Limb Scanning and Occultation. Ben Kravitz November 12, 2009

Transcription

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

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8 Refractivity (N) N = 77.6(p/T) x10 5 (e/t 2 ) x10 7 (n e /f 2 ) p = atmospheric pressure T = temperature e = water vapor pressure ne = electron density (number of electrons per m 3 ) f = carrier frequency of the GPS

9 N =4.03x10 7 (n e /f 2 ) In the ionosphere, pressure is negligible, so the refractivity gives us electron density.

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

11 N = 77.6(p/T) x10 5 (e/t 2 ) 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

12 N = 77.6(p/T) x10 5 (e/t 2 ) Ignoring this part gives us the dry temperature. This is very accurate in low humidity environments (like the stratosphere). dry temperature actual temperature

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

14 Verifying GPS RO Comparison with AMSU Comparison with radiosondes

15 Comparison with AMSU

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

17 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

18 Typhoon Jangmi approaching Taiwan

19 Determining Atmospheric Structure

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

21 Earth s Limb

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23 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

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

25 Nadir. Limb and Occultation Measurements with SCIAMACHY 1821 Fig. 2. Nominal limb scan mode of SCIAMACHY.

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29 onion-peeling method

30 Solar Occultation Instruments Stratospheric Aerosol and Gas Experiment (SAGE): SAGE II: SAGE III: Optical Spectrograph and Infrared Imager System (OSIRIS): 2001-present

31 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.

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

33 Stratospheric Aerosols

34 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

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

36 umn to heights of 18 to 20 ated from a NASA aircraft s, only one more eruption, sent debris into the strato- ASA's SAGE (Stratospherd Gas Experiment) satellite d on 18 February 1979 to al measurements of the verof aerosols in the stratohe instrument makes solarsurements at four different for each satellite sunrise these measurements are ine proffles of aerosol extince a vertical resolution of 1 ccuracy of about 10 percent k of the stratospheric aerose data are also inverted to nd nitrogen dioxide concenfunction of altitude. Meade on successive orbits are about 240 of longitude and o 0.3 of latitude. As the it precesses, geographical tween approximately 65 N obtained, a cycle taking h to complete. ht detection and ranging) s made during the Soufriere 17 April are also available. rements were taken from an tem onboard a NASA P-3 raft returning from a SAGE mission in Brazil. The airected to the neighborhood of approaching it at the time pril eruption. Special mislown on 18 and 19 April to he height and location of. 216, 4 JUNE 1982 E so 30% I c--- ~~~~'=rtx- -18N ufriere ^I, I--~--r r F- -- I I------, _ t- o9 25 = A 23 c 21 a Fig. 1. (a) Normal aerosol extinction profile as determined by the SAGE satellite system. (b) Enhanced aerosol profile observed near Soufriere on 24 April (c) Map showing SAGE measurements near Soufriere in April The latitudes for each day of SAGE measurements are shown by the dashed lines. Events showing enhanced aerosol extinction in the stratosphere (50 percent or more above normal) are marked by x's; the altitude (in kilometers) of each layer peak is shown /82/ $01.00/0 Copyright 1982 AAAS.Z

37 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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42 SAGE Intercomparison

43 properties observed by SAGE II and HALOE were reported by Lu et al. [1997, 2003]. A comparison of the aerosol optical depths observed by SAGE III, POAM III, and Figure 3. Comparison of SAGE II and SAGE III extinction coefficient profiles at four wavelengths measured on 2 March Tropopause heights are indicated by arrows. Figure 4. Comparison of SAGE II and SAGE III average extinction coefficient profiles at four wavelengths measured in of10

44 the Southern Hemisph surement pairs varies f Because of missing me both satellite experime groups in that year. [9] In comparing th Table 1 with Table 1 o may be noted for 2002 that the coincidence g included in the previou coincidences for group 2.2. SAGE II and SA Comparison [10] The extinction during 2004 and 2005 respectively. The measu selected data points a locations of the tropo distance between SAG locations is about 230 for the most part, aeros satellites is in good agr within the measureme differences between S ments are found eithe 30 km where measur

45 YUE ET AL.: COMPARING SAGE II AND SAGE III AEROSOLS D07 Figure 11. Differences between SAGE II and SAGE III measured stratospheric optical depth defined as SAGE III measurement minus SAGE II measurement divided by their mean value at four different wavelengths. ences when measurement locations on the same day y close to each other. These coincidences occurred in and Taha, 2003;Thomason et al., 2007;Yue et al.,20 are all based on the version 3.00 or an earlier versio

46 Other Sources of Radiation

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