Stability Lab WX 352

1. Stability LabWX 352

2. Adiabatic Charts • The adiabatic chart is a valuable tool for anyone who studies the atmosphere • The chart itself is a graph that shows how the various atmospheric elements change with altitude • These charts are utilized to plot and display vertical profiles of temperature and moisture. • SkewT-Log P is a common tool used for vertical plots

3. Vertical axis is pressure in mb so horizontal lines are equal pressure. Skew-T Log-P Diagram Note the vertical axis has a logarithmic spacing.

4. The Skew-T Log-P Diagram Constant temperature lines are skewed and go diagonally on chart.

5. Combined pressure and temperature lines. The Skew-T Log-P Diagram

6. The Skew-T Log-P Diagram Dry adiabatic lines of constant potential temperature.

7. The Skew-T Log-P Diagram Moist adiabatic lines of equal pseudo-adiabatic potential temperature.

8. The Skew-T Log-P Diagram Combined lines moist and dry adiabats with skewT-log P.

9. The Skew-T Log-P Diagram Lines of constant mixing ratio. Dew Point for a vertically moving parcel is dependent on moisture and pressure so is not constant with height. Mixing ratio for a vertically moving parcel (not mixing with the environment) is constant with height Lifted parcels maintain their original mixing ratio.

10. The Skew-T Log-P Diagram Total Skew T log P chart

11. Skew-T Log-P

12. Skew-T Log-P (courtesy F. Remer)

13. Norman, Oklahoma 4/26/91 - 0000 UTC Example of use Temperature Dew Point Winds • Current Soundings

14. Lifting Condensation Level (LCL) • The level at which a parcel lifted dry adiabatically will become saturated. • Find the temperature and dew point temperature (at the same level, typically the surface). Follow the mixing ratio up from the dew point temp, and follow the dry adiabat from the temperature, where they intersect is the LCL.

15. Finding the LCL

16. Lifted Index • Lifted Index The lifted index (LI) is used for assessing/predicting the potential for thunderstorm activity. It identifies instability and low-level moisture, but does not address whether or not lifting mechanisms will be present. • The lifted index essentially is the difference between the environmental temperature at 500 mb  and the temperature of a parcel of air theoretically lifted from the surface or some reference layer to the 500 mb level under a number of simplifying assumptions: • LI = T500 mb - Tparcel at 500 mb

17. Lifted Index • If the parcel is warmer than the environment at that level, then LI is negative (unstable), and the parcel would have a tendency to keep on rising, producing a thunderstorm. • In practice, even LIs at a positive value of about +2 or less should be considered for convective development, as mechanisms may act to destabilize the atmosphere. • The more negative the LI, the stronger thunderstorms are likely to be, if they develop. • Even with a negative or unstable LI, thunderstorms may not develop if(1) there is a "capping" inversion above the surface, limiting or suppressing vertical motion.(2) no lifting mechanism ("trigger") is present.(3) the large scale pattern is one of subsidence (sinking air) so that lifting by trigger mechanisms is suppressed

18. Using Skew-T to calculate the LI • 1. Start with temperature at the surface on a thermodynamic diagram.  Draw a line paralleling the dry adiabats (potential temperature lines) from this temperature upward to around 850 mb (higher if necessary). • 2. Draw a second line from the surface dewpoint temperature following parallel to the mixing ratio lines up to where it crosses your dry adiabatic line from the surface temperature. • 3. The pressure at this level is the Lifting Condensation Level (LCL). • 4. From the LCL, follow parallel to a saturation or moist adiabatic line up to 500 mb.  The temperature at which this line crosses 500 mb is the parcel temperature at that level. • 5. The Lifted Index (LI) is given by using the formula on the previous page.

19. LI = T500 mb – T parcel at 500 mb T parcel T500mb T500mb Lifted Condensation Level

20. Lifted Index Scale • SCALE • 0 to -2RASH/SNSH possible • -3 to -5TS possible • -6 TS+ possible • -7 Tornado possible • -10 Move to Alaska

21. Level of Free Convection (LFC) • The LFC, or Level of Free Convection • height at which a parcel lifted dry adiabatically to saturation at the LCL and moist adiabatically above the LCL would first become warmer (less dense) than the surrounding air. • At the LFC, the parcel experiences positive buoyancy and starts to accelerate upward without further need for forced lifting

22. Example of LFC

23. LFC

24. Equilibrium Level • On a sounding, the level above the level of free convection (LFC) at which the temperature of a rising air parcel again equals the temperature of the environment. • The height of the EL is the height at which thunderstorm updrafts no longer accelerate upward. Thus, to a close approximation, it represents the height of expected (or ongoing) thunderstorm tops. However, strong updrafts will continue to rise past the EL before stopping, resulting in storm tops that are higher than the EL. This process sometimes can be seen visually as an overshooting top or anvil dome. • The EL typically is higher than the tropopause, and is a more accurate reference for storm tops

25. Example of finding the EL

26. Convective Condensation Level (CCL) • The level at which cumulus clouds form due to surface warming • Finding the CCL • Start at the surface dewpoint temperature • Follow a mixing ratio line up until it meets the ELR

27. Convective Temperature (CT) • The surface temperature that must be reached for purely convective clouds to develop. (If the CT is reached at the surface then convection will be deep enough to form clouds at the CCL.) • Determine the CCL, then follow the dry adiabat down to the surface.

28. Finding the CCL and CT

29. K-Index • K – Index is an index that assess convective potential • It evaluates mid level moisture and the potential for air-mass thunderstorms • K = t850 - t 500 + td850 – ( t700 - td700) • The first term is a lapse rate term • the second and third are related to the moisture between 850 and 700 mb • 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

30. K Index • K = t850 - t 500 + td850 – ( t700 - td700)

31. The Stuve Diagram (Pseudoadiabatic Diagram) • Axes are • Adiabats, isobars and Isotherms are straight lines • Adiabats intersect the isotherms at ~45º. Isobars and isotherms are perpendicular • The area/energy relationship is not strictly met, but it is a good approximation

32. Stuve (Pseudoadiabatic)

33. Stuve Stuve (Joe)

34. Stuve Diagram (courtesy F. Remer)

35. Note large dry Area at 700mb This is typical of a severe storm

36. Notice moisture at both 850 and 700mb This is a typical sounding for an airmass thunderstorm. Compare the K index of this sounding to the previous one.

37. Assignment • Today’s lab will involve manipulating air “parcels” via the Skew-T Log-P diagram, and comparing them to a sounding of temperature and dew point to better understand how stability is analyzed for the potential of thunderstorm development. • The sounding will be chosen by the instructor at the following web site: • http://www-frd.fsl.noaa.gov/mab/soundings/java/ • You will be directed what to do IN CLASS.