Journal Article

Evaluation of continental carbon cycle simulations with North American flux tower observations

Huntzinger, Deborah, Tian, Hanqin, Law, Beverly, Schaefer, Kevin, Verbeeck, Hans, Davis, Kenneth J., Neilson, Ronald P., Poulter, Benjamin, Viovy, Nicolas, Post, Wilfred M., Raczka, Brett M., Keenan, Trevor F., Xiao, Jingfeng, Ciais, Philippe, Tomelleri, Enrico, Ricciuto, Daniel, Baker, Ian, Richardson, Andrew D.

2013

Overview

Terrestrial biosphere models can help identify physical processes that control carbon dynamics, including land-atmosphere CO2 fluxes, and have great potential to predict the terrestrial ecosystem response to changing climate. The skill of models that provide continental scale carbon flux estimates, however, remains largely untested. This paper evaluates the performance of continental-scale flux estimates from 17 models against observations from 36 North American flux towers. Fluxes extracted from regional model simulations are compared with co-located flux tower observations at monthly and annual time increments. Site-level model simulations are used to help interpret sources of the mismatch between the regional simulations and site-based observations. On average the regional model runs overestimate the annual gross primary productivity (5%) and total respiration (15%), and significantly underestimate the annual net carbon uptake (64%) during the time period 2000-2005. Comparison with site-level simulations implicate choices specific to regional model simulations as contributors to the gross flux biases, but not the net carbon uptake bias. The models perform the best at simulating carbon exchange at deciduous broadleaf sites; likely because a number of models use prescribed phenology to simulate seasonal fluxes. The models do not perform as well for crop, grass and evergreen sites. The regional models match the observations most closely in terms of seasonal correlation and seasonal magnitude of variation, but have very little skill at inter-annual correlation and minimal skill at inter-annual magnitude of variability. The comparison of site versus regional level model runs demonstrate that 1) the inter-annual correlation is higher for site-level model runs but the skill remains low, and 2) the underestimation of year-to-year variability for all fluxes is an inherent weakness of the models. The best performing regional models that do not use flux tower calibration are CLM-CN, CASA-GFEDv2 and SIB3. Two flux tower calibrated, empirical models, EC-MOD and MOD17+, perform as well as the best process-based models. This suggests that 1) empirical, calibrated models can perform as well as complex, process-based models, and 2) combining process-based model structure with relevant constraining data could significantly improve model performance.

• Publisher: ap,ECOLOGICAL SOCIETY OF AMERICA,Ecological Society of America
• Subject: Animal and plant ecology / Animal, plant and microbial ecology / Biological and medical sciences / Biosphere / carbon / Carbon cycle / Carbon dioxide / carbon fluxes / Climate change / Climate models / Continental interfaces, environment / Correlation analysis / Crossovers / Ecological modeling / Ecosystem models / empirical models / Fluctuations / flux towers / Fundamental and applied biological sciences. Psychology / General aspects / grasses / gross primary productivity / Growing seasons / Meteorology / model validation / Modeling / model–data comparison / Net ecosystem exchange / Ocean, Atmosphere / phenology / Sciences of the Universe / Simulation / simulation models / Simulations / Spatial models / terrestrial biosphere models / Terrestrial ecosystems