METR 361                                                                                                                         Spring, 2016   

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 CAPE.   When the parcel is cooler than the sounding, subtract the area from CAPE. Values around 1500-2000 are high.  CAPE has substantial variability.

 

                   CIN (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 THRESHOLD AND CRITICAL VALUES

 

 

General Thunderstorms

 

Severe Thunderstorms

 

Unlikely

Possible

See Severe ->

 

Unlikely

Possible

Very Likely

LI

> 0

-1 to -2           

< -2

 

> -2

-2 to  -3

< -4

K

< 16

> 16

---

 

NA

NA

NA

TTI

< 44

44 to 50

> 50

 

< 50

50 to 55

> 55

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