METR 361 Spring,
2018
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 principal 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 CAPE.
When the parcel is cooler than the sounding, subtract the area from
CAPE. Values around 1500-2000 are high.
CAPE has substantial variability.
CINH (Convective Inhibition)
The
convective inhibition is a measure of the cap or lid. As in CAPE, lift a
parcel. Before it gets to its level of free convection (LFC), it will have
negative buoyant energy, i.e., negative CAPE. That’s CINH.
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 Thresholds and Critical Values
These are only suggested
values that should be applied with caution (or refined) to local areas.
|
|
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 |
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 (hPa) |
T (°C) |
Td (°C) |
DDFFF (knots) |
|
Level (hPa) |
T (°C) |
Td (°C) |
DDFFF (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 |
|||
|
Level (hPa) |
T (°C) |
Td (°C) |
DDDFF (knots) |
|
100.0 |
-69.1 |
-75.1 |
24545 |
|
145.0 |
-72.1 |
-77.1 |
25559 |
|
150.0 |
-70.5 |
-75.5 |
25556 |
|
200.0 |
-56.7 |
-62.7 |
25080 |
|
250.0 |
-46.1 |
-52.1 |
26077 |
|
300.0 |
-37.7 |
-50.7 |
25588 |
|
355.0 |
-27.6 |
-60.0 |
25590 |
|
400.0 |
-21.5 |
-64.5 |
25587 |
|
462.0 |
-15.3 |
-41.3 |
25281 |
|
500.0 |
-10.7 |
-17.7 |
24572 |
|
560.0 |
-5.7 |
-12.8 |
22589 |
|
617.0 |
0.4
|
-9.6 |
22581 |
|
652.0 |
4.4
|
-7.2
|
22578 |
|
685.0 |
8.0
|
-5.0
|
22577 |
|
700.0 |
9.0
|
-3.0
|
22576 |
|
728.6 |
11.0 |
0.8
|
22576 |
|
754.0 |
12.8 |
5.8
|
22572 |
|
783.3 |
14.3 |
9.0
|
22563 |
|
850.0 |
17.8 |
16.2 |
21560 |
|
893.0 |
20.0 |
19.0 |
20866 |
|
935.0 |
22.2 |
20.5 |
19553 |
|
983.0 |
26.0 |
22.0 |
18024 |