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Stability Index Values

Introduction

The stability measures displayed below are taken from a study by Rasmussen and Blanchard (1998). This study examined over 6,000 soundings that occurred close in time and space to severe weather. Several indices and parameters were evaluated for these soundings, divided into three classifications, and then compared. values are presented below.

Basic Definitions

The following criteria were used to define the three classes of storms.

Category	No. of Soundings	Criteria
TOR	51	One or more tornadoes having damage rated as F2 or greater
SUP	119	One or more reports of hail > 2 inches in diameter, but no tornadoes have damage > F1 (F0 and F1 allowed)
ORD	2767	10 or more CG lightning flashes, but no reports of large hail, tornadoes, or wind damage allowed

Boundary Layer to 6-km Shear

The magnitude of the shear vector between the mean wind in the 0-500 m AGL layer and the wind at 6 km AGL was evaluated.

m/s	Percentiles
Category	10%	25%	median	75%	90%
ORD	3.0	5.7	10.8	15.7	22.0
SUP	8.1	12.1	19.1	22.1	25.8
TOR	4.7	13.6	18.4	21.8	29.0

Storm-Relative Helicity

This parameter was calculated over the lowest 3 km layer.

m²/s²	Percentiles
Category	10%	25%	median	75%	90%
ORD	-2	17	55	100	168
SUP	25	64	124	208	304
TOR	68	100	180	279	411

Mean Shear in 04 km AGL Layer

km x 10^-3/s	Percentiles
Category	10%	25%	median	75%	90%
ORD	2.79	3.61	5.32	6.42	8.09
SUP	4.52	5.23	6.92	7.83	9.44
TOR	4.52	5.60	8.02	9.44	10.29

Storm-Relative Upper-Tropospheric Wind Speed

This parameter evaluates the storm-relative winds for a 2 km-deep layer centered around -40^oC level of the sounding.

m/s	Percentiles
Category	10%	25%	median	75%	90%
ORD	3.0	6.3	12.0	19.0	27.2
SUP	5.0	8.7	17.7	25.7	36.1
TOR	5.6	10.3	19.5	25.4	29.5

Convective Available Potential Energy (CAPE)

kJ/kg	Percentiles
Category	10%	25%	median	75%	90%
ORD	0	0	537	1094	1821
SUP	0	283	1152	1821	2453
TOR	66	519	1314	1877	3028

CAPE in the First 3 km above the LFC

J/kg	Percentiles
Category	10%	25%	median	75%	90%
ORD	70	182	294	390	480
SUP	174	307	397	541	636
TOR	216	313	394	531	719

Bulk Richardson Number

This parameter combines CAPE with the boundary layer-6 km shear previously presented.

	Percentiles
Category	10%	25%	median	75%	90%
ORD	0.19	1.5	9.6	40	140
SUP	0.94	2.0	6.6	17.3	34
TOR	1.13	4.2	7.5	13.7	20.8

Energy-Helicity Index

This parameter combines CAPE with the storm-relative helicity previously presented.

	Percentiles
Category	10%	25%	median	75%	90%
ORD	-0.03	0.0	0.14	0.34	0.77
SUP	0.0	0.20	0.64	1.46	2.87
TOR	0.11	0.42	1.48	2.87	4.05

Lifting Condensation Level

km	Percentiles
Category	10%	25%	median	75%	90%
ORD	0.30	0.56	0.88	1.41	2.02
SUP	0.52	0.93	1.23	1.57	1.87
TOR	0.16	0.45	0.78	1.18	1.47

Convective Inhibition

J/kg	Percentiles
Category	10%	25%	median	75%	90%
ORD	0	0	18.0	35.9	84
SUP	0	0	35.0	69.9	135
TOR	0	0	12.0	20.8	117

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References

Rasmussen, Erin N., and David O. Blanchard, 1998: A Baseline Climatology of Sounding-Derived Supercell and Tornado Forecast Parameters. Weather and Forecasting, 13, 4, 1148-1164.

last updated on 3/09/10