Ben KravitzNovember 12, 2009

Limb Scanning and Occultation

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

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

How it works

•GPS Radio Occultation

•Limb Emission/Sounding

•Solar Occultation

Techniques

GPS Radio Occultation

•Fairly new technique (first applied in 1995)

•Requires a constellation of GPS satellites and (at least one) Low Earth Orbit satellite

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

N =4.03x107(ne/f2)

In the ionosphere, pressure is negligible, so the refractivity gives us electron density.

N = 77.6(p/T)

In the stratosphere, electron density is negligible, as is water vapor pressure, so the

refractivity gives us temperature.

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

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

GPS RO systems

•GPS/Met

•COSMIC/FORMOSAT-3 - Constellation Observing System for Meteorology, Ionosphere, and Climate

Verifying GPS RO

•Comparison with AMSU

•Comparison with radiosondes

Comparison with AMSU

•Radiosondes are the only technology that has provided us with over three decades of continuous data

•Radiosondes have an emissivity

Radiosondes

•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

Typhoon Jangmi approaching Taiwan

Determining Atmospheric Structure

Determining Atmospheric Structure

Can also determine tropopause height (using some very complicated algorithms) - this is very

recent research

Earth’s Limb

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

Limb Emission/Sounding

•SCIAMACHY - coordinates with nadir measurements to give total column profiles of greenhouse gases

•OSIRIS

•Microwave Limb Sounder (MLS)

“onion-peeling” method

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

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.

Minor tests of OSIRIS

Carbonyl sulfide (OCS)

OSIRIS can compare its background sulfate aerosol measurements to those from SAGE

Stratospheric Aerosols

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

Soufriere Volcano

•Eruption on St. Vincent (in the Caribbean)

•April 1979 (SAGE launched in February 1979)

•The first satellite observed volcanic eruption

Mount Pinatubo

•Eruption in the Philippines

•June 1991 (SAGE II)

•The largest eruption in recent history (20 megatons of SO2 injected into the stratosphere)

SAGE Intercomparison

Other Sources of Radiation

•Moonlight (lunar occultation)

•Starlight: Global Ozone Monitoring by Occultation of Stars (GOMOS)

Other Sources of Radiation