CarbonWatch-NZ: Atmospheric inverse modelling of terrestrial carbon exchange in New Zealand’s forests and grasslands

The implementation and verification of national greenhouse gas emissions reduction strategies requires accurate estimates of biosphere-atmosphere CO₂ exchange. Atmospheric inverse modelling uses atmospheric mole fraction observations and an atmospheric transport model to derive the spatial and temporal distribution of surface fluxes. Inverse models can be used to support and verify national reporting of greenhouse gas emissions and carbon uptake, but few countries have developed this capability for CO₂, due in part to challenges in capturing the diurnal cycle of fluxes and separating the fossil and terrestrial biosphere components. New Zealand is an ideal natural laboratory to develop this capability, given its expanding observation network, high resolution modelling capability, relatively small anthropogenic signal, and isolation from outside CO₂ sources. We will present new results from the CarbonWatch-NZ national scale CO₂ inversion system. Previous results, based on observations from just two in situ stations, suggested a larger net sink compared to both terrestrial biosphere models and the national inventory report. Here, we extend these results by incorporating observations from three additional in situ stations, developing modelling tools to represent the diurnal cycle and use nighttime observations, and increasing the spatial and temporal resolution of the inverse model by estimating fluxes on a 12 km grid, with 3 hourly time resolution. We performed a series of observing system simulation experiments (OSSEs) to assess the ability of the inversion system to constrain fluxes at regional, ecoregional, and national scales. We then quantify the differences between inversion and bottom-up flux estimates for New Zealand’s indigenous forests, exotic forests, and grassland environments.