Comparing Lagrangian and Eulerian models for CO 2 transport – a step towards Bayesian inverse modeling using WRF/STILT-VPRM

We present simulations of atmospheric CO2 concentrations provided by two modeling systems, run at high spatial resolution: the Eulerian-based Weather Re- search Forecasting (WRF) model and the Lagrangian-based Stochastic Time-Inverted Lagrangian Transport (STILT) model, both of which are coupled to a diagnostic biospheric model, the Vegetation Photosynthesis and Respiration Model (VPRM). The consistency of the simulations is assessed with special attention paid to the details of horizontal as well as vertical transport and mixing of CO2 concentrations in the atmosphere. The dependence of model mismatch (Eulerian vs. Lagrangian) on models' spatial resolution is further in- vestigated. A case study using airborne measurements during which two models showed large deviations from each other is analyzed in detail as an extreme case. Using aircraft observa- tions and pulse release simulations, we identified differences in the representation of details in the interaction between tur- bulent mixing and advection through wind shear as the main cause of discrepancies between WRF and STILT transport at a spatial resolution such as 2 and 6 km. Based on observa- tions and inter-model comparisons of atmospheric CO2 con- centrations, we show that a refinement of the parameteriza- tion of turbulent velocity variance and Lagrangian time-scale in STILT is needed to achieve a better match between the Eulerian and the Lagrangian transport at such a high spatial resolution (e.g. 2 and 6 km). Nevertheless, the inter-model differences in simulated CO2 time series for a tall tower ob- servatory at Ochsenkopf in Germany are about a factor of two smaller than the model-data mismatch and about a fac- tor of three smaller than the mismatch between the current global model simulations and the data.