Sensitivity analysis of the potential impact of discrepancies in stratosphere–troposphere exchange on inferred sources and sinks of CO<sub>2</sub>

Abstract

Abstract. The upper troposphere and lower stratosphere (UTLS) represents a transition region between the more dynamically active troposphere and more stably stratified stratosphere. The region is characterized by strong gradients in the distribution of long-lived tracers, whose representation in models is sensitive to discrepancies in transport. We evaluate the GEOS-Chem model in the UTLS using carbon dioxide (CO 2 ) and ozone (O 3 ) observations from the HIAPER (The High-Performance Instrumented Airborne Platform for Environmental Research) Pole-to-Pole Observations (HIPPO) campaign in March 2010. GEOS-Chem CO 2 /O 3 correlation suggests that there is a discrepancy in mixing across the tropopause in the model, which results in an overestimate of CO 2 and an underestimate of O 3 in the Arctic lower stratosphere. We assimilate stratospheric O 3 data from the Optical Spectrograph and InfraRed Imager System (OSIRIS) and use the assimilated O 3 fields together with the HIPPO CO 2 /O 3 correlations to obtain an adjustment to the modeled CO 2 profile in the Arctic UTLS (primarily between the 320 and 360 K isentropic surfaces). The HIPPOderived adjustment corresponds to a sink of 0.60 Pg C for March-August 2010 in the Arctic. Imposing this adjustment results in a reduction in the CO 2 sinks inferred from GOSAT observations for temperate North America, Europe, and tropical Asia of 19, 13, and 49 %, respectively. Conversely, the inversion increased the source of CO 2 from tropical South America by 23 %. We find that the model also underestimates CO 2 in the upper tropical and subtropical troposphere. Correcting for the underestimate in the model relative to HIPPO in the tropical upper troposphere leads to a reduction in the source from tropical South America by 77 %, and produces an estimated sink for tropical Asia that is only 19 % larger than the standard inversion (without the imposed source and sink). Globally, the inversion with the Arctic and tropical adjustment produces a sink of −6.64 Pg C, which is consistent with the estimate of −6.65 Pg C in the standard inversion. However, the standard inversion produces a stronger northern land sink by 0.98 Pg C to account for the CO 2 overestimate in the high-latitude UTLS, suggesting that this UTLS discrepancy can impact the latitudinal distribution of the inferred sources and sinks. We find that doubling the model resolution from 4 • × 5 • to 2 • × 2.5 • enhances the CO 2 vertical gradient in the high-latitude UTLS, and reduces the overestimate in CO 2 in the extratropical lower stratosphere. Our results illustrate that discrepancies in the CO 2 distribution in the UTLS can affect CO 2 flux inversions and suggest the need for more careful evaluation of model errors in the UTLS.

Quantifying the Impact of Atmospheric Transport Uncertainty on CO2 Surface Flux Estimates

“…The strong signals of CO 2 difference in the upper layers of the atmosphere at around 88 hPa appear to be tied to a discontinuity in layer pressure thicknesses in the stratosphere in the TM5 model (see GEOS-Chem and TM5 pressure thickness plot in Figure S1). We note that UTLS mixing in the GEOS-Chem model was discussed in detail in a recent model-data comparison paper (Deng et al, 2015) and is a fundamental part of an upcoming paper by Dr. Brad Weir at NASA-GMAO. It will not be discussed further here.…”

Using airborne HIAPER Pole-to-Pole Observations (HIPPO) to evaluate model and remote sensing estimates of atmospheric carbon dioxide

“…As before, the error bar for GOSAT is derived as the standard error in the mean and the model error bar by using the variability of HIPPO XCO 2 using the two different models to extrapolate to the top-of-atmosphere (and the average of the two is defined as HIPPO XCO 2 . The center box spans a range from −0.5 to 0.5 ppm, a strict requirement for systematic biases (GHG-CCI, 2014). The green and red shaded areas indicated regions where either the GOSAT data meet the 0.5 ppm requirement but the models do not (green) or vice versa (red).…”

Top-down constraints on global N<sub>2</sub>O emissions at optimal resolution: application of a new dimension reduction technique

“…This approach therefore implicitly assumes that the sign of the a priori flux (which can be negative over the ocean) is correct for each model grid square. The GEOS-Chem adjoint has previously been applied to a wide range of inverse problems for atmospheric composition, including constraining sources and sinks of long-lived greenhouse gases such as CO 2 (Deng et al, 2014;Liu et al, 2014;Deng et al, 2015;Liu et al, 2015), methane (Wecht et al, 2014;Turner et al, 2015a), and N 2 O (Wells et al, 2015), as well as aerosols and reactive trace gases (e.g., Henze et al, 2007;Kopacz et al, 2009;Wells et al, 2014).…”