for CLM4 (and CLM4.5), as a generic template for modeling of multi-tracer and multi-phase reactive transport.
Application of CLM4-BeTR to analyze carbon-nitrogen dynamics at the Harvard Forest site indicates CLM4-BeTR was able to accurately reproduce measured soil CO2 gas profiles. In our time series analysis of CO2 effluxes and soil respiration, we found the prevailing assumption held in the single layer biogeochemical models, that is “surface CO2 efflux equals soil respiration” did not hold at many temporal scales. The differences were larger ( 20%~20%) at hourly scales, smaller (-4%~4%) at daily scales, and persisted into monthly scales (<4%). In addition, we simulated episodic CO2 emissions due to freeze-thaw events. These emissions can be three to four times as high as peak growing season respiration. CLM4-BeTR is also able to simultaneously track the soil surface effluxes and soil profiles for trace gases produced from different biogeochemical processes (e.g., N2O). In an illustrative analysis for belowground CO2 production at Harvard Forest, we showed that autotrophic and heterotrophic respiration have distinct temporal and spatial patterns, indicating that measurement of soil CO2 gas profiles could provide useful constraints to the model structure and parameterization.
We expect that these developments, and their future extensions, will help constrain uncertainty in land biogeochemical processes in CESM, and enable better mechanistic assessment of climate-land-biogeochemistry feedbacks.
This research was supported by the Director, Office of Science, Office of Biological and Environmental Research of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 as part of their Regional and Global Climate Modeling Program. The authors appreciate Ms. Kathleen Savage and Dr. Eric Davidson at the Woods Hole Research Center for providing the soil CO2 profile data and soil moisture and temperature data at the Harvard Forest site.