When I was first introduced to potential vorticity, I was told that it is a very important tool for meteorologists to learn. Sure, it solved a simple problem in dynamics, but it wasn't apparent to me at the time on why it was important.

As I progress, I find that it becomes more and more useful in finding notable features, such as tropical cyclones and fronts. That being said, why isn't it plotted more often? I don't find plots of potential vorticity often, but when I do, it is often plotted for higher in the troposphere, and not where features are commonly found.

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    $\begingroup$ @BaroclinicCplusplus - perhaps the difficulty in calculating it. You essentially want an isentropic PV which is not available on model charts. Either you have sigma coordinates(GFS), model levels(ERA ECWMF), SLEVE coordinate(ICON), isobaric levels(NOAA NCAR reanalysis). So you are now talking of a interpolation from these vertical coordinate formats to an isentropic coordinate. Some do a two step interpolation from terrain based coordinate (such as sigma, model levels) to isobaric to isentropic. So errors are compounded. $\endgroup$
    – user1066
    Jul 7, 2018 at 5:44
  • $\begingroup$ Interpolation is non trivial- elimination of superadiabatic layers , intersections of isentropic surfaces with the earth surface, on a daily basis the isentropic surface of interest varies i.,e. it could be 350 K or 315 K or 320 K. So there are many many difficulties. I have a github project I can share in which I highlight these difficulties. If you are interested I can share a link offline $\endgroup$
    – user1066
    Jul 7, 2018 at 5:47
  • $\begingroup$ @BaroclinicCplusplus - let me know what you think and I can then write it up as an answer. $\endgroup$
    – user1066
    Jul 7, 2018 at 11:54
  • $\begingroup$ @gansub You don't have to calculate PV on an isentropic surface. Sure, it eliminates the horizontal components of $\nabla \theta$, but those can be calculated. The vertical components can be calculated using the geopotential height of adjacent surfaces. I do know of one product that calculates PV (specifically Ertel's PV), - MERRA2 from NASA. But that is a reanalysis, not a model used for forecasting. $\endgroup$ Jul 7, 2018 at 20:03
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    $\begingroup$ This is long overdue, but with @gansub bringing it up, I thought I'd share this: atmos.washington.edu/~hakim/tropo/info_php.html. These are the maps that were referenced in all my dynamics classes. $\endgroup$ Apr 23, 2019 at 10:02

1 Answer 1


This one has not been answered for a long time and I am going to summarize what I wrote in the comments. If I understood OP's question correctly I believe it is asking why Potential Vorticity is not shown in weather maps in a operational sense.

From this old(but still very useful reference) Isentropic Potential Vorticity presuming isentropic surface are available operationally(AFAIK GFS provides isobaric PV surfaces Warning: You need to register to access the GFS link) and you need to interpolate from isobaric to isentropic.

Generally, as in constant pressure analyses, one would like to use at least two or three isentropic surfaces to analyze. However, unlike constant pressure analyses, one has to choose different isentropic levels depending upon the time of year.

| Season | Low Level Potential Temperature |

| Winter | 290-295 K

| Spring | 295-300 K

| Summer | 310-315 K

|Fall | 300-305 K

As one can see the correct isentropic surface to choose for a particular day can be quite a challenge. Also each part of the world may have it's own low level potential temperature complicating the problem.

Isentropic surfaces tend to intersect the ground at steep angles causing analyses near where they intersect the ground to be suspect. One must be careful about over-interpreting analyses made near where the isentropic surface intersects the earth. One would do best to choose isentropic levels which never reach the ground. However, to depict moist/dry tongues associated with synoptic scale systems one tries to go as low as possible without hitting the ground. So there are trade-offs; as usual, there are no quick-fixes!

In my part of the world(tropics) an all season isentropic surface that does not intersect with the earth's surface globally is the 370 K isentropic level. From a isobaric perspective this turns out to be around 200 hPa from a perspective of climatology.

As one can note this is at a very high level and corresponds to the level of the jet stream. As one analyzes lower isentropic levels one can see maps (you need 3-D to plot PV on an isentropic surface) that intersect the earth's surface and when that happens you need sophisticated extrapolation methods to calculate PV especially along orography(and these turn to be generally inaccurate). Over the tropics generally at lower isentropic levels PV surfaces tend to be perpendicular and their utility is restricted to as OP puts it to the inside of tropical cyclones and mesoscale convective systems.

An example of using PV at lower levels of the atmosphere can be seen in this youtube video Sandy PV filaments

  • $\begingroup$ I'm coming into this pretty late, but I am also curious about the focus on isentropic surfaces. I know the dynamics of PV (where features will move to in the future) makes more sense on isentropic surfaces, but I would think snapshots of PV on any kind of surface would still be useful for just identifying the features. $\endgroup$ Apr 23, 2019 at 10:01
  • $\begingroup$ @JarethHolt The document that I cited by Dr. Moore goes through the value of isentropic surfaces in great detail. In the absence of diabatic heating vertical motion along a isentropic surface captures the adiabatic "lift" $\endgroup$
    – user1066
    Apr 23, 2019 at 10:20

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