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Applications of GPS Derived data to the Atmospheric Sciences Jaclyn Secora Trzaska
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Overview History of GPS How GPS occultations work 3 GPS campaigns Applications of GPS Characterizing the Atmosphere using GPS: Zonal Means and Arctic
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Global Positioning System (GPS) 24 Operational Satellites currently in orbit 12 hour, 20,000km circular orbits Inclination angle, i = 55 ˚ Transmits at 2 frequencies, 1575MHz and 1227MHz (19 and 24.4 cm)
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GPS Satellite
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GPS Orbits
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History of GPS Originally called Navigation System with Timing And Ranging (NAVSTAR) Developed by the US Department of Defense to provide all-weather round- the-clock navigation capabilities for military ground, sea, and air forces
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Uses of GPS Recreational Uses (boating,aircraft, hiking) Surveying Fleet tracking Roadside Assistance (OnStar) GeoCaching: people hunt for treasure with only coordinates as a clue
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Radio Occultations Been used for over 30 years to characterize planetary atmospheres Occultation occurs as satellite “rises or sets” on the horizon as viewed by receiver Uses a microwave transmitter (GPS) to send a signal to a receiver (LEO) on the opposite side of some medium of interest (atmosphere) Medium characterized by effect it has on radio signal
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Features of GPS Occultations No long term drift—ideal for global warming detection Global coverage (~500 soundings/day) All-weather remote sensing system Measures profiles of refractivity, density, temperature and pressure from surface to 50 km Measures water vapor profiles in the troposphere, with accuracy of 0.2 g/kg 0.5K accuracy for individual profiles 100 meters vertical resolution
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Some Theory Assume spherical symmetry (no horizontal variations in temperature or moisture) Relationship formed between refractive index and bending angle Assume dry atmosphere, pressure and temperature are found
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Occultation Geometry
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Derivation of Geophysical Parameters
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Occultation Movie http://genesis.jpl.nasa.gov/zope/GENESIS/Background/Movie
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GPS/MET: The First Campaign April 3, 1995 to March, 1997 100 to 150 occultations per day 1 Low Earth Orbiting Receiver orbiting at ~775km
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GPS/MET Coverage June 30, 1995 www.cosmic.ucar.edu/gpsmet
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GPS/MET Coverage June 21, 1995 to July 4, 1995 www.cosmic.ucar.edu/gpsmet
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GPS/MET Profiles genesis.jpl.nasa.gov/html/missions/gpsmet
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Location of GPS Occultations 1 2 3 4 5 6
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456 --- GPS --- ECMWF 123 4 56
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456 --- GPS --- ECMWF 1 23 4 5 6
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CHAMP German satellite, launched in 2000 Collecting data since February 2001 Approximately 250 occultations per day Scheduled to be in orbit for 5 years Used for gravity field magnetic field and electric field recovery and atmospheric limb sounding
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CHAMP Orbit http://op.gfz-potsdam.de/champ/index_CHAMP.html
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CHAMP Temp Profile
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SuomiNet Network of GPS receivers located at or near universities GPS receivers are ground based provide realtime atmospheric precipitable water vapor measurements and other geodetic and meteorological information. http://www.suominet.ucar.edu
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SuomiNet Worlwide
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SuomiNet US
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Passage of Javier Remnants over Tucson http://www.gst.ucar.edu/gpsrg/realtimehttp://www.gst.ucar.edu/gpsrg/realtime/
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Hurricane Katrina http://www.suominet.ucar.edu/katrina/katrina.mov
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Applications of GPS Temperature Measurement Water Vapor Measurement Planetary Boundary Layer Ionosphere
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Temperature Measurement October 2001
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Water Vapor Measurements C. Minjuarez-Sosa
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Planetary Boundary Layer F. Xie
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PBL top
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Ionosphere S. Syndergaard max
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S. Syndergaard max
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Some Other Applications Climate research all weather viewing Global dataset Unaffected by aerosols Long term accuracy Assimilation into Weather Forecasts Tropopause dynamics Gravity field, magnetic field
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An Investigation into Observed and Modeled Global Atmospheric Stability Jaci Secora, Rob Kursinski, Andrea Hahmann, Dan Hankins
