Difference between Vertical Velocity and Lift Index

[Chris Harrell]

I am a bit curious as to what the differences between lifted index and vertical velocity are. Does lifted index (LI) affect upward vertical velocity (UVV) directly? Is the LI or UVV even vital forecasting units seeing how the UVV is directly linked within the CAPE measurements sqrt(CAPE * 2)?

 

[Blake Michaleski]

Lifted Index is an instability parameter. It is simply the temperature at 500mb minus the temperature of a parcel lifted from the surface to the 500mb level.

A negative number is unstable. as in the parcel temperature is warmer than the temperature of the air at 500mb. LI can be an observed or a forecast parameter


UVV is a forecast parameter that describes the general ascent (or descent) at a certain level. This is not convective upward motion, but rather isentropic ascent or descent. This is motion is in hydrostatic balance and is an indicator of forcing.

I dont use the UVV forecast fields to often, I prefer to simply look at Isentropic charts, you get an excellent idea of what the large scale ascent regime is.

So basically the two are not related, LI is a thermodynamic index where as UVV is a kinematic field.


Also, LI only gives you information about stability at one level.

CAPE is generally a better measure of instability as it basically takes into account the entire troposphere. The different ways CAPE is measured is useful to know. Surface based, mixed layer, and most unstable CAPE are all important to understand.

 

[Chris Harrell]

Thank you for the explanation Blake. When I was first doing my attempts at forecasting I looked at UVV quite a bit and never looked at LI. Luckily I never got shot in the foot while I did this. Now I've been looking at both, but I was somewhat curious if/how they were related. You told me exactly what I wanted to know.

I'll definitely look into the different measurements of CAPE. I always knew SBCAPE, MLCAPE, etc were different, but I didn't know how exactly.

 

[Bobby Prentice]

I am a bit curious as to what the differences between lifted index and vertical velocity are.

stability index—Any of several quantities that attempt to evaluate the potential for convective storm activity and that may be readily evaluated from operational sounding data. <table><tbody><tr><td><dd>In the definitions, T_p and D_p correspond to the temperature and dewpoint at pressure levels p. Those most commonly used are the following. </dd><dd><table><tbody><tr><td>Showalter stability index: An index given by

where T_L is the temperature (°C) of a parcel lifted from 850 to 500 mb, dry-adiabatically to saturation and moist-adiabatically above that. As the index decreases to zero andbelow, the likelihood of showers and thunderstorms is considered to increase (Showalter 1947).
</td></tr></tbody></table> </dd><dd><table><tbody><tr><td>K-index: This index is due to George (1960) and is defined by

The first term is a lapse rate term, while the second and third are related to the moisture between 850 and 700 mb, and are strongly influenced by the 700-mb temperature–dewpoint spread. As this index increases from a value of 20 or so, the likelihood of showers and thunderstorms is expected to increase.
</td></tr></tbody></table> </dd><dd><table><tbody><tr><td>Totals Indices: The Total Totals index is attributable to Miller (1972). It is defined as the sum of two indices:

where VT is the Vertical Totals index, defined by <center>

</center> Avalue of about 40 corresponds to a dry-adiabatic lapse rate. For a moist- adiabatic lapse rate it is about 20 for T₈₅₀ = 15°C, about 30 for T₈₅₀ = 0°C. The Cross Totals index, CT, is defined by

so is strongly influenced by the 850-mb moisture. Showers and thunderstorms become increasingly likely from TT values of about 30, and severe thunderstorms are considered likely for values of 50 or more.
</td></tr></tbody></table> </dd><dd><table><tbody><tr><td>Lifted index: This index, developed by Galway (1956), is

so is nominally identical to the Showalter index, except that the parcel being lifted (dry- adiabatically to saturation and then moist-adiabatically to 500 mb) is defined by the dry adiabat running through the predicted surface afternoon temperature maximum and the mean mixing ratio in the lowest 900 m of the sounding. If no further heating is expected, as with a sounding taken in the late afternoon, then the mean potential temperature in the lowest 900 m of the sounding defines the dry adiabat used for the parcel. Numerous variations, focused on how the lifted parcel is defined, have been used since the original definition. The values of this index tend to be somewhat lower than those of Showalter, and the interpretation depends to some extent on how the lifted parcel is defined.
</td></tr></tbody></table> </dd><dd><table><tbody><tr><td>SWEAT index: (Or severe weather threat index; also abbreviated SWI.) Another index attributable to Miller (1972), used mainly for analyzing the potential for severe thunderstorms, is defined as

where TT is the Total Totals index (set to zero if less than 49), V₈₅₀ and V₅₀₀ are the 850- and 500-mb wind speeds, and ΔV_{500 − 850} is the 500-mb wind direction minus the 850-mb wind direction, in degrees. The last term is set to zero if any of the following conditions are not met: 1) 850-mb wind direction is in the range from 130 to 250 degrees; 2) 500- mb wind direction is in the range 210 to 310 degrees; 3) the difference in wind directions is positive, or 4)both 850- and 500-mb wind speeds are at least 15 knots. No term in the formula is allowed to be negative. The severe thunderstorm threat is considered to increase from values of about 300 and higher; tornadoes are considered to increase in likelihoodfrom values of about 400 and up.
</td></tr></tbody></table>
Showalter, A. K., 1947: A stability index for forecasting thunderstorms. Bull. Amer. Meteor. Soc., 34, 250– 252.
George, J. J., 1960: Weather Forecasting for Aeronautics, Academic Press, 673 pp.
Miller, R. C., 1972: Notes on analysisand severe storm forecasting procedures of the Air Force Global Weather Central. Tech. Rept. 200(R), Headquarters, Air Weather Service, USAF, 190 pp.
Galway, J. G., 1956: The lifted index as a predictor of latent instability. Bull. Amer. Meteor. Soc., 528–529. </dd></td></tr></tbody></table>

vertical velocity—In meteorology, the component of the velocity vector along the local vertical.

Does lifted index (LI) affect upward vertical velocity (UVV) directly?

No.

Is the LI or UVV even vital forecasting units seeing how the UVV is directly linked within the CAPE measurements sqrt(CAPE * 2)?

The Lifted Index (LI) can be used to identify unstable air which is the second of three ingredients necessary for thunderstorm formation. Upward vertical velocities (UVV) can be used to identify lift which is the third of the three ingredients for thunderstorm formation.

The formula you mention can be used to estimate the speed of thunderstorm updrafts, but it assumes pure parcel theory which observational studies suggest often does not work in real thunderstorms. Updraft speeds in general (non-severe thunderstorms) is generally about 50 kt, but can approach 100 kt in supercells.

Generally, UVV's are referenced in regard to synoptic (several hundred to several thousand kilometers) and mesoscale weather (few to several hundred kilometers) systems, not thunderstorm updrafts which are meso-beta scale phenomena. UVV's are generally referenced on weather charts in units of centimeters per second.

 

[Chris Harrell]

Awesome Bobby! Thanks for all that information, this helps tremendously!