Evaluation of the boundary layer dynamics of the TM5 model over Europe
- 1European Commission Joint Research Centre, Ispra (Va), Italy
- 2Max-Planck-Institute for Biogeochemistry, Jena, Germany
- 3ICOS Carbon Portal, ICOS ERIC at Lund University, Lund, Sweden
- 4SRON Netherlands Institute for Space Research, Utrecht, the Netherlands
- 5Institute for Marine and Atmospheric Research Utrecht, Utrecht University, Utrecht, the Netherlands
- 6MAQ, Wageningen University and Research Centre, Wageningen, the Netherlands
- 7Netherlands Organisation for Applied Scientific Research (TNO), Utrecht, the Netherlands
- 8Institut für Umweltphysik, Heidelberg University, Heidelberg, Germany
- 9Energy research Center Netherlands (ECN), Petten, the Netherlands
- 10Royal Holloway, University of London (RHUL), Egham, UK
- 11Laboratoire des Sciences du Climat et de l'Environnement, LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay, 91191 Gif-sur-Yvette, France
- 12Royal Netherlands Meteorological Institute (KNMI), De Bilt, the Netherlands
- 13Centrum voor Isotopen Onderzoek (CIO), Rijksuniversiteit Groningen, Groningen, the Netherlands
- 14Atmospheric Chemistry Research Group, University of Bristol, Bristol, UK
- 15Department of Meteorology, Pennsylvania State University, State College, PA, USA
- 16Australian Nuclear Science and Technology Organisation (ANSTO) Environment Research Theme, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia
Abstract. 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 (222Rn). 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 222Rn activity concentrations, using a novel, process-based 222Rn flux map over Europe (Karstens et al., 2015), with harmonised 222Rn 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 222Rn activity concentration simulations based on the new 222Rn flux map show significant improvements especially regarding the average seasonal variability, compared to simulations using constant 222Rn fluxes. Nevertheless, the (relative) differences between simulated and observed daytime minimum 222Rn 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 222Rn nighttime maxima are actually larger at several continental stations. This counterintuitive behaviour points to potential deficiencies of TM5 to correctly simulate the vertical gradients within the nocturnal boundary layer, limitations of the 222Rn flux map, or issues related to the definition of the nocturnal BLH.
At several stations the simulated decrease of 222Rn activity concentrations in the morning is faster than observed. In addition, simulated vertical 222Rn 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.