1. Profiling the Antarctic Atmosphere Using the GPS Radio
Occultation Technique from Stratospheric Balloons
J. Maldonado Vargas1, J.S. Haase2, F. Rabier3, P. Cocquerez4, M. Minois5, V. Guidard3, P. Wyss6, A. Johnson2 and B. Murphy2
1 Department of Electrical and Computer Engineering, University of Puerto Rico, Mayaguez Campus; jayson.maldonado@upr.edu; 2 Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, IN
3 CNRM-GAME/GMAP Meteo-France and CNRS Toulouse, France; 4 Centre National d’Etudes Spatiales, Toulouse, France; 5 SOGETI High Tech, Blagnac, France; 6 Dept of Chemistry, Purdue University, West Lafayette, IN
Intercomparison of Radio Occultation, ARPEGE model and dropsondes
Figure 6. (Left) Excess Doppler (derivative of phase) for the GPSRO observations for PRN25 corrected by high elevation satellite PRN13,
and simulated excess Doppler calculated for the ARPEGE model profile and the dropsonde profile. The excess Doppler in the GPS signal
increases as the line of sight between the balloon and the occulting GPS satellite moves lower in the atmosphere. The difference between
observed and calculated Doppler is shown in the lower left panel.
(Right) Impact parameter vs retrieved bending angle obtained using the raytracing simulation (black line) for the ARPEGE model and the
retrieved bending angle from the occultation measurements with different assumptions for in-situ refractivity at the balloon height.
Refractivity retrieval
Figure 7. (left) ARPEGE, dropsonde, and retrieved refractivity. (center) Percent refractivity difference between retrieved and ARPEGE and
dropsonde profiles above 8km. (right) Difference between ARPEGE and dropsonde profiles for the full height of profiles.
Discussion and Conclusions
• The agreement between the ARPEGE model and retrieved refractivity is better than 2%.
• The agreement between dropsonde and retrieved refractivity is better than 1%.
• This indicates the potential for assmilated radio occultation data to provide further model
improvement in the Antarctic.
• With off-the shelf receiver and antenna components and limited transmission bandwidth, the system
succeeded in providing more 711 occultations, 32% of which reached within 4 km of the surface.
• Future technological developments, in particular higher gain antennas, could easily lead to even
more of this unique type of data, with deeper penetration depth.
Acknowledgements
This work was supported by NSF grants 0814290 and ANT-1043676. We thank the following people and
organizations for their support: NSF Office of Polar Programs for the instrument deployment field support; James
Zimmerman and Mike Everly at the Purdue University AMY Chemistry Facility for assistance with hardware
development; Olivier Gallien, Jean-Marc Nicot and the team at CNES for assistance with the GPS ROC integration
into the stratospheric balloons; Steve Cohn, Junho Wang, and NCAR EOL for supplying the dropsonde dataset;
University of Puerto Rico of Mayaguez for funding a summer internship for portions of this work; Paytsar Muradyan
for assistance with the data processing. Map graphics were produced using the GMT software. Concordiasi was built
by an international scientific group and is currently supported by the following agencies: Météo-France, CNES,
CNRS/INSU, NSF, NCAR, University of Wyoming, University of Colorado, the Alfred Wegener Institute, the Met Office
and ECMWF. Concordiasi also benefits from logistic or financial support of the operational polar agencies IPEV,
PNRA, USAP and BAS, and from BSRN measurements at Concordia. Concordiasi is part of the THORPEX-IPY cluster
within the International Polar Year effort.
References
[1] David W. J. Thompson and Susan Solomon (2002), Interpretation of Recent Southern Hemisphere Climate Change, Science VOL 296.
[2] Aparicio, J., G. Deblonde, L. Garand, and S. Laroche (2009), The signature of the atmospheric compressibility factor in COSMIC, CHAMP and
GRACE radio occultation data, J. Geophys. Res.
[3] Healy, S. B. (2011), Refractivity coefficients used in the assimilation of GPS radio occultation measurements, J. Geophys. Res., 116,D01106,
doi:10.1029/2010JD014013.
[4] G. A.Hajj, E. R. kursinski, L. J. Romans, W. I. Bertiger, S. S. Leroy (2002), A technical description of atmospheric sounding by GPS occultation,
Jet Propulsion Laboratory, California Institute of Technology, Mail Stop 238-600, 4800 Oak Grove Dr., Pasadena, CA 91109, USA.
[5] Feiqin Xie, Jennifer S. Haase and Stig Syndergaard (2008), Profiling the Atmosphere Using the Airborne GPS Radio Occultation Technique: A
Sensitivity Study, IEEE Transactions on Geoscience and Remote Sensing, VOL. 46, NO. 11.
[6] S. P. E. Keeley, N. P. Gillett, D. W. J. Thompson, S. Solomon, and P. M. Forster (2007), Is Antarctic climate most sensitive to ozone
depletion in the middle or lower stratosphere?, Geophysical Research Letters, VOL. 34, L22812, doi:10.1029/2007GL031238, 2007.
