Improvement in Regional Climate modeling using WRF coupled to the high complexity multilayer land surface model ACASA

Many GCMs have projected increase in both temperature and intensity of summer droughts over California. These changes in climate conditions will have large impacts on the state's snow water storage, water budget, and overall environmental conditions in both regional and local scale. Regional climate models in the last decade have been widely used for future climate change predictions. However, many of them lack interaction with a complex land surface scheme. In this study, we introduce a new framework with a regional atmospheric model, WRF, coupled to a high complexity microscale land surface model, ACASA. Although WRF is a state-of-art regional atmospheric model with high spatial and temporal resolutions, the land surface schemes available in WRF lack intricate biogeophysic processes and do not include carbon dioxide calculations. ACASA (Advanced Canopy-Atmosphere-Soil Algorithm) is complex multilayer land surface model with interactive canopy physiology and full surface hydrological processes that allows microenvironmental variables such as air and surface temperatures, wind speed, humidity, carbon dioxide concetration to vary vertically. Carbon dioxide, sensible heat, water vapor, and momentum fluxes between the atmosphere and land surface are estimated in the ACASA model through third order turbulence equations. It includes counter-gradient transport that low-order turbulence closure models are unable to simulate.

Therefore, the WRF-ACASA framework can simulate the carbon dioxide, water, and energy fluxes between the terrestrial system and the atmosphere for present and future conditions. In particular, the complex physiological processes in the WRF-ACASA model allow future climate conditions (changes in temperature and CO2 concentration) to modify plant behavior and thus the WRF-ACASA framework can simulate carbon, water, and energy fluxes more accurately. In this paper, results from several experiments are presented. Present day conditions are used to drive a transect region over Northern California, from the coastal region to the Sierra Nevada mountains, and results from WRF-ACASA and WRF-NOAH LSM are compared to observations. WRF-ACASA shows great improvements for simulations of surface temperature, snow water equivalence, planetary boundary layer height, and evapotranspiration. It also allows carbon dioxide flux calculation on a regional scale. Another experiment aims at investigating the impact of future conditions (2050) on the vegetation behavior in terms of CO2 and water budget. Finally, WRF-ACASA is used to study urban metabolism and heat island effect.