Ben KravitzNovember 12, 2009Limb Scanning and OccultationOccultationAn 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 astronomyIn 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.)OccultationHow it works•GPS Radio Occultation•Limb Emission/Sounding•Solar OccultationTechniquesGPS Radio Occultation•Fairly new technique (first applied in 1995)•Requires a constellation of GPS satellites and (at least one) Low Earth Orbit satelliteRefractivity (N)N = 77.6(p/T) + 3.73x105(e/T2) - 4.03x107(ne/f2)p = atmospheric pressureT = temperaturee = water vapor pressurene = electron density (number of electrons per m3)f = carrier frequency of the GPSN =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 globeN = 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 temperatureGPS RO systems•GPS/Met•COSMIC/FORMOSAT-3 - Constellation Observing System for Meteorology, Ionosphere, and ClimateVerifying GPS RO•Comparison with AMSU•Comparison with radiosondesComparison with AMSU•Radiosondes are the only technology that has provided us with over three decades of continuous data•Radiosondes have an emissivityRadiosondes•Useful for a very stable, accurate long-term climate record across the entire globe•Better numerical weather prediction•Determining atmospheric structureWhat we do with GPS RO dataTyphoon Jangmi approaching TaiwanDetermining Atmospheric StructureDetermining Atmospheric StructureCan also determine tropopause height (using some very complicated algorithms) - this is very recent researchEarth’s LimbLimb Emission/Sounding•The limb of the atmosphere emits radiation•We measure the limb at each vertical level which tells us about the atmospheric propertiesLimb 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 1821 Fig. 2. Nominal limb scan mode of SCIAMACHY. for this requirement is that the combination of these near-simultaneous limb and nadir measurements enables the retrieval of tropospheric columns by subtracting the limb stratospheric column from the total nadir column. A similar approach has been used to determine tropospheric ozone columns from the combination of TOMS total nadir columns with SAGE II occultation profiles or SBUV nadir profiles (Fishman et al., 1990, 1996). However, from SCIAMACHY measurements it will be possible to derive tropospheric columns not, only for 03 but also for NOz, CO: CH4, HzO, NzO, SOz, H&O, and BrO, and aerosol parameters. This limb-nadir-matching has significant implications for the operational concept. It requires an accurate synchronisation of the measurements, an appropriate scan speed, and even a special yaw steering correction to take into account the satellite attitude and compensate for the rotation of the Earth between the limb and nadir measurements. This is handled by the current operational concept and special correction algorithms for the instrument line-of-sight. Alternating limb and nadir measurements with appropriate limb-nadir matching will be performed throughout most of the sunlit part of the orbit. Solar Occultation Measurements The observational geometry in occultation mode is similar to limb. In solar occultation the sun is observed directly through the atmosphere, in lunar occultation the moon. Solar occultation measurements from space have been successfully performed by e.g. SAGE-II. Because of the forward viewing direction and the sun-fixed orbit of ENVISAT-1, SCIAMACHY can only observe the rising sun. The SCIAMACHY sun follower has a relatively small field of view of 0.72” x 2.2”. Thus, the approximate position of the sun has to be known in advance of the measurement. For the solar azimuth this is no problem as it can be calculated from the orbital position, but the solar elevation, i.e. the exact time of sunrise, depends on atmospheric refraction which is variable and may only be estimated by models. Moreover, the operational concept requires fixed time intervals, so it is of crucial importance to have a clear definition of ‘sunrise’ and the start time of a measurement. To solve this problem, a (nearly) refraction-independent definition of the sun-fixed event (sunrise) has been chosen. The time of sunrise is defined as the time when the centre of the geometrical sun reaches 17.2Geomon Annual Meeting 2008,GeneveLimb Sounding:“onion-peeling” methodSolar 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-presentTesting OSIRIS•Ran a climate model of Kasatochi volcano (same model used to simulate Pinatubo)•Output aerosol optical depth•Compared modeled optical depth with OSIRIS retrievalsThe agreement was pretty good, but there was an unresolved discrepancy which we cannot yet explain.Minor tests of OSIRISCarbonyl sulfide (OCS)OSIRIS can compare its background sulfate aerosol measurements to those from SAGEStratospheric AerosolsStratospheric 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 troposphereSoufriere Volcano•Eruption on St. Vincent