Overview
History of GPS How GPS occultations work 3 GPS campaigns Applications of GPS Characterizing the Atmosphere
using GPS: Zonal Means and Arctic
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)
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
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
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
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
Derivation of Geophysical Parameters
6 61 2 2 2
1 10 40.3 10w eP P nN n a a
T T f
Ph
= – g
1
m P mN
R T Ra
Occultation Movie
http://genesis.jpl.nasa.gov/zope/GENESIS/Background/Movie
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
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
CHAMP Orbit
http://op.gfz-potsdam.de/champ/index_CHAMP.html
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
Passage of Javier Remnants over Tucson
http://www.gst.ucar.edu/gpsrg/realtime/
Hurricane Katrina
• http://www.suominet.ucar.edu/katrina/katrina.mov
Applications of GPS
Temperature Measurement Water Vapor Measurement Planetary Boundary Layer Ionosphere
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
An Investigation into Observed and Modeled Global Atmospheric Stability
Jaci Secora, Rob Kursinski, Andrea Hahmann, Dan
Hankins
Overview
Motivation of Study GPS/MET Mission ECMWF Analysis NCAR Community Climate Model GPS/ECMWF/CCM3 Comparisons Conclusions
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
Purpose of Study
Study evaluates representation and variability of stability in climate models as well as characterizing the stability in the real atmosphere
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
Data Sets Used in this Study
GPS: Observations
ECMWF: Analysis = Model + Observations (Not GPS Observations)
CCM3: Model
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)
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
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)
Temperature Gradient Histograms from 40S to 50S
GPS
ECMWF
CCM3
375 - 300 300 - 250 250 - 200 200 - 150
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.
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
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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