Evaluation of the vertical diffusion coefficients from ERA-40 with 222Rn simulations
1Royal Netherlands Meteorological Institute, De Bilt, The Netherlands
2Eindhoven University of Technology, Eindhoven, The Netherlands
3European Centre for Medium-range Weather Forecasts, Reading, UK
Abstract. Boundary layer turbulence has a profound influence on the distribution of tracers with sources or sinks at the surface. The 40-year ERA-40 meteorological data set of the European Centre for Medium-range Weather Forecasts contains archived vertical diffusion coefficients. We evaluated the use of these archived diffusion coefficients instead of off-line diagnosed coefficients based on other meteorological parameters archived during ERA-40 by investigation of the effect on the distribution of the radioactive tracer 222Rn in the chemistry transport model TM3. In total four different sets of vertical diffusion coefficients are compared: (i) 3-hourly vertical diffusion coefficients archived during the ERA-40 project, (ii) 3-hourly off-line diagnosed coefficients from a non-local scheme based on Holtslag and Boville (1993), Vogelezang and Holtslag (1996), and Beljaars and Viterbo (1999), (iii) 6-hourly coefficients archived during the ERA-40 project, and (iv) 6-hourly off-line diagnosed coefficients based on a local scheme described in Louis (1979) and Louis et al. (1982). The diffusion scheme to diagnose the coefficients off-line in (ii) is similar to the diffusion scheme used during the ERA-40 project (i and iii).
The archived diffusion coefficients from the ERA-40 project which are time-averaged cause stronger mixing than the instantaneous off-line diagnosed diffusion coefficients. This can be partially attributed to the effect of instantaneous versus time-averaged coefficients, as well as to differences in the diffusion schemes. The 3-hourly off-line diagnosis of diffusion coefficients can reproduce quite well the 3-hourly archived diffusion coefficients.
Boundary layer heights are also available for the sets (ii) and (iii). Both were found to be in reasonable agreement with observations of the boundary layer height from Cabauw in the Netherlands and from the FIFE-campaign in the United States.
Simulations of 222Rn with the TM3 model using these four sets of vertical diffusion coefficients are compared to surface measurements of 222Rn in Freiburg, Schauinsland, Cincinnati and Socorro in order to evaluate the effect of these different sets of diffusion coefficients on the tracer transport. It is found that the daily cycle of the 222Rn concentration is well represented using 3-hourly diffusion coefficients. Comparison with observations of 222Rn data with the station in Schauinsland which is situated on a hill shows that all considered schemes underestimate the amplitude of the daily cycle of the 222Rn concentration in the upper part of the atmospheric boundary layer.
We conclude that the 3-hourly archived diffusion coefficients from ERA-40 are well suited for use in chemistry transport models.