Inverse modeling with aircraft observations for constraining CO2 flux estimates inAsia
Yosuke NIWA
Meteorological Research Institute, Japan
Estimates of carbon dioxide (CO2) fluxes at the earth surface (e.g., between atmosphere andterrestrial biospheres/oceans) have significant uncertainties, which limits our understanding of
the carbon cycle. Especially, uncertainties of the flux estimates for the Asian regions are
considerable because human activities such as fossil-fuel consumption and land-use changes
and frequent biomass burnings complicate the problem.

Globally distributed observations of atmospheric CO2 have been used to constrain estimates ofsurface CO2 fluxes and a statistical estimation has been often performed by a Bayesian
inversion analysis, which employs an atmospheric transport model that links surface fluxes
with the atmospheric observations. However, there are many challenges to perform a reliable
inversion analysis; for instance, it requires an accurate transport model, a dense observation
network, and a sophisticated inversion scheme.

Toward an accurate transport simulation, we have developed the atmospheric transport model
NICAM-TM (Nonhydrostatic ICosahedral Atmospheric Model-based Transport Model), whichhas promising capabilities for mass conservation and a high-resolution simulation. In theinversion analysis with NICAM-TM, we leverage aircraft observation data from the CONTRAIL(Comprehensive Observation Network for Trace gases by Airliner) project to fill the gaps of theconventional surface observation network. Since 2005, CONTRAIL has been continuouslyoperating in-situ measurements of atmospheric CO2 onboard commercial aircraft. Owing tothe observations on international flights, its observation network is worldwide and ranges fromthe boreal high latitudes to the austral mid-latitudes including many parts of Asia. Furthermore,in order to fully exploit a number of CONTRAIL data, we have newly developed an inversion
system, named NICAM-TM 4D-Var, by combining the four-dimensional method withNICAM-TM. This system provides high resolution flux estimates with nearly no limitation in thenumber of observations it can accommodate.

In the inverse modeling with the CONTRAIL data, we have demonstrated great utility of thoseaircraft data for constraining Asian flux estimates. Especially, the impact of the data is
noteworthy for South and Equatorial Asia, where the surface observation network is quitesparse. Over those regions, flux signals are uplifted by convective vertical transport and they
are more efficiently captured by the aircraft than by remote surface stations. In 2015, which is
the one of the biggest El Niño years for the last two decades, large-scale biomass burnings
occurred at Equatorial Asia. Anomalies of the CO2 fluxes induced by those biomass burnings
were clearly captured by the high-resolution inversion analysis with NICAM-TM 4D-Var. This
study could provide valuable insights on the mechanism of the CO2 fluxes in Equatorial Asia,
where a large amount of carbon is stored in tropical rainforests and peatlands and disturbanceson those reservoirs have strong impacts on the growth rate of the global atmospheric CO2.