Steve Zubrick asked:
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?

Dave answers:

Yes, this is a good question and one that we clearly didn't address as well as I would have liked within the webcast or the Schultz and Schumacher paper. Part of the problem is that, on one level, we don't know how to answer that question. On another level, it may not make any difference. Let me clarify.

The primary reason we got into this discussion of PSI versus CSI in the first place was because the terminology used was inappropriate. For the most part, we had people using theta-e to measure instability and calling it CONDITIONAL symmetric instability. Of course, when saturation is present, conditional and potential symmetric instabilities are the same, so in one sense, the point is moot. In another sense, the anal person in me is saying, "Hey, let's at least use the right terminology in the first place. Then we can debate over whether moist symmetric instability is occurring and whether the methodology is useful from a forecasting perspective."

Now, given that we use consistent terminology, what difference will making an assessment of CSI versus PSI make? That question remains unanswered. Phil suggested at one time that PSI (a layer instability) might be more appropriate for certain situations, whereas CSI (a parcel instability) might be more appropriate for other situations. Since no one has examined this from a research or operational perspective, I don't know what to tell you. This is an open question. I suspect that a knowledgeable forecaster will be able to identify when slantwise convection may occur using either PSI or CSI within the context of the ingredients-based methodology, therefore this point may be moot as well.

A different way to view the same question is the following. From Schultz and Schumacher:

The question naturally arises as to whether it is more appropriate to calculate CSI or PSI for assessing the possibility of moist slantwise convection in the atmosphere. Drawing an analogy to moist gravitational convection may help provide some insight. The necessary condition for moist gravitational convection is that a rising air parcel be saturated and the lapse rate be greater than the moist adiabatic lapse rate (CI), so that positive buoyancy exists. Meeting the lapse-rate requirement for PI or PSI is only a necessary condition for release of the instability; saturation must also occur. Consequently, the presence of PI is not necessary for moist gravitational convection to occur in the atmosphere (e.g., Emanuel 1994, 185). Therefore, throughout the rest of this article, we use the term CSI, computed with the proper thermodynamic variable theta-e*, as the appropriate measure of the susceptibility of the atmosphere to slantwise convection. Thus, many previous studies may not determine the true potential for slantwise convection because of their use of theta-e and PSI, rather than theta-e* and CSI. [Note also that you want to check for saturation with CSI, as well. Always use the ingredients-based methodology!]

Now, AWIPS is funny because Phil has looked at the fields of moist PV and gets answers different from that using GEMPAK on the same dataset! Since the source code is hidden for AWIPS, we have no way of knowing how the calculation is done. (Any progress on that front, Phil?) Therefore, I don't know if you can trust AWIPS to provide accurate assessments of moist symmetric instability. Is it using the geostrophic wind? Is it using theta-e, instead of theta-e*?

These points were worth making and I've made them about as clear and forceful as I ever have, so it may be worth adding this to the CSI page for others' benefit.

Thanks for the good question. Phil, you have any comments or differences of opinion?

Dave Schultz

Phil answers:

I agree with Dave. The research is really lacking on the difference between looking for PSI and its effect on assessing slantwise convection. My personal opinion is that until the research is done, there is nothing wrong with only examining PSI or CSI. Just like in your WAF paper [Weismueller and Zubrick 1998] (or with upright convection), I think the critical element is to identify when the atmosphere is primed for slantwise convection. I don't think any model (even at 10 km) is going to predict exactly where slantwise convection is going to occur. However, even the NGM can give indications that the potential exists when using the ingredients-based methodology. So once the forecaster recognizes the potential, they can recognize it more quickly. So when he/she sees banding developing in an area on the radar, one can "jump" on it with short-term forecasts, statements, etc, long before any significant snowfall has occurred.

From my experience, I have found that in most cases, using theta-E* tends to "brush" a larger area with CI and CSI than the "traditional" theta-E (PSI). Assuming that all the other ingredients are available, at times where CSI is indicated and and PSI is not, generally MPVg (using theta-E) will still be very near 0 so that one would anticipate that a single-band of heavy precipitation would occur due to a frontogenetical circulation. So I don't think we lose too much by looking at PSI but I have started to believe that CSI may be more appropriate from a theoretical standpoint.

As for AWIPS, I mentioned to Craig Hartsbough that I thought the EPV calculation was wrong in AWIPS. Of course, he has now moved onto a different job so I don't know how soon anyone will be checking it out. I also know of a couple of people trying to adapt the AWIPS PV calculation to do EPV. However, my concern is that AWIPS PV only uses Ertel PV (vertical stability * vertical vorticity). If that is the case (I am not sure if it is!!!), then EPV will only be less than zero when theta-E* decreases with height. That needs to be checked out.

Finally, I am on a AWIPS working group for improving Volume Browser. The group has indicated that it is necessary to get the correct EPV (with Saturated theta-E) calculation and change in Sat. theta-E with height into VB. We also are pushing for the ability to calculate any variable (but esp. EPV) within a user-defined layer (say 600-500 mb) since operational experience has shown that EPV is generally less than zero in a thin layer (< 200 mb thick) and using only set layers may cause one to misdiagnose the areal extent of the instability. (On a side note, we are also pushing to get Petersen frontogenesis included as well). Any fix is not until V5.0 at minimum.

I hope this was useful information. Take care.

Phil Schumacher
SOO, FSD