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Volume 11, issue 5 | Copyright
Geosci. Model Dev., 11, 1725-1752, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Development and technical paper 04 May 2018

Development and technical paper | 04 May 2018

Development of the WRF-CO2 4D-Var assimilation system v1.0

Tao Zheng1,4, Nancy H. F. French2, and Martin Baxter3 Tao Zheng et al.
  • 1Department of Geography and Environmental Studies, Central Michigan University, Mount Pleasant, MI, USA
  • 2Michigan Tech Research Institute, Michigan Technological University, Ann Arbor, MI, USA
  • 3Department of Earth and Atmospheric Sciences, Central Michigan University, Mount Pleasant, MI, USA
  • 4Institute for Great Lakes Research, Central Michigan University, Mount Pleasant, MI, USA

Abstract. Regional atmospheric CO2 inversions commonly use Lagrangian particle trajectory model simulations to calculate the required influence function, which quantifies the sensitivity of a receptor to flux sources. In this paper, an adjoint-based four-dimensional variational (4D-Var) assimilation system, WRF-CO2 4D-Var, is developed to provide an alternative approach. This system is developed based on the Weather Research and Forecasting (WRF) modeling system, including the system coupled to chemistry (WRF-Chem), with tangent linear and adjoint codes (WRFPLUS), and with data assimilation (WRFDA), all in version 3.6. In WRF-CO2 4D-Var, CO2 is modeled as a tracer and its feedback to meteorology is ignored. This configuration allows most WRF physical parameterizations to be used in the assimilation system without incurring a large amount of code development. WRF-CO2 4D-Var solves for the optimized CO2 flux scaling factors in a Bayesian framework. Two variational optimization schemes are implemented for the system: the first uses the limited memory Broyden–Fletcher–Goldfarb–Shanno (BFGS) minimization algorithm (L-BFGS-B) and the second uses the Lanczos conjugate gradient (CG) in an incremental approach. WRFPLUS forward, tangent linear, and adjoint models are modified to include the physical and dynamical processes involved in the atmospheric transport of CO2. The system is tested by simulations over a domain covering the continental United States at 48km × 48km grid spacing. The accuracy of the tangent linear and adjoint models is assessed by comparing against finite difference sensitivity. The system's effectiveness for CO2 inverse modeling is tested using pseudo-observation data. The results of the sensitivity and inverse modeling tests demonstrate the potential usefulness of WRF-CO2 4D-Var for regional CO2 inversions.

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Short summary
We developed WRF-CO2 4D-Var, a carbon dioxide data assimilation system based on the online atmospheric chemistry–transport model WRF-Chem. The accuracy of the model for sensitivity calculation and inverse modeling is assessed with pseudo-observation data. In this system, carbon dioxide is treated as an atmospheric tracer and its influence on meteorology is ignored. This system provides a useful model tool for regional-scale carbon source attribution and uncertainty assessment.
We developed WRF-CO2 4D-Var, a carbon dioxide data assimilation system based on the online...