Characterizing model errors in chemical transport modeling of methane: using GOSAT XCH 4 data with weak-constraint four-dimensional variational data assimilation

Stanevich, Ilya; Jones, Dylan B. A.; Strong, Kimberly; Keller, Martin; Henze, Daven K.; Parker, Robert J.; Boesch, Hartmut; Wunch, Debra; Notholt, Justus; Petri, Christof; Warneke, Thorsten; Sussmann, Ralf; Schneider, Matthias; Hase, Frank; Kivi, Rigel; Deutscher, Nicholas M.; Velazco, Voltaire A.; Walker, Kaley A.; Deng, Feng

We examined biases in the global GEOS-Chem chemical transport model for the period of February–May 2010 using weak-constraint (WC) four-dimensional variational (4D-Var) data assimilation and dry-air mole fractions of CHinline-formula4 (XCHinline-formula4) from the Greenhouse gases Observing SATellite (GOSAT). The ability of the observations and the WC 4D-Var method to mitigate model errors in CHinline-formula4 concentrations was first investigated in a set of observing system simulation experiments (OSSEs). We then assimilated the GOSAT XCHinline-formula4 retrievals and found that they were capable of providing information on the vertical structure of model errors and of removing a significant portion of biases in the modeled CHinline-formula4 state. In the WC 4D-Var assimilation, corrections were added to the modeled CHinline-formula4 state at each model time step to account for model errors and improve the model fit to the assimilated observations. Compared to the conventional strong-constraint (SC) 4D-Var assimilation, the WC method was able to significantly improve the model fit to independent observations. Examination of the WC state corrections suggested that a significant source of model errors was associated with discrepancies in the model CHinline-formula4 in the stratosphere. The WC state corrections also suggested that the model vertical transport in the troposphere at middle and high latitudes is too weak. The problem was traced back to biases in the uplift of CHinline-formula4 over the source regions in eastern China and North America. In the tropics, the WC assimilation pointed to the possibility of biased CHinline-formula4 outflow from the African continent to the Atlantic in the mid-troposphere. The WC assimilation in this region would greatly benefit from glint observations over the ocean to provide additional constraints on the vertical structure of the model errors in the tropics. We also compared the WC assimilation at 4inline-formulainline-formula× 5inline-formula and 2inline-formulainline-formula× 2.5inline-formula horizontal resolutions and found that the WC corrections to mitigate the model errors were significantly larger at 4inline-formulainline-formula× 5inline-formula than at 2inline-formulainline-formula× 2.5inline-formula resolution, indicating the presence of resolution-dependent model errors. Our results illustrate the potential utility of the WC 4D-Var approach for characterizing model errors. However, a major limitation ofpage9546 this approach is the need to better characterize the specified model error covariance in the assimilation scheme.

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Stanevich, Ilya / Jones, Dylan B. A. / Strong, Kimberly / et al: Characterizing model errors in chemical transport modeling of methane: using GOSAT XCH4 data with weak-constraint four-dimensional variational data assimilation. 2021. Copernicus Publications.

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