A Study of the Role of Filamentary PV Mixing on the Sharpness of the Extratropical Tropopause

Abstract:

Birner's work (2006) showed that the extratropical tropopause is extremely sharp when the mean tropopause structure is plotted using the tropopause altitude as the reference level. Wirth suggested that this sharpness is largely control by the upper tropospheric relative vorticity. It has been suggested that this effect, while important, is not sufficient to explain the observed extratropical tropopause sharpness, and that radiative processes provide the needed additional sharpening. Furthermore, there are large annual and latitudinal variations in this tropopause sharpness, but the reasons for these remain unexplained.

By analysis of observations and modeling, we suggest that dry baroclinic mixing is sufficient to explain the extratropical tropopause sharpness and its latitudinal and annual variations. We revisit some earlier work by Son and Polvani (2007), who showed that in their dry idealized global model, the tropopause sharpness depended more on horizontal resolution than on vertical resolution, but they gave no explanation. Our work has reproduced these resolution-dependent results, and we have sought to explain these results by a series of diagnostics. Our results suggest that while the Wirth control by upper tropospheric relative vorticity is true on synoptic time scales, the climatological control is much more related to the downward flux of potential vorticity by baroclinic mixing, with heat fluxes playing a lesser role. Our results suggest that baroclinic mixing of potential vorticity is the cause of the observed climatological extratropical tropopause sharpness, and that the resolution dependence of this sharpness is a result of more vigorous baroclinic activity for higher horizontal resolutions as well as the greater resolution of filamentary mixing of potential vorticity. Also, we suggest that the annual variation of the extratropical tropopause sharpness follows the annual migration of the latitude of the jet stream.

Birner (2006, J. of Geophys. Res., 111, D04,104, doi:10.1029/2005JD006,301)

Wirth (2001, J. Atmos. Sci., 58, 26-37)

Son, S.-W., and L. M. Polvani (2007, Geophys. Res. Lett., 34, L17806, doi:10.1029/2007GL030564)