March 14th, 2007

Supercell_Composite_Parameter Tool: Eric Evenson (WFO BTV), Josh Smith
(WFO BOI), and Tom LeFebvre (GSD)

IFPS build used to update the tool: IFPS 17+

Data requirements for the tool: NAM12, NAM40 and RUC80 model for the
following parameters and levels:

> Surface based CAPE, 0-3km Storm Relative Helicity, wind fields at 50
> mb intervals from the wind 10 meters above ground to the 500 mb wind.

Recent research by members of the Storm Prediction Center (SPC)
indicated utility in the use of a product called the Supercell Composite
Parameter (SCP). This technique is intended to help operational
forecasters in their efforts to discriminate between supercell and
non‑supercell environments. The SCP incorporates MUCAPE (CAPE based on
the most unstable parcel in the lowest 300 mb), 0‑3km storm‑relative
helicity (SRH, Davies‑Jones et al. (1990)), and the denominator of the
Bulk Richardson Number (BRN, Weisman and Klemp (1982), Stensrud et al
(1997)). Each component is normalized to supercell \"threshold\" values
based on these previous studies, as well as parameter distributions from
Thompson et al. (2002), with the following formulation for the SCP:

SCP = (MUCAPE/1000 Jkg‑1) \* (0‑3km SRH/150 m2s‑2) \* (BRN
denominator/40m2s‑2)

A sounding with 1000 J/kg MUCAPE, 150 m2/s2 0‑3km SRH, and 40 m2/s2 BRN
shear term will result in an SCP value of 1. This SCP formulation was
applied to 458 supercell proximity soundings and 75 discrete
non-supercell storms. To see the paper on this technique, go to
[http://www.spc.noaa.gov/publications/thompson/sigtor.pdf]{.underline}
(Note: this project has been submitted to Weather and Forecasting and in
the review process it was noted that the normalized value for 0-3km SRH
should be 100 m2/s2 rather than 150 m2/s2)

The Supercell_Composite_Parameter SmartTool uses the findings from the
Thompson, Edwards, and Hart (2002) paper to develop this tool. The tool
collects the model forecast fields of surface CAPE, 0-3km Helicity, and
surface to 500 mb wind data (to calculate a 0-6km mean shear value). The
0-6km mean shear value, which is not density weighted, is calculated by
adding the magnitudes of 50 mb layers from the wind 10 meters above the
ground to the 500 mb level. Using their database, the authors of the SCP
calculated a normalized value for 0-6km shear (15 m/s). In addition, the
SCP formula uses the most unstable CAPE (MUCAPE), but the CAPE used for
this tool is surface based. As a result, some discrepancy in CAPE values
can occur. For this smart tool, the SCP formula is as follows:

SCP = (SFC CAPE/1000 Jkg‑1) \* (0‑3km SRH/100 m2s‑2) \* (0-6km shear/15
m/s)

If the SCP is greater than 1, the tool will generate a weather grid that
adds user defined attributes of winds and hail to the thunderstorm
weather string (Damaging Winds and Large Hail or Gusty Winds and Hail).
If Damaging Winds and Large Hail is selected as an attribute, T+ will be
coded to represent a severe thunderstorm. A GUI allows the user to
ability to decide if the qualifying terms should be in uncertainty or
areal coverage terms. The qualifiers are determined from the user=s PoP
grid. If the SCP is less than 1, thunderstorms with no attributes will
be plotted where the PoP is greater than or equal to 15.

Limitations of this tool include model availability from the ETA and
MesoEta only. While the RUC can be useful in short term forecasting,
CAPE fields are not available in all versions of the RUC (RUC40 and
RUC80). Further development of the tool will include the addition of the
RUC once the necessary fields are available. Also, the GFS does not have
a CAPE field nor the detailed wind levels to create a 0-6km shear. In
addition, the meteorological data used in the ETA is only available in
six hour intervals and may not represent the most favorable environment.
Finally, non-supercell storms can produce severe weather, and the
appropriate attributes should be included in the Wx grid.

References

Davies‑Jones, R. P., D. W. Burgess, and M. Foster, 1990: Test of
helicity as a tornado forecast parameter. Preprints, 16th Conf. on
Severe Local Storms, Kananskis Park, Alberta, Canada, Amer. Meteor.
Soc., 588‑592.

Stensrud, D. J., J. W. Cortinas Jr., and H. E. Brooks, 1997:
Discriminating between tornadic and nontornadic thunderstorms using
mesoscale model output. Wea. Forecasting, 12, 613‑632.

Thompson, R.L., R. Edwards and J. A. Hart, 2002: Evaluation and
Interpretation of the Supercell

Composite and Significant Tornado Parameters at the Storm Prediction
Center. Preprints, 21st

Conf. Severe Local Storms, San Antonio.

Weisman, M. L., and J. B. Klemp, 1982: The dependence of numerically
simulated convective storms on wind shear and buoyancy. Mon. Wea. Rev.,
110, 504‑520.