Applications of GPS Derived data to the Atmospheric Sciences

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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 driftideal 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
a

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Derivation of Geophysical Parameters

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Occultation Movie
http//genesis.jpl.nasa.gov/zope/GENESIS/Backgroun
d/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
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5
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1
2
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4
5
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4
5
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--- GPS --- ECMWF
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2
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1
5
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5
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4
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--- GPS --- ECMWF
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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/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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C. Minjuarez-Sosa
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Planetary Boundary Layer
F. Xie
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F. Xie
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F. Xie
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PBL top
F. Xie
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Ionosphere
max
S. Syndergaard
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max
S. Syndergaard
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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 SSTs (NMC)

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Temperature vs Heights
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5
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--- GPS --- ECMWF
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GPS Zonal Mean Temperatures
Pressure
Latitude

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GPS Zonal Mean Temperature Gradients
Large displacement
Least stability
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ECMWF Zonal Mean Temperature Gradients
oscillations
Least stability
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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
375 - 300
300 - 250
250 - 200
200 - 150
GPS
ECMWF
CCM3
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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 (cont)

• 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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