1. Airborne Studies of Atmospheric Dynamics
Thomas R. Parish
Department of Atmospheric Science
University of Wyoming

2. Newton’s Second Law – The Equation of Motion
Total
Derivative
Local
Derivative
Advection
(Inertia) Term
Horizontal Pressure
Gradient Force
Coriolis Force
Geostrophic
component
Ageostrophic
component

3. Purely rotational (non-divergent) Often the largest component of wind
Geostrophic Wind
Balanced flow state
Purely rotational (non-divergent)
Often the largest component of wind
Relatively inert component of the wind Vg L H P C

4. Ageostrophic Wind
Unbalanced flow state
Often contains significant divergent component
Generally small component of wind
Isallobaric and inertia/advective components generally largest
Important forcing component of the wind

5. Measurement of Geostrophic Wind
Write Equation of Motion in isobaric coordinates
Variation of height on a pressure surface proportional to horizontal pressure gradient force
Airborne applications – use autopilot

6. Pre-GPS Era (before 2004)
Radar Altimeter measurements provide height above surface
Terrain maps (digital) provide terrain height assuming geographic position known with high accuracy
Height of isobaric surface is sum of above signals

7. Problems:
Two signals (radar altimeter heights, terrain height) large and of opposite sign
PGF is the sum of those terms, being quite small and noisy
Potential errors in both radar altimeter height, terrain height
“Artifact” problem for radar altimeter
Footprint issue for altimeter
Uncertainties in aircraft position estimates
Issues with terrain height data sets
Resulting uncertainty with “terrain registration”

8. Example: Great Plains Low-Level Jet
Nocturnal summertime jet maximum ~400 m agl
Competing theories for LLJ formation
Blackadar frictional decoupling
Holton sloping terrain influence

12. Results of PGF measurements at lowest level
Flight Strategy – Repeating isobaric legs

13. Conclusions:
Isallobaric component of wind ~4 m/s at level maximum wind
Large changes in turbulent intensity at jet level
Blackadar frictional decoupling dominant mechanism in
forcing Great Plains LLJ

14. GPS Era
Avoid “terrain registration” issues
GPS provides a means to accurately map isobaric surface
Position errors from standard GPS receiver insufficient to
resolve isobaric slopes
Differential GPS required
Requires fixed base station
Position errors at base station can be used to correct
position errors for rover platform (aircraft)
Importance of acceptable satellite constellation (5 or 6?)
Position accuracy on order of decimeters
Relative accuracy probably much better

15. GPS04 Study – Arcata CA
Frequent summertime LLJ at top of marine boundary layer
Comparison with altimetry-derived geostrophic wind
Tested GrafNav differential processing software

16. GPS04 LLJ Example
Isobaric east-west flight leg south of Cape Mendocino
Nearly identical signals
dGPS calculations of Vg from most legs within 1 m/s altimetry Vg
GPS04 validated dGPS technique

18. Application of dGPS on atmospheric dynamics – Coastally Trapped Wind Reversals (CTWRs, also CTDs, southerly surges)
0000 UTC 22 June UTC 26 June 2006

19. CTWR Forcing Issues Kelvin Wave Cross-coast PGF
Variations in MBL Height
Topographic Rossby Wave
Topographically-Trapped Wave
Density Current
Synoptic-scale response
Ageostrophic acceleration
Importance of synoptic-scale pressure field

20. 23 June 2006 Example
Isobaric east-west
flight leg
Little detectable
cross-coast PGF

21. 23 June 2006 Example

22. 23 June 2006 Example

23. 23 June 2006 Example

24. 24 June 2006 Example

25. 24 June 2006 Example

26. 23-25 June 2006 CTWR Conclusions
CTWR density current
No Kelvin-wave features observed during this event
Active propagation phase highly ageostrophic
Little detectable cross-coast PGF at any time during the life history
Onset and propagation dependent on synoptic pressure field

27. Application: CloudGPS08
May-June 2008, flights over high plains WY, NE, CO
Measure horizontal perturbation pressures associated with clouds
Clouds mostly in cumulus congestus phase
Differential GPS dependent on accurate measurement of static pressure

28. Application: CloudGPS08 (May 21)

29. θV Isobaric Height Liquid Water Content (g/kg) u W (m/s) v
Horizontal Pressure
Perturbation (mb) θV

30. Liquid Water
Content (g/kg)
W (m/s)
Horizontal Pressure Perturbation (mb)

31. Application: CloudGPS08 (June 17)

32. Leg 1 θV Liquid Water Content (g/kg) Isobaric Height W (m/s) u v
Horizontal Pressure
Perturbation (mb) θV

33. Leg 2 θV Liquid Water Content (g/kg) Isobaric Height W (m/s) u v
Horizontal Pressure
Perturbation (mb) θV

34. Leg 3 θV Liquid Water Content (g/kg) Isobaric Height W (m/s) u v
Horizontal Pressure
Perturbation (mb) θV

35. Leg 4 θV Liquid Water Content (g/kg) Isobaric Height W (m/s) u v
Horizontal Pressure
Perturbation (mb) θV

36. Leg 5 θV Liquid Water Content (g/kg) Isobaric Height W (m/s) u v
Horizontal Pressure
Perturbation (mb) θV

37. Leg 1

38. Leg 2

39. Leg 3

40. Leg 4

41. Leg 5

42. Application: Ocean Surface Topography
Differences between GPS height, radar altimeter signal measure of ocean surface topography

43. Application: Ocean Surface Topography
Reciprocal legs along 40.8°N
Consistent pattern of height
differences
Validate using multiple altimeters?
Gulf Stream flights?

44. Application: Ocean Surface Topography
Reciprocal legs along 40.8°N
Consistent pattern of height
differences
Validate using multiple altimeters?
Gulf Stream flights?

45. Application: Ocean Surface
Topography

46. dGPS can provide precise mapping of aircraft height
Conclusions
dGPS can provide precise mapping of aircraft height
Base station data rate 1 Hz
Baseline ~ 100 km?
dGPS accuracy within decimeters?
Relative accuracy higher?
Accurate measurement of static pressure permits PGF calculations
Assessment of atmospheric dynamics for a wide variety of flows
Thanks to Dave Leon, Larry Oolman, Dave Rahn and Eric Parish