Simulation of the transport of Rn222 using on-line and off-line global models at different horizontal resolutions: a detailed comparison with measurements,

Abstract

The short‐lived radionuclide Rn222 is emitted at a fairly constant rate from the continents and is a good surrogate for studying the transport of “air pollution” from polluted continental areas to clean, remote regions. The large concentration gradients of 2–3 orders of magnitude which exist between the continents and the remote atmosphere present a major challenge to the modelling of horizontal and vertical atmospheric transport. We use the global off‐line tracer transport model TM3 at 3 different resolutions. Input to the model consists of meteorological data for the year 1993 obtained from the European Centre for Medium Range Weather Forecasts (ECMWF). The same meteorological data is used to constrain the climate model ECHAM4‐T42‐L19. Using these meteorological data, Rn222 simulations are used to evaluate and document model performance and associated uncertainties. High time‐resolution measurements made at 2 continental stations, 2 stations under continental influence and 4 remote sites, and aircraft measurements obtained during the NARE aircraft campaign are used for a detailed comparison. Although in specific regions there are inter‐model differences of up to a factor of 2 in the calculated boundary layer concentrations, these differences are not translated into a better performance of either model for the stations used for comparison. We generally obtain high correlations of model results and measurements; these range from r= 0.6–0.8 for the continental and coastal stations and 0.5–0.6 for the remote sites. Calculated mean concentrations and corresponding standard deviations generally agree favourably with observations, lending credibility to the usefulness of our models for evaluating transport of air pollutants from continental sources to remote regions. The main cause of model deviations is probably related to uncertainties in the meteorological input data set provided by the ECMWF model and to a lesser extent by our knowledge of the spatial distribution of Rn222 emissions and uncertainties involving sub‐grid scale parameterization of vertical transport, e.g., diffusion and convection.

Evaluation of the new UKCA climate-composition model – Part 2: The Troposphere

“…It shows that for the control experiment, the zonal mean distribution from HadGEM2-UKCA is similar to the models that participated in the World Climate Research Program (WCRP) intercomparison (Jacob et al, 1997) and other models (e.g. Dentener et al, 1999;Hauglustaine et al, 2004). Surface concentrations show a maximum in the northern mid-latitudes of 20-30 ×10 −21 mol mol −1 , in close agreement with Jacob et al (1997).…”

“…This discrepancy has been found in other models (e.g. Stockwell et al, 1998;Gupta et al, 2004) and could be related to a lack of deep convection over Asia, insufficient long-range transport from Asia, and/or an underestimate in the Asian emissions in the simple emissions scenario used here (Jacob et al, 1997;Dentener et al, 1999). The Nova Scotia measurements were taken during 9 North Atlantic Regional Experiment (NARE) flights over the North Atlantic Ocean and close to Nova Scotia and New Brunswick, Canada, between 16 and 31 August 1993 .…”

Sensitivity of Global Modeling Initiative chemistry and transport model simulations of radon-222 and lead-210 to input meteorological data

“…Emission rates from frozen ground are significantly reduced, and the emission of 222 Rn from the oceans is negligible. The radionuclide has proved useful for investigating rapid transport processes such as convection (Mahowald et al, 1997;Stockwell et al, 1998), continental influence on remote ocean locations (Balkanski et al, 1992), the diurnal variability of the planetary mixed layer (Jacob and Prather, 1990), synoptic variability (Allen et al, 1996), and differences between on-line and off-line transport (Dentener et al, 1999). It has also been used in a major intercomparison of tropospheric transport models (Jacob et al, 1997).…”

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

“…Hourly measurements of 222 Rn at Freiburg and Schauinsland (both at 48 • N, 8 • E) for the year 1993 are used. These data have also been used in a study by Dentener et al (1999). Freiburg and Schauinsland are located at heights of 300 and 1200 m above sea level respectively.…”

“…222 Rn is an excellent tracer to study the transport characteristics on short time scales (hours to weeks) because it has an almost uniform emission rate over land and is only lost through radioactive decay with an e-folding lifetime of about 5.5 days (Dentener et al, 1999;Balkanski and Jacob, 1990;Kritz et al, 1990;Brost and Chatfield, 1989;Polian et al, 1986). Therefore 222 Rn has been used extensively to evaluate parameterisations of convective transport (Mahowald et al, 1997;Allen et al, 1996;Feichter and Crutzen, 1990;Jacob and Prather, 1990) and ABL diffusion (Stockwell and Chipperfield, 1999;Stockwell et al, 1998;Jacob et al, 1997;Lee and Larsen, 1997;Mahowald et al, 1997) in atmospheric models.…”