Figure 3. Geometry of the airborne GPSRO technique.
In this work, we compare occultation observations and retrieved refractivity with simulated
values of phase delays, bending angle, and refractivity using a geometrical optics raytracing
method. We compare values at several steps in the processing chain shown in figure 4 below.
Figure 4. Representation of the radio occultation processing procedure.
For a consistent comparison, geometric height coordinates must be used rather than pressure
coordinates, therefore geometric height is estimated from the atmospheric variables as follows:
ARPEGE model and dropsonde profiles
The Météofrance ARPEGE model has 70 vertical levels, and uses a stretched grid centered on
Antarctica, so that spectral truncation at T798 gives a horizontal resolution of 10–15 km over
Antarctica and 60 km at the antipodes. The model profile shown below is approximately 250 km
from the dropsonde location.
Figure 5 compares the ARPEGE model and dropsonde sounding. The geometric height takes into account the difference
between the true geoid and the WGS-84 ellipsoid, and height dependence of gravity.
Figure 5 shows small refractivity differences between the ARPEGE model and the dropsonde
profile with a maximum refractivity difference of 1.8 N-units, with the ARPEGE refractivity
higher than the dropsonde refractivity. Although relative humidity differences are large,
because of the extremely low temperatures, this does not correspond to a large difference in
specific humidity, and the moisture is not likely to be resolvable with occultation data.
Abstract
The second phase of the Antarctic Concordiasi campaign was carried out in the austral spring of
2010 to study ozone and polar stratospheric clouds, gravity waves over the Antarctic peninsula,
and data assimilation in numerical weather prediction models using observations made from
stratospheric superpressure balloons. For the first time, GPS radio occultation (GPSRO)
measurements were made from these stratospheric balloons to provide atmospheric profiles of
refractivity. Dropsondes were also deployed for assessing the quality of satellite data assimilation
over the Antarctic. We compare the GPS Radio occultation measurements near the Antarctic
Peninsula with refractivity derived from nearby dropsondes and the Méteofrance ARPEGE model
to demonstrate the potential for this data also to serve for model validation. We discuss in detail
the method used to map the dropsonde measurements and model profiles of temperature and
relative humidity as a function of pressure to refractivity as a function of geometric height.
Motivation for improving Antarctic numerical model analyses
• Climate models show that a CO2 increase in the atmosphere leads to a decrease in pressure
centered over Antartica, intensification of the Antarctic polar vortex and an increase in the
strength of the circumpolar westerlies that affect sea ice distribution and surface temperature
trends.
• However, stronger westerly flow in the Antarctic polar vortex is also correlated with trends in
total column ozone.
• To improve climate model and reanalysis quality in the Antarctic and understand these factors
forcing climate change, more independent observations of the atmosphere are required.
Concordiasi Balloon Campaign
19 long duration stratospheric balloons were deployed:
4 Balloons equipped with microphysics and ozone sensors.
2 Balloons equipped with Radio Occultation (ROC) and ozone sensors.
13 Balloons equipped with driftsonde packages for the deployment of up to 6 to 20
dropsondes profiles per day
.
A B C
Figure 1. Trajectories for all (A) and GPSRO (B) stratospheric balloons during the campaign. (C) Flight trajectories for PSC18 and
PSC19, the flights carrying the ROC instrument. PSC18 is shown in blue, PSC19 is shown in black. Diamond symbols indicate the
locations of the occultation recordings. Red diamonds show the locations of the dropsondes released from the 13 MSD balloons. The
blue star indicates the location of a dropsonde close in space and time to the occultation data analyzed below.
Figure 2. Histograms of the penetration depth of the occultation measurements for flight PSC18 (left) and PSC19 (right). Top figures
show penetration depth in terms of height above the WGS84 reference ellipsoid. Lower figures show penetration in terms of height
above the land surface.
The two balloons equipped with GPS radio occultation systems flew a combined total of 107 days
within the Antarctic polar vortex, and recorded more than 700 occultations, a comparable number
of profiles to the 647 dropsondes released by the 13 balloons equipped with driftsonde systems
during the campaign.
0 5 10 15
0
20
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height ( km above ellipsoid )
numberofoccultations
Flight PSC18
0 5 10 15
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20
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height ( km above ellipsoid )
numberofoccultations
Flight PSC19
0 5 10 15
0
20
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height above land surface (km)
numberofoccultations
0 5 10 15
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height above land surface (km)
numberofoccultations
GPS
GPS
Balloon
Tangent Points
GPS
rE
Center of
the Earth
rt
GPS Radio oocultation (GPSRO) is a
remote sensing technique that relies on
measurements of GPS dual-frequency
phase delays collected from an airborne
or spaceborne receiver which is tracking
a GPS satellite that is setting or rising
behind the Earth’s limb.
Methodology