The maps found on these pages are derived from the Ertel potential vorticity (PV), and are useful for understanding atmospheric dynamics. Because the PV usually jumps in value by approximately an order of magnitude over a short vertical distance near the tropopause, one may define the tropopause as a PV surface. This 'dynamical' definition of the tropopause is adopted here; specifically, we use the 1.5 PVU surface.
Tropopause maps are derived by a two step process. The first step involves computing the PV from gridded data fields on the sphere from 1000 hPa to 10 hPa. The second step involves searching for the tropopause surface in the 3D PV space. Here we search for the tropopause at each grid point by starting at 10 hPa (PV >> 1.5) and working down until 1.5 PVU is crossed. Any field [e.g., potential temperature ("theta")] may then be interpolated to this surface.
Isentropic maps are determined in a similar manner, by searching for the appropriate potential-temperature surface, and interpolating quantities to that surface.
The topography of the tropopause is summarized by both the pressure and potential temperature distributions. Often, these fields show elongated ribbons of very large gradients ("fronts") that normally define the location of jet streams. Unlike constant-pressure charts, all tropopause-based jets appear here on one map (e.g., polar jet and subtropical jet). Rossby waves appear as undulations of these gradients, and coherent vortices appear as regions of closed potential temperature contours (isolated fluid).
Assuming adiabatic and frictionless conditions, both the PV and potential temperature are conserved following the motion. Therefore, the dynamics of the tropopause may be visualized by the advection of potential temperature on the tropause. Conversely, regions of nonconservation associated with convection may be noticed as regions of high potential temperature that appear suddenly in a region where no such values existed earlier.
An example use of tropopause maps in a research setting, along with references to other papers, can be found in:
Hakim, G. J., L. F. Bosart, and D. Keyser, 1995: The Ohio Valley wave-merger cyclogenesis event of 25-26 January 1978. Part I: Multiscale case study. Mon. Wea. Rev, 123, 2663--2692.
A more general discussion of tropopause maps can be found in:
Morgan, M.C. and J.W. Nielsen-Gammon, 1998: Using tropopause maps to diagnose midlatitude weather systems, Mon. Wea. Rev, 126, 2555--2579.