1. Radio Occultation (ROC) Instrument for Strateole
Jennifer Haase and Weixing Zhang
Scripps Institution of Oceanography

2. GPS Radio Occultation Concept
GPS satellite at
positive elevation
Stratospheric balloon
Zero elevation
Atmospheric refractivity
Is retrieved at the indicated
tangent points
rballoon
rearth
rtangent
The GPS radio occultation concept is shown here. A receiver on the balloon measures the signal from a GPS Satellite as it sets beyond the horizon. The signal is delayed and refracted as it passes nearly horizontally through the atmosphere becaues it is not a vacuum. With assumptions on spherical symmetry we can attribute the signal refraction and bending to the properties at the point of closest approach to the surface. In this way we can derive a profile, albeit a slanted profile, of atmopsheric refractivity through the atmosphere.
ratmo
Negative elevation
(occultation)

3. Observation geometry Maximum of 115 occultations
per day, 58 of which are setting.
Actually transmitted to ground 6.
Large tangent drifts ~250km.
Prior to the campaign we simulated the type of observations we would record using a 110 day trajectory from a similar campaign in 2005.
Each line shows the horizontal extent of the slanted refractivity profile that would be retrieved for each setting GPS satellite. In green are all the occultations that would occur over a period of one day, on average 115 occultations. We had to limit our data collection because of limited bandwidth available on the Iridium satellite comunication link.

4. Important Points Data transmission may be limiting factor:
3 Mbytes per day => 6 profiles of 120 possible
Accuracy ~ 1% in refractivity ~2K (still working to improve this)
Penetration to the lower atmosphere will be less than for Concordiasi, expect 7-9 km altitude
Upgrades desired:
possible 2 way transmission for precise on- board positioning
on-board processing for excess phase

5. PSC18 and PSC19 711 occultations total 639 dropsondes total o PSC18
o dropsondes
In comparison for 13 other balloons that carried driftsonde packages, they released a total of about 650 profiles. So two balloons provided a comparable number of refractivity profiles, although the GPS radio occultation does not permit to distinguish unamibigously between moisture and temperature and of course do not measure wind. We will compare the observations from an occultation from flight 19 to a dropsonde released here and the nearest model grid node.

6. Penetration depth of occultations
With a simple commercial receiver and antenna we don’t always get very low in the atmosphere. For flight 18 we have the number of occultations that get below 8 km from the surface all the way to those profiles that get within 1km of the surface here about 40.
For flight 19, 15 are wihthin 1km. We have a very significant dataset.

7. Excess Doppler for PRN25 compared with simulated excess Doppler
First we did a forward raytracing calculation for the excess phase that is, given the dropsonde refractivity model we calculated the expected increase in observed phase due to the refractivity relative to the straight line vacuum path. We show here the derivative, or excess doppler from that calculation. You see that with time as the satellite sites the obseved and predicted excess phase and resulting excess Doppler shift increase as the ray path samples deeper and deeper into the atmosphere. The predicted excess phase very closely matches the observed for the Dropsonde as well as that predicted using the ARPEGE model, shown in Red. This demonstrates that the system successfullly samples observes the refractivity structure of the atmospehre.

8. Dropsonde/model comparison
Refractivity difference between a nearby dropsonde refractivity profile and:
retrieved ROC refractivity profile (blue),
NCEP refractivity (black),
AMPS refractivity (green) or ARPEGE (green)

9. Improved GPS analysis
Corrected phase data for millisecond time offsets New PPP with ambiguity resolution software Uses ground network to resolve corrections (ambiguities) then calculates absolute position Now reanalyzing occultation data

10. Estimating accuracy of position
Inclined GPS antennas for recording high elevation navigation signals and low elevation occultation signals.
Relative position of ant2 relative to ant1
Difference between absolute position of ant2 and ant1
Rms 7 cm horizontal, 20 cm vertical
ROC

