Evaluation of the boundary layer dynamics of the TM5 model over Europe

We evaluate the capability of the global atmospheric transport model TM5 to simulate the boundary layer dynamics and associated variability of trace gases close to the surface, using radon (Rn-222). Focusing on the European scale, we compare the boundary layer height (BLH) in the TM5 model with observations from the National Oceanic and Atmospheric Admnistration (NOAA) Integrated Global Radiosonde Archive (IGRA) and also with ceilometer and lidar (light detection and ranging) BLH retrievals at two stations. Furthermore, we compare TM5 simulations of Rn-222 activity concentrations, using a novel, process-based Rn-222 flux map over Europe (Karstens et al., 2015), with harmonised Rn-222 measurements at 10 stations. The TM5 model reproduces relatively well the daytime BLH (within 10-20% for most of the stations), except for coastal sites, for which differences are usually larger due to model representation errors. During night, however, TM5 overestimates the shallow nocturnal BLHs, especially for the very low observed BLHs (< 100 m) during summer. The Rn-222 activity concentration simulations based on the new Rn-222 flux map show significant improvements especially regarding the average seasonal variability, compared to simulations using constant Rn-222 fluxes. Nevertheless, the (relative) differences between simulated and observed daytime minimum Rn-222 activity concentrations are larger for several stations (on the order of 50 %) than the (relative) differences between simulated and observed BLH at noon. Although the nocturnal BLH is often higher in the model than observed, simulated Rn-222 nighttime maxima are actually larger at several continental stations. This counter-intuitive behaviour points to potential deficiencies of TM5 to correctly simulate the vertical gradients within the nocturnal boundary layer, limitations of the Rn-222 flux map, or issues related to the definition of the nocturnal BLH. At several stations the simulated decrease of Rn-222 activity concentrations in the morning is faster than observed. In addition, simulated vertical Rn-222 activity concentration gradients at Cabauw decrease faster than observations during the morning transition period, and are in general lower than observed gradients during daytime. Although these effects may be partially due to the slow response time of the radon detectors, they clearly point to too fast vertical mixing in the TM5 boundary layer during daytime. Furthermore, the capability of the TM5 model to simulate the diurnal BLH cycle is limited by the current coarse temporal resolution (3 h/6 h) of the TM5 input meteorology.