Presentation by Chris Stumpf

1. A Review for forecasters on the application of hodographs to forecasting severe weather:Paper by Charles Doswell Presentation by Chris Stumpf

2. Introduction • Basics of the Hodograph Research Objectives • Processes which create vertical wind shear • Interpreting and forecasting the hodograph in convective events

3. The hodographis a useful tool • Thunderstorm Forecasting • Forecasting type of thunderstorms • Potential for supercell thunderstorms • But we don’t get thunderstorms or supercells here that often! • Why should I care about a hodograph?

4. Introduction • Basics of the Hodograph Research Objectives • Processes which create vertical wind shear • Interpreting and forecasting the hodograph in convective events

5. What’s a sounding? TOPMANTOP WOUS00 KTOP 271200 72456 TTAA 77121 72456 99988 15658 32005 00158 ///// ///// 85557 17269 02027 70179 05662 32520 50582 15980 31022 40746 28380 33028 30945 451// 32530 25064 543// 33033 20204 645// 32026 15379 631// 30035 10631 601// 30529 88172 673// 30527 77999 51515 10164 00003 10194 01527 34521= TOPSGLTOP WOUS00 KTOP 271200 72456 TTBB 7712/ 72456 00988 15658 11984 17459 22958 18660 33938 21665 44894 19880 55764 10658 66601 05363 77589 06567 88560 09562 99500 15980 11438 23759 22400 28380 33356 35359 44327 39960 55226 601// 66172 673// 77145 637// 88126 577// 99110 613// 11100 601//=   PPBB 77120 72456 90012 32005 32508 02028 90345 03034 03032 02026 90678 01022 35019 32520 909// 32022 91246 33021 32016 31514 92056 31022 33029 33527 929// 32527 93058 32529 33033 33537 939// 32523 9427/ 30525 30542 9502/ 31032 30530=

6. Wind shear visualized on a sounding

7. Plotting wind Vectors

8. Wind vectors plotted on the hodograph Wind Vectors Wind Shear Vectors

9. Introduction • Basics of the Hodograph Research Objectives • Processes which create vertical wind shear • Interpreting and forecasting the hodograph in convective events

10. The thermal wind!! • Defined as the vertical shear of the geostrophic wind • Similar to the geostrophic wind • Plotted parallel to isotherms • Lower temperatures plotted to the left • Speed increases when isotherms are closer together • Combining the geostrophic wind vector and thermal wind vector = Real Wind

11. The thermal wind!! • No change in geostrophic wind with height = no thermal wind • Geopotential Height gradient does not change in the vertical • This is called a Barotropic Atmosphere. • In a Barotropic atmosphere the hodograph • A straight line parallel to the wind • Changes in direction and magnitude of the height gradient = • Baroclinic atmosphere

12. Wind shear in the planetary boundary layer • Real wind = Geostrophic wind only in the free atmosphere • In the boundary layer friction must be accounted for • Frictional effects decrease with height in the boundary layer

13. Veering winds as a result of friction • Ekman Spiral in the atmosphere • In the NH near the surface winds should veer with height • Result of an increasing influence on the wind by the Coriolis force • Soundings which show veering winds near the surface • If no large veering with height above the surface • Veering could be accounted for by the Ekman Spiral

14. Ageostrophic processes • Jet streaks • Meteorological “bombs” • Rapidly moving cyclones

15. Introduction • Basics of the Hodograph Research Objectives • Processes which create vertical wind shear • Interpreting and forecasting the hodograph in convective events

16. Supercells and the hodograph • Supercells account for a small fraction of the severe thunderstorms • They account for the majority of the damage and casualties • Certain wind shear patterns are associated with supercells • Got to be a Magic number of shear to = supercell and tornadoes • Not quite, however values on the order of 10^-3/s and larger ( 6 kts/10,000 ft)or above 40 knots of shear

17. CAPE and wind shear • Given a certain amount of CAPE • There is a range of values for shear within which long-lived convective storms are likely • Weak wind shear • Cold downdrafts and outflows undercut the updraft and block the inflow of the storms • Very strong wind shear • Updrafts will get sheared away and lose contact with their base/inflow • Studies have shown that there is a certain “sweet spot” or perfect combination of CAPE and shear to get long-lived storms

18. Thunderstorm propagation • Storms are not solid unchanging things • Made up of many different types of air parcels • Air parcels which make up an updraft change ~ every 20 min. • Interaction between the storm and the environmental wind create new storms • Storm motion is a combination of two things • Advection and Propagation

19. Straight Hodographs • Regardless of strength of shear • Storms will split and become mirror images of each other • No storm is favored and both persist for similar lengths of time

20. Curved hodograph

21. How do curved hodographs develop? • Got to have veering of winds with height • Presence of low-level jet can result in a larger turning of the winds with height • Also serves to bring warm moist air northward…more unstable environment • Middle-level minimum in the (700-500) mbwinds

22. Changes in surface wind fields • Paying close attention to changes in the surface wind field • Can anticipate the effects on the curvature of the hodograph • Studies have shown that backing and strengthening of the surface wind field • Recognized as a clue to the development of tornadic storms • Strengthening/development of low-level jet • While shear in the morning may be weak • Upstream shear could be strong and it is reasonable to expect that those values to reach you

23. Perfect setup = tornado every time…? • Large scale environments with the “correct” hodograph will not always produce a tornado • “bad” hodographs will occasionally produce a tornado • With weak shear local effects can still produce a tornado (Plainfield F5 event) • While the proper shear setup is required • Observations reveal that the forcing/lifting mechanism plays a large role on storm maintenance and development • Supercells and Squall lines form in the similar wind shear environments but the actual forcing mechanism for each is completely different

24. Questions?