11. Complete time series
PSC18 54 days PSC19 42 days

12. Gravity wave analysis
Case study when balloon passing over Antarctic Peninsula
PSC19 on

13. Gravity wave analysis Perturbation time series
Comparison between PPPAR and CNES solutions
Fit the trend using
3-order polynomial
41 points window
(Vincent & Hertzog, 2014)
CNES-based results
PPAR-based results

14. Gravity wave analysis Perturbation time series Zoom in
PPAR-based results
CNES-based results

15. Gravity wave analysis Perturbation time series Zoom in
PPAR-based results
CNES-based results

16. Pressure gradient depends on height accuracy

17. Pressure gradient depends on height accuracy

18. Gravity wave analysis Perturbation time series FFT results 285 sec
PPAR-based results
285 sec
CNES-based results

19. Comparison with WRF simulation
WRF simulation Domain Two-way nesting Horizontal resolution: 21 km (d01) and 7 km (d02) Vertical: 61 levels from the ground to 10 hPa Period From: _00:00:00 To : _00:00:00 Physical scheme
Item
scheme
Microphysics
Ferrier (new Eta)
Cumulus
Kain-Fritsch (new Eta) scheme
Boundary-layer
Mellor-Yamada-Janjic TKE scheme
Surface-layer
Monin-Obukhov (Janjic) scheme
Land-surface
Unified Noah land-surface model
Longwave radiation
RRTMG scheme
Shortwave radiation
Goddard short wave
Upper level damping flag
With diffusive damping

20. Comparison with WRF simulation
The horizontal and vertical wind speed at 17.4 km height is ~consistent with PPPAR results

21. Comparison with WRF simulation
Cross-section view of potential temperature and wind speed along the balloon trajectory

22. WRF model / dropsonde comparison
Note dropsonde not on the same day Looking for gravity wave characteristics

23. Next steps in preparation for Strateole
Next steps are to raytrace through the atmospheric model perturbed by gravity waves in 2D to calculate accumulated delay Perform an inversion to see how mapped refractivity represents actual refractivity Estimate probability of recording gravity wave from balloon position simultaneously with temperature profile variations from occultation measurements

24. Balloon Rotation Study
Two receivers (V181 and V182) on Get the baseline solutions using GrafNav The baseline azimuth time series (angles vs. north)
3 order polynomial fit
41 points window

25. Balloon Rotation Study
Panel temperature time series on

26. Refractivity Retrieval
We performed a retrieval by inverting the excess doppler by turning it into an estimate of the geometric bending angle and then inverting it for refractivity structure. Left shows the refractivity profile for model, dropsonde and observed. The are very close. On the right we show the diference between the retrieved refractivity minus that from the arpege model in red, less than 2 % and also the difference between the observed dropsonde and radio occultation. This is less than 1 percent, which indicates the RO data is close enough to the true atmospehre (prepresented by the dropsonde) to provide information to improve the model.

27. Conclusions
Successful mission – exceeded expectations for a prototype mission
Retrieved 6-9 profiles per day for each balloon as expected with equally good quality for rising and setting occultations
During the two balloon flights:
a combined total of 107 days,
more than 700 occultations were recorded (number limited by the data transmission rates)
More than 32% of the profiles (227) descended within 4km of surface
Very good outlook for contributing to the goal of improving atmospheric models in the Antarctic
In summary

28. PSC18 and PSC19 flights
We recorded a totle of 711 occultations with duration greater tan 7 minutes of continuous data below the horizon
Flight 18 was up for 54 days and flight 19 was up for 42 days
We estimated how many of these were of sufficient duration to provide an occultation that extended within 4km of the surface:
43% for flight 18 and 22% for flight 19, probably because these flights were over the ocean, and propagation through the moister boundary layer structure could have contributed to reduction in signal strength and continuity
155 occultations rising 570 sec mean for both rising and setting
182 occultations setting
Total of 711 occultations with duration greater than 7 minutes of continuous data below the horizon
639 total dropsondes on 13 balloons

29. Gravity wave analysis Perturbation time series Zoom in
PPAR-based results
CNES-based results