METR 361 Spring,
2017
Stability Indices
Assessing
stability of a sounding on a thermodynamic diagram is usually too
time-consuming a process considering the daily routine of a weather
office. This is especially true during
times when thunderstorms are likely to occur. For this reason indices have been
devised to give a rapid means for making stability assessments. The various
stability indices described below have been statistically linked to shower and
thundershower activity. When using any
of these indices for forecasting purposes, one must be careful to anticipate
changes that might occur in the environmental lapse rate between the sounding
time (usually 12Z) and the expected time of convective activity (usually 5-10
hours after 12Z).
Lifted Index (LI)
This is
probably the most well-known index, although its simplicity means it is no
longer the principle index used by the SPC.
The NWS definition is found at http://w1.weather.gov/glossary/index.php?word=lifted+index:
“A common measure
of atmospheric instability. Its value is obtained by computing the temperature
that air near the ground would have if it were lifted to some higher level
(around 18,000 feet, usually) and comparing that temperature to the actual
temperature at that level.”
To calculate
the LI, lift a parcel from the surface to its LCL. Then lift it moist- adiabatically to 500 hPa. Subtract the lifted parcel temperature from
the environmental 500 hPa temperature.
LI
= T500 - Temperature of lifted parcel
The LI works
well in severe weather situations with a conditionally unstable atmosphere and
a low level trigger for lifting, such as a cold front or pressure trough.
K Index
Subtract the
environmental 500 hPa temperature from the 850 hPa temperature. Add the 850 hPa dew point. Subtract the 700 hPa dew point depression.
K
= T850 - T500 + Td 850 - (T700 - Td
700)
The K Index
does not work well in severe weather situations because convectively unstable
atmospheres usually have mid-tropospheric dry tongues at 700 hPa. To maximize the K Index, the low level (850 hPa)
should be warm and moist, the upper level (500 hPa) cold and the 700 hPa dew
point depression should be small, indicating deep low level moisture.
Total-Totals Index
This index
consists of two sub-indices, the Vertical Totals and the Cross Totals. The Vertical Totals index is the 850 hPa
temperature minus the 500 hPa temperature.
The Cross Totals index is the 850 hPa dew point minus the 500 hPa
temperature. The Total-Totals index is
the sum of the Vertical and Cross Totals.
TT
= (T850 - T500) + (Td 850 - T500) =
T850 + Td 850 - 2T500
For the TT
to work well, the lower level (850 hPa) should be warm and moist and the upper
level (500 hPa) cold. It is a fairly
reliable severe thunderstorm predictor but is generally not used by the SPC.
Severe Weather Threat Index (SWEAT)
The SWEAT
Index was developed by the Air Force Global Weather Center severe storm group. It
puts more information into an index than any of the others listed. Wind speeds and shear are included. To
calculate the SWEAT Index, first multiply the 850 hPa dew point by 12. Then subtract 49 from the Total-Totals Index
and multiply the result by 20. Add twice
the 850 hPa speed to the 500 hPa wind speed.
Finally, subtract the 850 hPa wind direction from the 500 hPa wind
direction, take the sine, add 0.2 and multiply the result by 125. All negative terms are set to zero for the
SWEAT index.
SWEAT
= 12Td 850 + 20(TTI - 49) + 2w850 + w500 +
125(S + 0.2)
In SWEAT, Td
850 = 850 hPa dew point in °C
w850,
w500 = 850hPa and 500 hPa wind speeds in knots
S
= sin(500 hPa wind direction - 850 hPa wind direction).
This term is set to zero if either w850 or w500
are less than 15 knots. S is not
computed unless the 500 hPa wind direction is within the range 210° to 310° and
850 hPa wind direction is in the range 130° to 250°.
CAPE
(Convective Available Potential Energy)
This index
is calculated from a sounding on the thermodynamic diagram. Lift a parcel from the surface to the
tropopause. The parcel will saturate at
the LCL and rise moist-adiabatically.
Whenever the parcel is warmer than the sounding, add the area between
the parcel and the sounding to
CIN (Convective Inhibition)
The
convective inhibition is a measure of the cap or lid. As in
LCL (Lifting Condensation Level)
Not an index
but important. When lifting a parcel
from the surface, this is where condensation first occurs.
CCL (Convective Condensation Level)
This is the level where condensation occurs if a parcel lifts
itself from the surface due to positive buoyancy. You will need to heat the surface air until it
breaks the cap. The CCL will be higher
than the LCL. To find it, start at the
surface dewpoint. From there, lightly
draw a line parallel to the mixing ratio lines upward until it intersects the
temperature sounding line. That pressure
is the CCL because heating surface air to its Convective Temperature causes
rising along the dry adiabat.
LFC (Level of Free Convection)
This is
where negative buoyancy becomes positive.
