Abstract
Abstract. We use an atmospheric general circulation model (AGCM) driven chemistry-transport model (ACTM) to simulate the evolution of sulfur hexafluoride (SF6) in the troposphere. The model results are compared with continuous measurements at 6 sites over 71° N–90° S. These comparisons demonstrate that the ACTM simulations lie within the measurement uncertainty over the analysis period (1999–2006) and capture salient features of synoptic, seasonal and interannual SF6 variability. To understand transport timescales of SF6 within the troposphere, transport times of air parcels from the surface to different regions of the troposphere ("age") are estimated from a simulation of an idealized tracer. The age estimation error and its sensitivity to the selection of reanalysis meteorology for ACTM nudging or the tracer transport by deep cumulus convection as represented in the model are discussed. Monthly-mean, 2-box model exchange times (τex) are calculated from both the observed and simulated SF6 time series at the 6 observing sites and show favorable agreement, suggesting that the ACTM adequately represents large-scale interhemispheric transport. The simulated SF6 variability is further investigated through decomposition of the mixing ratio time-tendency into advective, convective, and vertical diffusive components. The transport component analysis illustrates the role of each process in SF6 synoptic variability at the site level and provides insight into the seasonality of τex.
Highlights
Sulfur hexafluoride (SF6) represents a powerful tracer of atmospheric transport (Maiss et al, 1996)
Because there are relatively few sites for which high frequency SF6 data exist, we focus on the continuous observations of SF6 from 6 sites (WDCGG, 2008; see Table 1 for a summary of site information), namely Point Barrow (BRW), Schauinsland (SCH), Niwot Ridge (NWR), Mauna Loa (MLO), Samoa (SMO) and South Pole (SPO)
Simulation of all tracers was initialized on 1 January 1960, and the analysis presented here covers the period of continuous SF6 observations (ACTM nudged to NCEP2 meteorology; data available from 1979)
Summary
Sulfur hexafluoride (SF6) represents a powerful tracer of atmospheric transport (Maiss et al, 1996). To compute the heat and moisture exchange fluxes at the earth’s surface, the AGCM is supplied with interannually varying monthly-mean sea ice and sea-surface temperature (SST) fields from the Met Office Hadley Centre observational datasets (Rayner et al, 2003) This forward transport model has been adapted for simulations of greenhouse gases (CO2, CH4, N2O, CFCs, SF6 etc.) that have. We decompose the flux divergence term into three components in this study: (1) advection by grid-scale air flow calculated from the flux-form transport scheme (Lin and Rood, 1996); (2) lifting through cumulus convection as parameterized by the Arakawa and Schubert (1974) scheme; and (3) vertical diffusion calculated using the turbulent closure method of Mellor and Yamada (1974). The component tendencies, which are positive (negative) if a model grid gains (loses) mass, are utilized to examine the impact of different transport mechanisms on the temporal evolution of SF6
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