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Dynamic tropopause and potential vorticity (2 PVU)

Also called: dynamic tropopause map, DT map, theta on 2 PVU, potential temperature on the dynamic tropopause, potential vorticity, PV

The dynamic tropopause is the surface of constant potential vorticity (PV) that separates stratospheric air (high PV) from tropospheric air (low PV), conventionally the 2-PVU surface, where 1 PVU = 10⁻⁶ K m² kg⁻¹ s⁻¹. In isentropic coordinates Ertel's PV is

$$P = -g\,(\zeta_\theta + f)\,\frac{\partial\theta}{\partial p}$$

It is conserved following the flow in the absence of heating and friction and, given a balance condition, the whole balanced flow can be recovered from it. A map of the dynamic tropopause therefore shows the upper-level circulation in a single field.

How to read a dynamic tropopause map

• Potential temperature on 2 PVU is the usual variable. Low values (cold colours) are the stratosphere reaching down: troughs and cut-off lows. High values (warm colours) are tropical air lifting the tropopause: ridges.
• A tight gradient of θ on the tropopause is the jet stream.
• A tongue of low θ that thins and wraps up is a Rossby wave breaking; cyclonic and anticyclonic breaking lead to different downstream weather.
• PV on the 330 K and 350 K isentropes shows the same structure on surfaces near the winter jet and near the summer and subtropical jet.

The live forecast

Ertel PV is computed from the temperature and wind of AIFS ensemble member 0 on nine levels from 700 to 100 hPa, every 12 hours to day 10. A single member is used on purpose: potential vorticity does not survive ensemble averaging, because the mean of fifty differently placed folds is a smear. The 2-PVU surface is found by searching upward from 700 hPa, and isentropic PV by interpolation in θ.