In practical terms, this is where a lifted surface parcel (not one which
was heated as in the CCL) will rise on its own due to buoyancy.
SUGGESTED INDEX THRESHOLD AND
CRITICAL VALUES
|
GENERAL THUNDERSTORMS |
|
SEVERE THUNDERSTORMS |
||||
|
UNLIKELY |
POSSIBLE |
See Severe (à) |
|
UNLIKELY |
POSSIBLE |
VERY LIKELY |
|
|
|
|
|
|
|
|
LI |
> 0 |
-1 to -2 |
< -2 |
|
> -2 |
-2 to -3 |
< -4 |
TTI |
< 44 |
44 to 50 |
> 50 |
|
NA |
NA |
NA |
SWEAT |
< 150 |
150 to 300 |
>300 |
|
< 300 |
300 to 400 |
> 400 |
These are only suggested values that should be applied with caution (or
refined) to local areas.
ASSIGNMENT (due next Wednesday)
1. On thermodynamic diagrams, plot the two given
soundings for stations AAA and BBB. Plot
temperature, dew point and wind barbs to 100 hPa, using the usual format. For both AAA and BBB, do this:
2. Find the LCL and LFC of surface air, the Lifted
Index, the K Index, the Total-Totals Index, and the Severe Weather Threat
Index. Write these indices on a separate
sheet of paper. Show your work where possible.
3. Using the indices and levels from part 2, assess
the potential for severe weather by writing a discussion for a meteorologist to
read. Write your assessment on your separate sheet of
paper.
4. The last sounding is from Birmingham, AL, taken at 00Z on April
28, 2011. Plot the temperature, dew point,
and wind flags. Find the LCL of surface
air, the LFC, the CCL, the Lifted Index using surface air, the Total-Totals
Index, and the Severe Weather Threat Index (no K). Show your work where
possible. You must also estimate the surface temperature required for air
parcels to acquire the CCL. On the sounding, draw a parcel ascent line to 140
hPa which is the Equilibrium Level for this sounding. On your separate sheet of paper, assess the potential for severe
weather by writing a discussion for a meteorologist to read.
Station AAA at 12Z Station
BBB at 12Z
Level T Td DDFFF Level T Td DDFFF
(hPa) (°C) (°C)
(knots) (hPa) (°C) (°C) (knots)
100 -64.0 - 100 -72.1
- 26576
150 -59.7 -
24048 150 -62.7
- 26604
191 -59.1 - 200 -55.1
- 26626
200 -63.5 -
24084 250 -46.3
- 26621
250 -53.0 -
23597 300 -39.9 -48.9 27068
300 -44.6 - 23599 353 -30.9 -40.9
317 -40.0 -44.7 400 -26.3 -56.3 26540
385 -32.9 -33.8 458 -17.7 -47.7
400 -30.1 -30.2 23084 500 -13.7 -43.7 27029
444 -27.5 -36.2 579 -6.5 -36.5
500 -16.2 -20.7 23060 700 2.2 -2.8 27516
550 -10.0 -22.2 22540 729 2.6 1.6
700 7.2 -24.8
21542 766 5.6 5.1
757
12.0 -18.2 850 8.8 6.8 15521
769 7.3 7.2 872
10.8 6.4
850
13.2 12.8 19046 902
12.6 11.0
900
16.1 15.3 1000 17.6
16.5
11015
932
17.8 16.2 1016 18.8
17.5
10009
992
20.2 16.7 17015
Sounding
for Birmingham, AL (KBMX) for 00Z April 28, 2011
-------------------------------------------------------------------------------
PRES TEMP DEWP DIR SPD
PRES TEMP DEWP DIR SPD
hPa °C °C deg
knots
hPa °C °C deg knots
--------------------------------------------------------------------------------
983.0
26.0 22.0 180 24
462.0
-15.3
-41.3 252 81
935.0
22.2 20.5 195 53 400.0
-21.5 -64.5 255 87
903.0
20.6 19.5 205 67
355.0
-27.6 -60.0 255 90
893.0
20.0 19.0 208 66
300.0
-37.7 -50.7 255 88
871.7
18.5 17.2 215 65 250.0 -46.1
-52.1 260 77
850.0
17.8 16.2 215 60 200.0 -56.7
-62.7 250 80
817.0 16.4
12.2 224 62 150.0 -70.5 -75.5
255 56
783.3 14.3
9.0 225 63 145.0
-72.1 -77.1 255 59
754.0
12.8 5.8 225 72
135.0
-70.3 -75.3 260 61
728.6
11.0 0.8 225 76 100.0
-69.1 -75.1 245 45
700.0 9.0
-3.0 225 76
685.0
8.0 -5.0
225 77
652.0
4.4 -7.2
225 78
617.0
0.4 -9.6 225 81
560.0
-5.7 -12.8 225 89
500.0
-10.7 -17.7 245 72