Evaluation of archived and off-line diagnosed vertical diffusion coefficients from ERA-40 with <sup>222</sup>Rn simulations

Abstract

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 versus off-line diagnosed coefficients based on other meteorological parameters archived during ERA-40 by examining the influence on the distribution of the radionuclide 222 Rn 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.Atmospheric boundary layer heights of the sets (ii) and (iii) are also compared. Both were found to be in reasonable agreement with observations of the atmospheric boundary layer height in Cabauw (the Netherlands) and from the FIFE-campaign (United States) during summer.Simulations of 222 Rn with the TM3 model using these four sets of vertical diffusion coefficients are compared to surface measurements of 222 Rn at the mid-latitude continental stations 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 222 Rn concentration is well represented using 3-hourly diffusion coefficients. Comparison with observations of 222 Rn 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 222 Rn concentration in the upper part of the atmospheric boundary layer.

Vertical mixing in atmospheric tracer transport models: error characterization and propagation

“…However, this requires the fluxes to be known at least as well as we already have knowledge about the fluxes targeted in the inversion. Given the prior knowledge of CO 2 fluxes and their spatial and temporal patterns, and given the problems in properly simulating the emanation rate of 222 Rn (Olivié et al, 2004), we see only limited potential in this tracer. For SF 6 , the location and strength of local emissions are often dominating variability of measurements in the continental boundary layer (Rivier et al, 2006).…”

Simulation of tropospheric chemistry and aerosols with the climate model EC-Earth

“…Vertical diffusion of tracers in TM5 is described with a first-order closure scheme, where the diffusion coefficient is assumed to be the same as for heat (Olivié et al, 2004a). In the free troposphere it is computed based on wind shear and static stability following Louis (1979).…”

Improvement of Vertical Diffusion Analytic Schemes Under Stable Atmospheric Conditions

“…It has been shown that vertical diffusion has significant impacts on simulated chemical concentrations (e.g. Oliviè et al 2004;Nowacki et al 1996;Biswas and Rao 2000). On the other hand, the evaluation of K (z) is not an easy task because the diffusion coefficients cannot be measured directly, and their numerical representations are only a simplification of complex physical processes (e.g.…”