1. Hodograph Analysis Thermal Advection Stability

3. Geostrophic Thermal Wind Equation

5. Total Thermal Wind Vector
VT (vector quantity)
vector wind between two different levels
kts / degrees

6. Vertical Wind Shear VT/ ΔZ (scalar quantity)
average wind shear through the layer
kts/ 1000 ft
correlated with turbulence experienced by aircraft

8. Relationship Between VT and the Horizontal T Gradient

9. Relationship Between VT and the Horizontal T Gradient
this relationship holds only if the actual winds are nearly geostrophic
operationally we identify the actual thermal wind as the geostrophic thermal wind
free atmosphere above the friction layer only
not accurate in highly curved flows

10. Height of the Gradient Wind Level
hodograph can be used to determine the friction layer or PBL (planetary boundary layer)
wind in the PBL is a combination of:
gradient wind (geostrophic + curvature)
friction
horizontal temperature gradient
Ekman spiral result of the first two

12. Identifying the Top of the PBL
in the PBL the hodograph shows a combination of Ekman spiral and vertical wind shear due to thermal wind (difficult to separate out the latter)
at the top of the PBL (typically ~ 3000 ft):
no more frictional effects
veering of wind stops
increase of wind speed
wind becomes more nearly geostrophic

14. Horizontal Temperature Advection

15. Warm and Cold Advection on a Hodograph
warm advection - veering winds with height
cold advection - backing winds
no temperature advection - no direction change
B.C. Volkswagon
backing cold air advection / veering warm air advection

16. Magnitude of the Horizontal Advection

17. Stability Tendency

18. Cold Advection Over Warm Advection

19. Non Frontal Inversions
wintertime shallow radiation inversions in cA air
surface calm
full gradient wind a few hundred feet above the surface
large vertical wind shear is not normally associated with horizontal temperature gradient

20. Shallow cold air radiation inversion

21. Interpretation of Hodographs
geostrophic thermal wind fronts
an air mass is normally characterized by relatively small horizontal temperature gradient vertical wind shear is small
small means less than ~2-4 kt/1000 ft

22. Information in a Wind Shear Hodograph
height of the frontal surface, base of mixing zone
orientation of front
type of front
speed and direction of frontal motion
instantaneous changes in vertical stability
vertical motion in warm air

23. Identification of Fronts
relative maximum of wind shear in a layer is indicative of the mixing zone of a front (say ~5kt/1000ft or greater)
top of the layer corresponds to the top of the mixing zone
should be consistent with the tephi

24. Orientation of the Front
isotherms are approximately parallel to fronts in the mixing zone (assumption)
direction of thermal wind vector indicates orientation of front
with thermal wind at your back, cold air is on the left

25. Direction & Speed of Frontal Motion
take a vector from the origin perpendicular to the thermal wind vector
VN indicates the motion of the front
direction of motion of the cold air determines type of front
winds veering with height warm front
winds backing with height cold front

26. Vertical Motion in the Warm Air Mass
the shear pattern can indirectly indicate the field of vertical motion in a warm air mass above a frontal surface
assume the frontal slope does not change for a short period of time
equation of continuity principle

27. Active or Anabatic Cold front

28. Cross Section of Active Cold Front

29. Inactive or Katabatic Cold Front

30. Cross Section of Inactive Cold Front

31. Active or Anabatic Warm Front

32. Inactive or Katabatic Warm Front

33. Example of Hodograph Analysis

34. Exercise find a hodograph to illustrate a) warm front b) a cold front
are the fronts active or inactive?
find an example of a hodograph showing a trend toward a) decreasing stability b) increasing stability
find an example of an Ekman spiral and the top of the PBL