Modelling CO₂ weather – why horizontal resolution matters

• Main Authors: A. Agustí-Panareda, M. Diamantakis, S. Massart, F. Chevallier, J. Muñoz-Sabater, J. Barré, R. Curcoll, R. Engelen, B. Langerock, R. M. Law, Z. Loh, J. A. Morguí, M. Parrington, V.-H. Peuch, M. Ramonet, C. Roehl, A. T. Vermeulen, T. Warneke, D. Wunch
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-06-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/19/7347/2019/acp-19-7347-2019.pdf
• ISSN: 1680-7316; 1680-7324

<p>Climate change mitigation efforts require information on the current greenhouse gas atmospheric concentrations and their sources and sinks. Carbon dioxide (<span class="inline-formula">CO₂</span>) is the most abundant anthropogenic greenhouse gas. Its variability in the atmosphere is modulated by the synergy between weather and <span class="inline-formula">CO₂</span> surface fluxes, often referred to as <span class="inline-formula">CO₂</span> 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 <span class="inline-formula">CO₂</span> fluxes. This paper assesses the impact of horizontal resolution on the simulated atmospheric <span class="inline-formula">CO₂</span> variability with a numerical weather prediction model. The simulations are performed using the Copernicus Atmosphere Monitoring Service (CAMS) <span class="inline-formula">CO₂</span> forecasting system at different resolutions from 9 to 80&thinsp;<span class="inline-formula">km</span> and are evaluated using in situ atmospheric surface measurements and atmospheric column-mean observations of <span class="inline-formula">CO₂</span>, as well as radiosonde and SYNOP observations of the winds.</p> <p>The results indicate that both diurnal and day-to-day variability of atmospheric <span class="inline-formula">CO₂</span> are generally better represented at high resolution, as shown by a reduction in the errors in simulated wind and <span class="inline-formula">CO₂</span>. Mountain stations display the largest improvements at high resolution as they directly benefit from the more realistic orography. In addition, the <span class="inline-formula">CO₂</span> 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.</p> <p><span id="page7348"/>We conclude that increasing horizontal resolution matters for modelling <span class="inline-formula">CO₂</span> weather because it has the potential to bring together improvements in the surface representation of both winds and <span class="inline-formula">CO₂</span> 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 <span class="inline-formula">CO₂</span> and wind modelling error with resolution is not linear, making it difficult to quantify the improvement beyond the tested resolutions.</p> <p>Finally, we show that the high-resolution simulations are useful for the assessment of the small-scale variability of <span class="inline-formula">CO₂</span> 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 <span class="inline-formula">CO₂</span>. For these reasons, the high-resolution <span class="inline-formula">CO₂</span> 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.</p>