Implementation of Improved Parameterization of Terrestrial Flux in WRF-VPRM Improves the Simulation of Nighttime CO₂ Peaks and a Daytime CO₂ Band Ahead of a Cold Front

Enhanced CO₂ mole fraction bands were often observed immediately ahead of cold front during the Atmospheric Carbon and Transport (ACT)-America mission and their formation mechanism is undetermined. Improved understanding and correct simulation of these CO₂ bands are needed for unbiased inverse CO₂ flux estimation. Such CO₂ bands are hypothesized to be related to nighttime CO₂ respiration and investigated in this study using WRF-VPRM, a weather-biosphere-online-coupled model, in which the biogenic fluxes are handled by the Vegetation Photosynthesis and Respiration Model (VPRM). While the default VPRM satisfactorily parameterizes gross ecosystem exchange, its treatment of terrestrial respiration as a linear function of temperature was inadequate as respiration is a nonlinear function of temperature and also depends on the amount of biomass and soil wetness. An improved ecosystem respiration parameterization including enhanced vegetation index, a water stress factor, and a quadratic temperature dependence is incorporated into WRF-VPRM and evaluated in a year-long simulation before applied to the investigation of the frontal CO₂ band on August 4, 2016. The evaluation shows that the modified WRF-VPRM increases ecosystem respiration during the growing season, and improves model skill in reproducing nighttime near-surface CO₂ peaks. A nested-domain WRF-VPRM simulation is able to capture the main characteristics of the August 4 CO₂ band and informs its formation mechanism. Nighttime terrestrial respiration leads to accumulation of near-surface CO₂ in the region. As the cold front carrying low-CO₂ air moves southeastward, and strong photosynthesis depletes CO₂ further southeast of the front, a CO₂ band develops immediately ahead of the front.