ben kravitz november 12, 2009 limb scanning and occultation
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Ben KravitzNovember 12, 2009
Limb Scanning and Occultation
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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
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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.)
Occultation
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How it works
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•GPS Radio Occultation
•Limb Emission/Sounding
•Solar Occultation
Techniques
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GPS Radio Occultation
•Fairly new technique (first applied in 1995)
•Requires a constellation of GPS satellites and (at least one) Low Earth Orbit satellite
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Refractivity (N)
N = 77.6(p/T) + 3.73x105(e/T2) - 4.03x107(ne/f2)
p = atmospheric pressureT = temperature
e = water vapor pressurene = electron density (number of electrons per m3)
f = carrier frequency of the GPS
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N =4.03x107(ne/f2)
In the ionosphere, pressure is negligible, so the refractivity gives us electron density.
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N = 77.6(p/T)
In the stratosphere, electron density is negligible, as is water vapor pressure, so the
refractivity gives us temperature.
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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
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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
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GPS RO systems
•GPS/Met
•COSMIC/FORMOSAT-3 - Constellation Observing System for Meteorology, Ionosphere, and Climate
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Verifying GPS RO
•Comparison with AMSU
•Comparison with radiosondes
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Comparison with AMSU
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•Radiosondes are the only technology that has provided us with over three decades of continuous data
•Radiosondes have an emissivity
Radiosondes
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•Useful for a very stable, accurate long-term climate record across the entire globe
•Better numerical weather prediction
•Determining atmospheric structure
What we do with GPS RO data
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Typhoon Jangmi approaching Taiwan
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Determining Atmospheric Structure
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Determining Atmospheric Structure
Can also determine tropopause height (using some very complicated algorithms) - this is very
recent research
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Earth’s Limb
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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
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Limb Emission/Sounding
•SCIAMACHY - coordinates with nadir measurements to give total column profiles of greenhouse gases
•OSIRIS
•Microwave Limb Sounder (MLS)
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“onion-peeling” method
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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
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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.
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Minor tests of OSIRIS
Carbonyl sulfide (OCS)
OSIRIS can compare its background sulfate aerosol measurements to those from SAGE
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Stratospheric Aerosols
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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
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Soufriere Volcano
•Eruption on St. Vincent (in the Caribbean)
•April 1979 (SAGE launched in February 1979)
•The first satellite observed volcanic eruption
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Mount Pinatubo
•Eruption in the Philippines
•June 1991 (SAGE II)
•The largest eruption in recent history (20 megatons of SO2 injected into the stratosphere)
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SAGE Intercomparison
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Other Sources of Radiation
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•Moonlight (lunar occultation)
•Starlight: Global Ozone Monitoring by Occultation of Stars (GOMOS)
Other Sources of Radiation