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 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 How it works Techniques GPS Radio Occultation Limb Emission Sounding Solar Occultation 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 pressure T temperature e water vapor pressure ne 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 Radiosondes are the only technology that has provided us with over three decades of continuous data Radiosondes have an emissivity 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 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 Nadir Limb and Occultation Measurements with SCIAMACHY Fig 2 Nominal limb scan mode of SCIAMACHY 1821 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 L 216 4 JUNE 1982 Downloaded from www sciencemag org on No lumn to heights of 18 to 20 mated from a NASA aircraft is only one more eruption sent debris into the stratoNASA s SAGE Stratosphernd Gas Experiment satellite ed on 18 February 1979 to bal measurements of the verof aerosols in the stratoThe instrument makes solarasurements at four different for each satellite sunrise these measurements are inve proffles of aerosol extincve a vertical resolution of 1 accuracy of about 10 percent ak of the stratospheric aeroese data are also inverted to and nitrogen dioxide concena function of altitude Meaade on successive orbits are y about 240 of longitude and to 0 3 of latitude As the bit precesses geographical etween approximately 65 N s obtained a cycle taking th to complete ght detection and ranging ts made during the Soufriere 17 April are also available urements were taken from an stem onboard a NASA P 3 craft returning from a SAGE mission in Brazil The airrected to the neighborhood of approaching it at the time April eruption Special misflown on 18 and 19 April to the height and location of 40 so 30 E 0 20 10 29 1 18 2 27 to9 c 20 4 25 11 rtx ufriere I A 18N 23 c Z I 21 a I r r F I I 908 19 Fig 1 a Normal aerosol extinction profile as determined by the SAGE satellite system b Enhanced aerosol profile observed near Soufriere on 24 April 1979 c Map showing SAGE measurements near Soufriere in April 1979 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 0036 8075 82 0604 1 115 01 00 0 Copyright 1982 AAAS 1115 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 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 2005 Tropopause heights are indicated by arrows Figure 4 Comparison of SAGE II and SAGE III average extinction coefficient profiles at four wavelengths measured in 2005 3 of 10 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