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Limb Scanning and Occultation

Limb Scanning and Occultation. Ben Kravitz November 12, 2009. Occultation. An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. very commonly used in astronomy. Occultation.

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Limb Scanning and Occultation

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  1. Limb Scanning and Occultation • Ben Kravitz • November 12, 2009

  2. Occultation An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. very commonly used in astronomy

  3. Occultation In the case of atmospheric observations, we pick a source of some kind and measure how the radiation from that source passes through the atmosphere. (The signal gets occulted by the atmosphere.)

  4. How it works

  5. Techniques • GPS Radio Occultation • Limb Emission/Sounding • Solar Occultation

  6. GPS Radio Occultation • Fairly new technique (first applied in 1995) • Requires a constellation of GPS satellites and (at least one) Low Earth Orbit satellite

  7. Refractivity (N) N = 77.6(p/T) + 3.73x105(e/T2) - 4.03x107(ne/f2) p = atmospheric pressure T = temperature e = water vapor pressure ne = electron density (number of electrons per m3) f = carrier frequency of the GPS

  8. N =4.03x107(ne/f2) In the ionosphere, pressure is negligible, so the refractivity gives us electron density.

  9. N = 77.6(p/T) In the stratosphere, electron density is negligible, as is water vapor pressure, so the refractivity gives us temperature.

  10. N = 77.6(p/T) + 3.73x105(e/T2) In the troposphere, only electron density is negligible, giving us profiles of temperature and humidity. GPS can determine precipitable water at sub-mm accuracy over the globe

  11. N = 77.6(p/T) + 3.73x105(e/T2) Ignoring this part gives us the “dry temperature.” This is very accurate in low humidity environments (like the stratosphere). dry temperature ≤ actual temperature

  12. GPS RO systems • GPS/Met • COSMIC/FORMOSAT-3 - Constellation Observing System for Meteorology, Ionosphere, and Climate

  13. Verifying GPS RO • Comparison with AMSU • Comparison with radiosondes

  14. Comparison with AMSU

  15. Radiosondes • Radiosondes are the only technology that has provided us with over three decades of continuous data • Radiosondes have an emissivity

  16. What we do with GPS RO data • Useful for a very stable, accurate long-term climate record across the entire globe • Better numerical weather prediction • Determining atmospheric structure

  17. Typhoon Jangmi approaching Taiwan

  18. Determining Atmospheric Structure

  19. Determining Atmospheric Structure Can also determine tropopause height (using some very complicated algorithms) - this is very recent research

  20. Earth’s Limb

  21. Limb Emission/Sounding • The limb of the atmosphere emits radiation • We measure the limb at each vertical level which tells us about the atmospheric properties

  22. Limb Emission/Sounding • SCIAMACHY - coordinates with nadir measurements to give total column profiles of greenhouse gases • OSIRIS • Microwave Limb Sounder (MLS)

  23. “onion-peeling” method

  24. Solar Occultation Instruments • Stratospheric Aerosol and Gas Experiment (SAGE): 1979-1981 • SAGE II: 1984-2005 • SAGE III: 2002-2005 • Optical Spectrograph and Infrared Imager System (OSIRIS): 2001-present

  25. Testing OSIRIS • Ran a climate model of Kasatochi volcano (same model used to simulate Pinatubo) • Output aerosol optical depth • Compared modeled optical depth with OSIRIS retrievals The agreement was pretty good, but there was an unresolved discrepancy which we cannot yet explain.

  26. Minor tests of OSIRIS Carbonyl sulfide (OCS) OSIRIS can compare its background sulfate aerosol measurements to those from SAGE

  27. Stratospheric Aerosols

  28. Stratospheric Aerosols • Tropospheric aerosols get scavenged by rain - have an atmospheric lifetime of about two weeks (or less) • Stratospheric aerosols have an atmospheric lifetime of 1-3 years until they fall into the troposphere

  29. Soufriere Volcano • Eruption on St. Vincent (in the Caribbean) • April 1979 (SAGE launched in February 1979) • The first satellite observed volcanic eruption

  30. Mount Pinatubo • Eruption in the Philippines • June 1991 (SAGE II) • The largest eruption in recent history (20 megatons of SO2 injected into the stratosphere)

  31. SAGE Intercomparison

  32. Other Sources of Radiation

  33. Other Sources of Radiation • Moonlight (lunar occultation) • Starlight: Global Ozone Monitoring by Occultation of Stars (GOMOS)

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