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Overview Motivation of Study GPS/MET Mission ECMWF Analysis NCAR Community Climate Model GPS/ECMWF/CCM3 Comparisons Conclusions
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Motivation of Study Sinha, 1995 showed that lapse rate feedback is important in determining the equilibrium surface temperature when the climate system is perturbed 6% reduction in LR produces a 40% amplification in water vapor feedback, while a 12% increase extinguishes it 2000 study by Gaffen et al. looked at the observed decadal change in lapse rate and determined that some climate models were not correctly depicting it
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Purpose of Study Study evaluates representation and variability of stability in climate models as well as characterizing the stability in the real atmosphere
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Gaffen et al. (2000) Examined 2 time periods: 1960 -1997 and 1979 - 1997 1960 - 1997: Overall stabilization of atmosphere 1979 - 1997: Overall destabilization of atmosphere 3 models showed no change in stability, over both time periods
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Gaffen et al. Study
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Data Sets Used in this Study GPS: Observations ECMWF: Analysis = Model + Observations (Not GPS Observations) CCM3: Model
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GPS/MET Data GPS occultation data offers unique combination of high vertical resolution, accuracy and global coverage needed for this study GPS/MET Mission from April 1995 - February 1997 Current study focused on June 21 to July 4, 1995 - Anti - Spoofing encryption turned off - Over 800 occultations collected during period - Period falls during the northern summer/ southern winter near the solstice (24 hours of day/night in the poles)
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Coverage of Occultations June 21 – July 4, 1995
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ECMWF Data Global 6 hour analyses (not reanalyses) 1º x 1º horizontal resolution 31 vertical levels (up to 30mb) High resolution and accuracy make it a good comparison to GPS Interpolated to GPS occultation locations in the JPL Processing System
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NCAR Community Climate Model (CCM3) 18 vertical levels, ranging from the surface up to 2.9 mb horizontal resolution of 2.8° x 2.8° CCM3 data both horizontally and vertically interpolated to GPS occultation locations Uses Zhang and McFarlane deep convection scheme, Slingo expression for shortwave radiation Model forced by observed SST’s (NMC)
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Temperature vs Heights 456 --- GPS --- ECMWF
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GPS Zonal Mean Temperatures Latitude Pressure
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Large displacement Least stability GPS Zonal Mean Temperature Gradients
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ECMWF Zonal Mean Temperature Gradients Least stability oscillations
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GPS - ECMWF Zonal Mean Gradient Differences
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ECMWF - CCM3 Zonal Gradient Difference
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-2K/km 0K/km
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NH/SH Asymmetry
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ECMWF Zonal Mean Gradient Standard Deviations
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CCM3 Temperature Gradient Standard Deviation
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GPS Temp. Gradient Frequency
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Temperature Gradient Histograms from 40S to 50S GPS ECMWF CCM3 375 - 300300 - 250250 - 200200 - 150
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GPS/ECMWF/CCM3 Histograms Width and shape of variability differs greatly between GPS/ECMWF and CCM3 300 - 250 mb level: CCM3 variability much smaller than GPS or ECMWF Observed transition between stratosphere and troposphere GPS and ECMWF have significantly different distributions 250 - 200 mb level: CCM3 peak is more negative than observations indication of too high tropopause in CCM3 CCM3 has no skew while GPS/ECMWF have negative skew 200 - 150 mb level: CCM3 transition between troposphere and stratosphere GPS and ECMWF have a positive skew, CCM3 has a slightly negative skew.
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Conclusions GPS and ECMWF are quite similar though they are completely independent CCM3 tropical/subtropical upper troposphere temperature gradients are similar to the observed temperature gradients CCM3 Polar tropopause is much too high CCM3 has a smooth transition from the tropics to the poles in the SH while the observations show a very steep drop around 35S
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Conclusions (con’t) GPS observations exhibit larger lapse rate variability than CCM3 in general * Peak std dev. ~4.5 K/km (GPS) much larger than 2.0 K/km (CCM3) *CCM3 shows almost no variability associated with the tropical tropopause whereas GPS observations indicate it is a local maximum *In SH high latitudes, CCM3 has a local maximum in std dev while the observational std dev is decreasing towards the poles
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Acknowledgements Rob Kursinski Feiqin Xie Stig Syndergaard Carlos Minjuarez-Sosa
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GPS in the Arctic
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GPS ECMWF ARM
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