Three-dimensional variations of atmospheric CO<sub>2</sub>: aircraft measurements and multi-transport model simulations

Abstract

Abstract. Numerical simulation and validation of threedimensional structure of atmospheric carbon dioxide (CO 2 ) is necessary for quantification of transport model uncertainty and its role on surface flux estimation by inverse modeling. Simulations of atmospheric CO 2 were performed using four transport models and two sets of surface fluxes compared with an aircraft measurement dataset of Comprehensive Observation Network for Trace gases by AIrLiner (CONTRAIL), covering various latitudes, longitudes, and heights. Under this transport model intercomparison project, spatiotemporal variations of CO 2 concentration for 2006-2007 were analyzed with a three-dimensional perspective. Results show that the models reasonably simulated vertical profiles and seasonal variations not only over northern latitude areas but also over the tropics and southern latitudes. From CONTRAIL measurements and model simulations, intrusion of northern CO 2 in to the Southern Hemisphere, through the upper troposphere, was confirmed. Furthermore, models well simulated the vertical propagation of seasonal variation in the northern free troposphere. However, significant model-observation discrepancies were found in Asian regions, which are attributable to uncertainty of the surface CO 2 flux data. In summer season, differences in latitudinal gradients by the fluxes are comparable to or greater Correspondence to: Y. Niwa (yniwa@mri-jma.go.jp) than model-model differences even in the free troposphere. This result suggests that active summer vertical transport sufficiently ventilates flux signals up to the free troposphere and the models could use those for inferring surface CO 2 fluxes.

Long‐term change of CO2 latitudinal distribution in the upper troposphere

“…Thus, in addition to the enhancement of the negative gradient in the upper troposphere by the impact of the northern ocean sink, this explanation requires the cancellation of the positive gradient at the surface (caused by the rectifier effect) by the existence of an additional northern land sink. However, there are large uncertainties in estimating the magnitude of the northern land sink, because the modeled latitudinal gradient strongly depends on the rectifier effect in the lower troposphere [e.g., Gurney et al, 2003] as well as in the upper troposphere [Niwa et al, 2011]. Our understanding of these proposed processes could be improved when the negative gradients in both the upper and lower troposphere are consistently explained.…”

“…These long-term changes in the interhemispheric gradient provide useful information for estimating global land/ocean carbon fluxes [Keeling et al, 2011;Wang et al, 2013], although with large uncertainties. To improve these flux estimations, previous studies have pointed out the need to measure CO 2 in the free troposphere above the planetary boundary layer (PBL) [Conway and Tans, 1999;Stephens et al, 2007;Niwa et al, 2011]. However, long-term variations in the latitudinal CO 2 distribution above the PBL are not well delineated due to sparse airborne observations.…”

Carbon flux estimation for Siberia by inverse modeling constrained by aircraft and tower CO2 measurements

“…Vertical mixing was represented in the model by cumulus convection based on the convective precipitation rate, provided by JCDAS and calculated using a Kuo-type scheme following Grell et al [1995], and turbulent diffusion with explicitly parameterized physical processes in the PBL. This new version of the model is one of the participating models of the "Comprehensive Observation Network for Trace gases by Airliner (CONTRAIL)" transport model intercomparison for CO 2 [Niwa et al, 2011] and the Trans-Com-CH 4 experiment [Patra et al, 2011].…”

Bias assessment of lower and middle tropospheric CO<sub>2</sub> concentrations of GOSAT/TANSO-FTS TIR version 1 product

“…Locations of airports at which CONTRAIL CME ascending and descending observations were collected used in this study. and source-sink inversion studies of long-lived species such as CO 2 (Niwa et al, 2011a(Niwa et al, , b, 2012(Niwa et al, , 2017.…”

“…CO 2 is chemically inactive, and thus longrange transport processes as well as surface fluxes determine its horizontal distribution and seasonal cycle in the atmosphere (Miyazaki et al, 2008;Barnes et al, 2016). The treatment of vertical transport of CO 2 also produces differences in simulated CO 2 concentrations in the free troposphere among transport models unrelated to surface fluxes (Niwa et al, 2011a). Therefore, it is needed to observe CO 2 concentrations over land that are not strongly affected by local point sources of CO 2 emissions, as well as CO 2 concentrations in the free troposphere that can evaluate vertical CO 2 transport from the surface in transport models.…”