Comparing atmospheric transport models for future regional inversions over Europe – Part 1: mapping theatmospheric CO2 signals

[摘要] The CO₂ source and sink distribution across Europe can beestimated in principle through inverse methods by combining CO₂observations and atmospheric transport models. Uncertainties of suchestimates are mainly due to insufficient spatiotemporal coverage ofCO₂ observations and biases of the models. In order to assess thebiases related to the use of different models the CO₂concentration field over Europe has been simulated with fivedifferent Eulerian atmospheric transport models as part of theEU-funded AEROCARB project, which has the main goal to estimate thecarbon balance of Europe. In contrast to previous comparisons, hereboth global coarse-resolution and regional higher-resolution modelsare included. Continuous CO₂ observations from continental,coastal and mountain sites as well as flasks sampled on aircraftsare used to evaluate the models' ability to capture thespatiotemporal variability and distribution of lower troposphereCO₂ across Europe. ¹⁴CO₂ is used in addition toevaluate separately fossil fuel signal predictions. The simulatedconcentrations show a large range of variation, with up to ~10 ppm higher surface concentrations over Western and Central Europe inthe regional models with highest (mesoscale) spatial resolution.

The simulation – data comparison reveals that generallyhigh-resolution models are more successful than coarse models incapturing the amplitude and phasing of the observed short-termvariability. At high-altitude stations the magnitude of thedifferences between observations and models and in between models isless pronounced, but the timing of the diurnal cycle is not wellcaptured by the models.

The data comparisons show also that the timing of the observedvariability on hourly to daily time scales at low-altitude stationsis generally well captured by all models. However, the amplitude ofthe variability tends to be underestimated. While daytime values arequite well predicted, nighttime values are generally underpredicted.This is a reflection of the different mixing regimes during day andnight combined with different vertical resolution between models. Inline with this finding, the agreement among models is increased whensampling in the afternoon hours only and when sampling the mixedportion of the PBL, which amounts to sampling at a few hundredmeters above ground. The main recommendations resulting from thestudy for constraining land carbon sources and sinks usinghigh-resolution concentration data and state-of-the art transportmodels through inverse methods are given in the following: 1) Lowaltitude stations are presently preferable in inverse studies. Ifhigh altitude stations are used then the model level that representsthe specific sites should be applied, 2) at low altitude sites onlythe afternoon values of concentrations can be representedsufficiently well by current models and therefore afternoon valuesare more appropriate for constraining large-scale sources and sinks incombination with transport models, 3) even when using only afternoonvalues it is clear that data sampled several hundred meters aboveground can be represented substantially more robustly in models thansurface station records, which emphasize the use of tower data ininverse studies and finally 4) traditional large scale transportmodels seem not sufficient to resolve fine-scale features associatedwith fossil fuel emissions, as well as larger-scale features likethe concentration distribution above the south-western Europe. It istherefore recommended to use higher resolution models forinterpretation of continental data in future studies.

[学科分类] 大气科学