airborne studies of atmospheric dynamics
DESCRIPTION
Airborne Studies of Atmospheric Dynamics. Thomas R. Parish Department of Atmospheric Science University of Wyoming. Newton’s Second Law – The Equation of Motion. Total Derivative. Local Derivative. Advection (Inertia) Term. Horizontal Pressure Gradient Force. Coriolis Force. - PowerPoint PPT PresentationTRANSCRIPT
Airborne Studies of Atmospheric Dynamics
Thomas R. ParishDepartment of Atmospheric Science
University of Wyoming
Newton’s Second Law – The Equation of Motion
HHHHHH VkfpVVtV
dtVd
1
TotalDerivative
LocalDerivative
Advection(Inertia) Term
Horizontal Pressure Gradient Force
Coriolis Force
aggH VVV
Geostrophiccomponent
Ageostrophiccomponent
Geostrophic Wind
• Balanced flow state• Purely rotational (non-divergent)• Often the largest component of wind• Relatively inert component of the wind
kpf
V
Vkfp
Hg
HH
1
10
L H
Vg
P C
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
dtVdk
fV Hag
1
agHgH
HHH
VkfVkfVkfdtdV
VkfpdtdV
1
Measurement of Geostrophic Wind
HpH VkfzgdtVd
• Write Equation of Motion in isobaric coordinates
• Variation of height on a pressure surface proportional to horizontal pressure gradient force
• Airborne applications – use autopilot
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
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”
600
700
800
900
10000 2 4 6 8 10 12 14
Wind Speed (m/s)
Pres
sure
(hPa
)
4 am
1 pm
7 am
10 pm
7 pm
1 am
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
Flight Strategy – Repeating isobaric legs
Results of PGF measurements at lowest level
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
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
GPS04 Study – Arcata CA
• Frequent summertime LLJ at top of marine boundary layer
• Comparison with altimetry-derived geostrophic wind
• Tested GrafNav differential processing software
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
Application of dGPS on atmospheric dynamics – Coastally Trapped Wind Reversals (CTWRs, also CTDs, southerly surges)
0000 UTC 22 June - 0000 UTC 26 June 2006
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
23 June 2006 Example
• Isobaric east-west flight leg
• Little detectable cross-coast PGF
23 June 2006 Example
23 June 2006 Example
23 June 2006 Example
24 June 2006 Example
24 June 2006 Example
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
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
Application: CloudGPS08 (May 21)
Liquid Water Content (g/kg)
Isobaric Height
W (m/s) u
v
Horizontal Pressure Perturbation (mb)
θV
Liquid Water Content (g/kg)
W (m/s)
Horizontal Pressure Perturbation (mb)
Application: CloudGPS08 (June 17)
Leg 1Liquid Water Content (g/kg)
W (m/s)
Horizontal Pressure Perturbation (mb)
u
v
θV
Isobaric Height
Leg 2Liquid Water Content (g/kg)
W (m/s)
Horizontal Pressure Perturbation (mb)
u
v
θV
Isobaric Height
Leg 3Liquid Water Content (g/kg)
W (m/s)
Horizontal Pressure Perturbation (mb)
u
v
θV
Isobaric Height
Leg 4Liquid Water Content (g/kg)
W (m/s)
Horizontal Pressure Perturbation (mb)
u
v
θV
Isobaric Height
Leg 5Liquid Water Content (g/kg)
W (m/s)
Horizontal Pressure Perturbation (mb)
u
v
θV
Isobaric Height
Leg 1
Leg 2
Leg 3
Leg 4
Leg 5
Application: Ocean Surface Topography
• Differences between GPS height, radar altimeter signal measure of ocean surface topography
Application: Ocean Surface Topography
• Reciprocal legs along 40.8°N
• Consistent pattern of height differences
• Validate using multiple altimeters?
• Gulf Stream flights?
Application: Ocean Surface Topography
• Reciprocal legs along 40.8°N
• Consistent pattern of height differences
• Validate using multiple altimeters?
• Gulf Stream flights?
Application: Ocean Surface Topography
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