Stratospheric Brewer-Dobson Circulation and Age of Air
Beatriz Monge-Sanz, Sandip Dhomse, Wuhu Feng, Martyn Chipperfield
Background
Chemical transport models (CTMs) rely on the accuracy of the forcing wind and temperature fields to obtain realistic tracer distributions. Long-term stratospheric studies need fields thataccurately represent the stratospheric circulation, i.e. the combined effects of the Brewer-Dobson (B-D) circulation and the mixing processes.
Using analysed (or assimilated) fields has the advantage of making CTM results directly comparable with observations. However, several studies based on long-term CTM simulations have shown that winds from (re)analyses (e.g. ERA-40, Met Office ‘UARS’, GEOS-4) can produce too strong a stratospheric circulation [e.g., van Noije et al., 2004; Meijer et al., 2004; Scheele et al., 2005; Chipperfield, 2006]. The underestimation of the stratospheric age-of-air for a wide range of altitudes and latitudes implies that models overestimate the strength of the B-D circulation, which clearly affects their ability to reproduce observed tracer distributions.
We have evaluated recent ECMWF (re)analyses products to assess their accuracy in terms of representing stratospheric transport processes.
Results
Figure 1 shows the annual mean of the mean age-of-air at 20 km obtained from the TOMCAT/SLIMCAT CTM driven by different ECMWF products and by UKMO ‘UARS’ reanalyses. The modelled age-of-air is compared to ER-2 aircraft observations of CO2 (Andrews et al., 2001) and SF6 (Ray et al., 1999). It can be seen how the recent EXP471 dataset (experimental dataset basis for ERA-Interim) produces age-of-air values in much better agreement to observations. EXP471 also results in a more realistic latitudinal gradient, indicating additional improvements in subtropical mixing processes.
Conclusions
Developments in the ECMWF data assimilation system have contributed to a significant improvement in the stratospheric circulation. TOMCAT/SLIMCAT simulations with ERA-Interim experiemental datasets result in much more realistic age-of-air distributions and more constrained (realistic) subtropical mixing, compared to any previous (re)analyses products.
Publications
Dee DP; Uppala SM; Simmons AJ; et al. (2011) The ERA-Interim reanalysis: Configuration and performance of the data assimilation system, Q.J. Roy. Meteorol. Soc., 137, pp.553-597. doi: 10.1002/qj.828.
Monge-Sanz, B., M.P. Chipperfield, A. Simmons, and S. Uppala,
Mean age of air and transport in a CTM: Comparison of different ECMWF analyses
Geophys. Res. Lett., 34, L04801, doi:10.1029/2006GL028515, 2007.