A Study of the Extratropical Tropopause: Related to the Upper Tropospheric Relative Vorticity and the Distance from the Jet
Wang, Shu Meir
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The extratropical tropopause is a familiar feature in meteorology; however, the understanding of the mechanisms for its existence, formation, maintenance and sharpness is still an active area of research. In order to examine the sharpness of the tropopause, Birner et al. (2002) proposed the concept of the TIL (Tropopause Inversion Layer), which is based upon the characteristics of the thermal profile. Bell and Geller (2008), believing that the resulting stability profile was more fundamental to dynamics and transport, defined the ESTL (Extratropical Stability Transition Layer) based on the stability characteristics. Wirth (2004) suggested that the cyclonic/anticyclonic asymmetry plays a large role in the sharpening of the climatological TIL. Son and Polvani (2007) also showed that TIL sharpness was greater when the upper troposphere relative vorticity was anticyclonic than when there was cyclonic relative vorticity in their simplified atmospheric general circulation models. In contrast to this picture, we are finding that the location of the point where the extratropical tropopause is being determined with respect to the jet might be a more important factor in determining the height of extratropical tropopause. Furthermore, since Bell and Geller (2008) showed that the depth of the ESTL is mainly determined by the height of the extratropical tropopause, the same would be true of the sharpness. This may at first seem like a semantic argument, but we believe that there is an important distinction of the physical processes involved if the upper tropospheric vorticity is the dominant mechanism compared with the situation when the location with respect to the jet is dominant. In this study, three stations in the United States are used to examine the association between the tropopause height and the distance from the jet, and by comparison of the correlations between the tropopause height and the distance from the jet and that between the tropopause height and the upper tropospheric relative vorticity, the one which is more important in determining the tropopause height can be seen. We found that the distance from the jet is a more important in determining the tropopause height for the two northern stations, and for the south most station, the two correlations are statistically similar.