Abstract
Abstract. Climate change mitigation efforts require information on the current greenhouse gas atmospheric concentrations and their sources and sinks. Carbon dioxide (CO2) is the most abundant anthropogenic greenhouse gas. Its variability in the atmosphere is modulated by the synergy between weather and CO2 surface fluxes, often referred to as CO2 weather. It is interpreted with the help of global or regional numerical transport models, with horizontal resolutions ranging from a few hundreds of kilometres to a few kilometres. Changes in the model horizontal resolution affect not only atmospheric transport but also the representation of topography and surface CO2 fluxes. This paper assesses the impact of horizontal resolution on the simulated atmospheric CO2 variability with a numerical weather prediction model. The simulations are performed using the Copernicus Atmosphere Monitoring Service (CAMS) CO2 forecasting system at different resolutions from 9 to 80 km and are evaluated using in situ atmospheric surface measurements and atmospheric column-mean observations of CO2, as well as radiosonde and SYNOP observations of the winds. The results indicate that both diurnal and day-to-day variability of atmospheric CO2 are generally better represented at high resolution, as shown by a reduction in the errors in simulated wind and CO2. Mountain stations display the largest improvements at high resolution as they directly benefit from the more realistic orography. In addition, the CO2 spatial gradients are generally improved with increasing resolution for both stations near the surface and those observing the total column, as the overall inter-station error is also reduced in magnitude. However, close to emission hotspots, the high resolution can also lead to a deterioration of the simulation skill, highlighting uncertainties in the high-resolution fluxes that are more diffuse at lower resolutions. We conclude that increasing horizontal resolution matters for modelling CO2 weather because it has the potential to bring together improvements in the surface representation of both winds and CO2 fluxes, as well as an expected reduction in numerical errors of transport. Modelling applications like atmospheric inversion systems to estimate surface fluxes will only be able to benefit fully from upgrades in horizontal resolution if the topography, winds and prior flux distribution are also upgraded accordingly. It is clear from the results that an additional increase in resolution might reduce errors even further. However, the horizontal resolution sensitivity tests indicate that the change in the CO2 and wind modelling error with resolution is not linear, making it difficult to quantify the improvement beyond the tested resolutions. Finally, we show that the high-resolution simulations are useful for the assessment of the small-scale variability of CO2 which cannot be represented in coarser-resolution models. These representativeness errors need to be considered when assimilating in situ data and high-resolution satellite data such as Greenhouse gases Observing Satellite (GOSAT), Orbiting Carbon Observatory-2 (OCO-2), the Chinese Carbon Dioxide Observation Satellite Mission (TanSat) and future missions such as the Geostationary Carbon Observatory (GeoCarb) and the Sentinel satellite constellation for CO2. For these reasons, the high-resolution CO2 simulations provided by the CAMS in real time can be useful to estimate such small-scale variability in real time, as well as providing boundary conditions for regional modelling studies and supporting field experiments.
Highlights
Over synoptic weather timescales of hours to days and spatial scales less than 1000 km, the assumption that atmospheric CO2 is well-mixed into a homogeneous background does not hold, as shown by the observed variability at baseline in situ stations (e.g. Halter and Harris, 1983)
Most stations are on the World Meteorological Organization (WMO) CO2 scale, the inter-calibration of standard gases is not critical for this study because the focus is on the relative difference between the high- and lowresolution simulations to quantify the sensitivity of modelled CO2 to horizontal resolution in the model
This paper addresses the importance of horizontal resolution in the representation of CO2 variability at diurnal and synoptic scales, referred to here as CO2 weather
Summary
Over synoptic weather timescales of hours to days and spatial scales less than 1000 km, the assumption that atmospheric CO2 is well-mixed into a homogeneous background does not hold, as shown by the observed variability at baseline in situ stations (e.g. Halter and Harris, 1983). It reflects a complex combination of anthropogenic and natural CO2 fluxes near the Earth’s surface and transport by weather systems in the atmosphere (Geels et al, 2004; Patra et al, 2008). This synergy of CO2 fluxes and weather results in intricate atmospheric CO2 patterns of positive and negative anomalies, collocated with weather variations on top of the well-mixed CO2 background that varies slowly on timescales of weeks to years (Keeling et al, 1976). NWP models for weather forecasting have been doubling the global horizontal resolution approximately every 8 years (Wedi, 2014)
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