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

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)

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.

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

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.

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.

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.

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)

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

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

Complete time series PSC18 54 days PSC19 42 days

Gravity wave analysis Case study when balloon passing over Antarctic Peninsula PSC19 on 2010.311

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

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

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

Pressure gradient depends on height accuracy

Pressure gradient depends on height accuracy

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

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: 2010-11-06_00:00:00 To : 2010-11-08_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

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

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

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

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

Balloon Rotation Study Two receivers (V181 and V182) on 2010.286 Get the baseline solutions using GrafNav The baseline azimuth time series (angles vs. north) 3 order polynomial fit 41 points window

Balloon Rotation Study Panel temperature time series on 2010.286

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.

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

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

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