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These are the most popular questions that we get asked about CSI and how it works. Feel free to ask any questions of us.

Dave and Phil

Saturation Equivalent Potential Temperature and "Saturation"

Recently, we have been seeing a number of people using theta-e over theta-es to assess "CSI" because they argue that using theta-es to evaluate CSI implies some assumption about the atmosphere being saturated. The purpose of this FAQ is to address this concern.

For a definition of theta-e (equivalent potential temperature), see any good thermodynamics textbook. Theta-es is the theta-e that air *would have* if it were saturated. The theta-es of unsaturated air can be measured and and it has meaning. If you look at the mathematical expression of theta-es in Schultz and Schumacher (1999, footnote 1), you'll see that none of the variables are functions of moisture. Theta-es is a function of temperature and pressure only. Theta-e, on the other hand, is a function of temperature, pressure, and moisture.

I believe this is where the confusion over "saturation" comes from. Schultz and Schumacher (1999) are advocating using theta-es to assess CSI because we are just following the traditional definitions for upright instability: when theta-e decreases with height, that is PI. When theta-es decreases with height (equivalently defined as the lapse rate being between the moist and dry adiabatic lapse rates), that is conditional instability. Therefore, BY DEFINITION, CSI is assessed using theta-es and PSI is assessed using theta-e. There is no wiggle room to assert otherwise. Using theta-es does not imply that we make the assumption that RH is everywhere 100%.

An analogy can be drawn to dewpoint temperature. Fields of dewpoint temperature can be drawn in regions where the relative humidity is not 100%, but that does not imply that the whole field is saturated. Just because fields of potential temperature can be drawn does not imply that the whole field is at 1000 mb (the pressure at which the actual temperature of the air would be the potential temperature).

Hope that helps clarify this issue!


Q: In your "Use and Misuse of CSI" presentation, you indicate the requirement of using theta-e* (saturated equivalent potential temperature) versus theta-e (equivalent potential temperature) in evaluating the presence of CSI. However, theta-e is the available variable in AWIPS, not theta-e*. Which leads to my question: How much practical difference is there in replacing theta-e* with theta-e? It would seem that, in an area where adequate moisture is available for slantwise convection to be initiated, theta-e and theta-e* values should be reasonably close together. Please comment. - Val MacBlain, SOO Santa Teresa, NM (11/30/99)

Q: I had a question from a forecaster here...if one assesses the "Potential Symmetric Instability" (PSI) rather than CSI...what good does that do for ones' forecast?-Steve Zubrick, SOO Sterling, VA (11/30/99)

A: These are good questions, ones that Phil and I struggle with often. Here is our collective attempt to come to grips with this question.

Q: I wanted to ask a question about constructing cross sections to diagnose CSI. How critical is it to orient the line exactly perpendicular to the thickness lines? I know it must be perpendicular, but if it's off by 10 or 20 degrees, say, how much does that affect what you see on the cross sections? It would seem to me that the line needs to be as perpendicular as possible, but I would like to confirm that. The reason I'm asking is that one of our forecasters is trying to set up a procedure on AWIPS to do cross sections for CSI and he wants to set these up with 3 of 4 predefined baselines. I told him that might be risky at times, but wanted to ask the expert. Thanks very much for your time.-Loren Phillips, SOO, NWSFO Lubbock, TX(1/28/00)

A: Concerning orienting the cross-section line: As long as you are using negative MPVg* to ascertain the susceptibility of the atmosphere for slantwise convection, the cross section orientation is not particularly important. Of course, the easiest way to interpret cross sections is when they are perpendicular to the front or the jet. The problem that we point out in our paper with choosing cross-section orientations is *IF* you are using Mg-theta-e* diagnostics. In that case, your interpretation of the situation could be highly dependent on how your cross section orientation lies. That's why Phil and I (as well as Moore and others) advocate the moist PVg field as THE way to assess MSI.

With regards to the preplanned orientations that your forecaster wants to use, I don't see a problem with that for expedience, if you're using negative MPVg/MPVg*.

Hope that helps,


Q: Think you could easily explain the concept of geostrophic momentum to me? The textbooks treaments on it seem to be highly mathematical.

I was reading over a CSI paper (Wiesmuller/Zubrick) in the March 98 WAF journal, and one of the comments was:

"Frontogenetic forcing increases geostrophic vertical wind shear as the thermal wind responds to the enhanced thermal gradient. This in turn, makes geostrophic momentum surfaces flatter, making the region more prone to CSI."

Would a flat geostrophic momentum surface relate to a stable atmosphere?-Matt Rosier

A: Despite my thinking about it a lot, I do not have a nice description of geostrophic momentum for you (or for me). Rather than think about what geostrophic momentum is, I find it more satisfying to think about what the horizontal and vertical gradients of geostrophic momentum represent. The horizontal gradient of geostrophic momentum is equal to the geostrophic absolute vorticity. Thus, if you are looking at a cross section of Mg surfaces, areas where the Mg surfaces are packed in tight represent regions of high geo vorticity. The vertical gradient of geostrophic momentum is equal to the vertical shear of the geostrophic wind, which we know is roughly proportional to the horizontal temperature gradient (if in thermal wind balance). This explains the WZ98 quote.

Stable in what sense? An Mg surface that is nearly vertical would simply represent a barotropic atmosphere (no thermal gradient or vertical wind shear). A flat Mg surface would represent a region of zero vorticity, which might actually be inertially unstable.

Hope that helps!


Last update: 16 May 2001