Simulating Present and Future Water, Energy, and Carbon Dioxide Fluxes on a Regional Scale using the WRF/ACASA Coupled Model

Carbon sequestration from terrestrial systems in California is likely to change due to modifications in future climate conditions. Future increases in temperature and intensity of summer droughts in California are projected by GCMs. Moreover, the diminishing snow cover will release less water for plant growth during the snow melt period. In order to prevent water loss under high temperature and dry conditions, plants will close their stomata and less carbon will be fixed by photosynthesis whereas plant and soil respiration will increase. If the drought persists, the overall carbon fluxes from some ecosystems will change from a carbon sink to a carbon source when respiration exceeds the total photosynthesis. The current terrestrial ecosystems in California are important carbon sinks to offset the anthropogenic carbon emission, however, whether they will continue to be carbon sinks or some will become sources of carbon under warming and climatic variations in the future is uncertain.

Therefore the WRF and ACASA models are coupled to simulate the carbon dioxide, water, and energy fluxes between the terrestrial system and the atmosphere for the present and future conditions. Although WRF is a state-of-art regional atmospheric model with high spatial and temporal resolutions and it can be forced using reanalysis data and GCM outputs, there is no carbon dioxide calculation. Hence, the ACASA model is coupled to the WRF model as a surface-layer scheme. Carbon dioxide, sensible heat, water vapor, and momentum fluxes between the atmosphere and land surface are calculated in the ACASA model through third order turbulent equations. It allows counter-gradient transport that lower order turbulent closure models are unable to simulate. Additionally, the complex physiological processes in the WRF/ACASA model could permit plant behaviors to adopt the future changes in temperature and CO2 concentration, and to accurately simulate carbon flux. Simulation of future snow water equivalent will be crucial to water availability in California. Preliminary results showed that the WRF/ACASA coupled model has a better agreement with the reanalysis/input data than the WRF's pre-existed LSMs in both temperature and energy simulations.