Toward regional-scale modeling using the two-way nested global model TM5: Characterization of transport using SF6

Abstract

We present an evaluation of transport of sulfur hexafluoride (SF 6) in the two-way nested chemistry-transport model "Tracer Model 5" (TM5). Modeled SF6 values for January 2000 to November 2003 are compared with NOAA CMDL observations. This includes new high-frequency SF6 observations, frequent vertical profiles, and weekly flask data from more than 60 sites around the globe. This constitutes the most extensive set of SF6 observations used in transport model evaluation to date. We fmd that TM5 captures temporal variability on all timescales well, including the relatively large SF6 signals on synoptic scales (2-5 days). The model overestimates the meridional gradient of SF6 by 19%, similar to previously used transport models. Vertical profiles are reproduced to within the standard error of the observations, and do not reveal large biases. An important area for future improvements is the mixing of the planetary boundary layer which is currently too slow, leading to modeled SF6 mixing ratios that are too large over the continents. Increasing the horizontal resolution over North America from 6×4°, to 3×2°, to even 1×1° (lon×lat) does not affect the simulated global scale SF 6 distribution and potentially minimizes representation errors for continental sites. These results are highly relevant for future CO2 flux estimates with TM5, which will be briefly discussed. Copyright 2004 by the American Geophysical Union.