
CALCULATING CSI
PROPER METHODOLOGY
In a followup manuscript (Schumacher and Schultz, in preparation), we
will advocate a process to correctly assess conditional symmetric
instability. This methodology can be applied either in an operational
environment to forecast slantwise convection or in a research mode to
assess whether slantwise convection may have occurred in a particular
case study.
These seven steps are:
1. Construct a cross section of frontogenesis.
2. Generate a horizontal map of frontogenesis at the level determined
from step 1 and MPVg* slightly above that level.
3. Construct a horizontal map of frontogenesis and an upperlevel
forcing mechanism.
4. Construct a cross section of MPVg* and thetae*.
5. Choose a level(s) where MPVg* is negative and create a horizontal
map of MPVg* and d thetae*/dp on the same level(s).
6. Generate cross section and horizontal maps overlaying MPVG* and
relative humidity.
7. Compare the characteristics associated with the band(s) with those
predicted by moist symmetric instability theory.
There are better ways to do this in a single panel or two. See the COMET CSI Webcast for
some easy diagnostic ideas.
AWIPS
Phil has created the ability to examine MPVg* (using the geostrophic
wind and thetaes) in AWIPS. The code for AWIPS is available from Dan
Baumgardt's Volume Browser homepage (http://www.crh.noaa.gov/arx/vb/index.htm).
A comparison between the results in GEMPAK and in AWIPS is favorable.
An exact match, however, is not possible because AWIPS uses the bell
filter on the height field when computing the geostrophic wind whereas
GEMPAK does not. But, the match was close enough that Phil has
confidence in the results.
GEMPAK
Below we make available GEMPAK scripts that create figures similar to
those in our manuscript that can be used to assess the susceptibility
of the atmosphere to slantwise convection in the same manner as we do.
The code is selfexplanatory and welldocumented.
PCGRIDDS
As far as we are aware, PCGRIDDS cannot calculate the fully
threedimensional form of MPVg*. Therefore, the formulas in the
appendix of Wiesmueller and Zubrick (1998) are not the most accurate
way of assessing MSI, particularly in regions of strong curvature or
alongfront variations.
If you have your own code or tips on how to assess moist symmetric
instability that you would like to contribute, please let us know.
We're happy to share ideas amongst the internet community, if they
lead to good science and forecasting practice.
Last update: 22 May 2001
