The conventional definition of the Tropopause is the Lapse–rate Tropopause but more recently the so-called dynamical tropopause has become popular. Dynamic tropopause is used with potential vorticity instead of vertical temperature gradient as the defining variable. There is no universally used threshold, but the most common ones are that the tropopause lies at the 2 PVU (potential vorticity unit) or 1.5 PVU surface. This threshold will be taken as a positive or negative value (e.g., 2 and -2 PVU), giving surfaces located in the northern and southern hemisphere respectively. To define a global tropopause in this way, the two surfaces arising from the positive and negative thresholds need to be joined near the equator using another type of surface such as a constant potential temperature surface as illustrated in Fig. 1.8. A PV anomaly is produced by the intrusion of stratospheric air into the upper troposphere. An upper level PV anomaly advected down to middle troposphere is called tropopause dynamic anomaly or folding of the dynamical tropopause. Due to PV conservation, the anomaly leads to deformations in vertical distribution of potential temperature and vorticity. In a baroclinic flow increasing with height, the intrusion of PV anomaly in the troposphere produces a vertical motion. The deformation of the isentropes imposes ascending motion ahead of the anomaly and subsiding motion behind it.
For conservative flow, the dynamical tropopause (unlike the thermal tropopause) is a material surface; this is an advantage for instance when considering the exchange of mass across the tropopause (Wirth 2003; Wirth and Szabo 2007). Despite overall similarities between the thermal and dynamical tropopause, they are certainly not identical and in specific situations there may be significant differences. For an atmosphere at rest potential vorticity (PV) is essentially a measure of static stability, and one can basically enforce both tropopauses to be at the same altitude through a suitable choice of the PV value for the dynamical tropopause.
A current example