Validation of atmospheric temperature profiles derived from CHAMP radio occultation measurements

1
Validation of atmospheric temperature profiles
derived from CHAMP radio occultation
measurements

• M. Lange1, Ch. Jacobi2
• 1 Deutscher Wetterdienst, Offenbach
• 2 Institute for Meteorology, University of
  Leipzig
• email martin.lange_at_dwd.de

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Outline

• Motivation for the campaign
• GPS Radio Occultation principle
• The problem of detecting gravity waves by radio
  occultation measurements
• Experimental setup
• Results from the campaign
• Conclusions

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Motivation

• Gravity waves transport energy from the
  troposphere into the middle atmosphere
  influencing the large scale circulation.
• Champ provides temperature profiles in the range
  5-50 km on a global scale aiming to derive
  gw-characteristics from the perturbations.
• Validation of these measurements is necessary to
  assess the quality of the temperature profiles
  for that purpose.

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Radio Occultation measurement
GPS Sat.
LEO
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Temperature retrieval for small perturbations
Assumptions Dry atmosphere, ideal gas, adiabatic
conditions
Refractive index
Refractivity
?N106 ?n k1 R ??
Density
??-p0 /(RT0) ?T/T0 ? p/(RT0).
Temperature perturbation
?T 1/k1 ? 106? 2/5 ?n T02 / p0.
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Phase shift from GWs
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The problem of detecting gravity waves by RO
measurements
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Increased phase lag for short ?x
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Good resolution of wave amplitudes
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Good phase resolution for ?xgt200 km
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Experimental setup

• Three radiosondes are released consecutively
  around occultation events to detect vertical
  propagating gw structures.
• During Champ occultation The 2. balloon is
  centered at a height of 15 km.
• The time delay between balloon releases is set to
  2.5 hours corresponding to the shortest release
  timedifference including the preparation phase.

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Deviation about 1-2 K from average
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Periodic perturbations due to gws ?
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2. Balloon doesnt fit Champ patterns
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Correspondence for ?z6 km (3 km)
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Partial correspondence in amplitude
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Stationary phase, Amplitude damping
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No distinct GW behaviour
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Unrealistic Champ retrieval
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Conclusions

• Comparison of Temperature sounding by radiosonde
  and RO measurements at 8-20 km show mean
  deviation around 1-2 K overlayed with small scale
  perturbations of about 1-2 K. This approves the
  quality of concept and the ability to detect long
  term trends on a global scale.
• As no gravity wave event could be clearly
  identified from the measurements no conclusion
  could be drawn if Champ is able to reproduce
  realistic phase and amplitudes as assumed from
  theoretical considerations.
• To derive 3D wave parameter from RO measurements
  additional information is necessary. Future
  satellite missions may provide this by higher
  measurement density.

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Acknowledgement

• Thanks to all the Alomar people who provided very
  good support for the realisation of the project.
• Especially to the technical staff who carried out
  the balloon measurements remotely even during
  nighttimes.