Improving CLM5.0 Biomass and Carbon Exchange across the Western US Using a Data Assimilation System

Abstract:

Most of the carbon stocks and fluxes in the western United States are found in mountainous terrain, where observations are limited and the complex terrain presents modeling challenges. To overcome this we use the Community Land Model (CLM5.0) combined with the Data Assimilation Research Testbed (DART) to provide an observation-constrained estimate of carbon stock and fluxes. We use a meteorological data set designed specifically for complex terrain in combination with a CLM version that represents plant hydraulic water stress. The assimilation of remotely sensed observations of leaf area index (LAI) and above-ground biomass (AGB) significantly reduces the simulated biomass and switches the estimated land-atmosphere carbon exchange from a carbon sink to the land to a carbon source to the atmosphere. This CLM5-DART estimate of carbon exchange is similar to the FLUXCOM estimate in terms of ecosystem respiration, however, FLUXCOM projects much stronger gross primary productivity than CLM5-DART and thus a strong net carbon sink overall. We found the CLM5-DART estimate of carbon flux is sensitive to the state variables that are directly adjusted by the assimilation system. Observations of LAI generally have a higher acceptance rate than AGB observations as a result of low productivity vegetation regions across the Desert SouthWest and Great Basin regions.