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EHI and lapse rates
06-25-2010, 04:56 PM (This post was last modified: 06-25-2010 04:57 PM by NewEnglandWeatherMan.)
Post: #1
EHI and lapse rates
Can somebody explain to what EHI and lapse rates and helicity are just learned about cape j/kg the other day. Thanks!
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06-27-2010, 07:34 AM
Post: #2
EHI and lapse rates
Helicity is a mathematical quantity derived from: 1. speed shear (how much wind speed increases with height) between the surface and 2 km or 3 km above, 2. directional shear (how much wind speed changes direction with height) between the surface and 2 km or 3 km above, 3. The strength of the low level wind directly into the speed and directional wind shear. The stronger each of these components are, the higher the helicity. It is measured in meters per second.

Guidelines

150-300 Possible supercell
300-400 Supercells favorable
400+ Tornadic possible

A lapse rate is the rate of temperature change with height. The faster the temperature decreases with height, the steeper the lapse rate and the more unstable the atmosphere becomes. Lapse rates are typically displayed in ranges from 850-500-mb (4,500-18,000-ft above sea level) and 700-500-mb (10,000-18,000-ft above sea level). Lapse rates are shown in terms of degrees Celcius change per kilometer in height.


Guidlines

Values less than 5.5-6.0 degrees C/km are stable conditions.
Values near 9.5 degrees C/km are considered absolutely unstable
In between these two values, lapse rates are considered conditionally unstable. Conditional instability means that if enough moisture is present, the atmosphere will be unstable.

EHI (Energy-Helicity Index). The basic premise is that storm rotation will be maximized when CAPE is large and helicity is large. If you multiple cape and helicity and divide that sum by 160,000, you will get the EHI.

Guideline

0-1-km EHI values greater than 1-2 have been associated with significant tornadoes in supercells.

I hope this helps.
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06-27-2010, 07:45 AM
Post: #3
EHI and lapse rates
Thanks so much Steve!
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