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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union

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Atmos. Meas. Tech., 11, 763-780, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
08 Feb 2018
Sensitivity of airborne radio occultation to tropospheric properties over ocean and land
Feiqin Xie1, Loknath Adhikari1, Jennifer S. Haase2, Brian Murphy3,a, Kuo-Nung Wang4, and James L. Garrison5 1Department of Physical and Environmental Sciences, Texas A&M University – Corpus Christi, Corpus Christi, Texas, USA
2Scripps Institution of Oceanography, University of California, San Diego, California, USA
3Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, Indiana, USA
4Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
5School of Aeronautics and Astronautics, Purdue University, West Lafayette, Indiana, USA
anow at: Edison State Community College, Piqua, Ohio, USA
Abstract. Airborne radio occultation (ARO) measurements collected during a ferry flight at the end of the PRE-Depression Investigation of Cloud-systems in the Tropics (PREDICT) field campaign from the Virgin Islands to Colorado are analyzed. The large contrast in atmospheric conditions along the flight path from the warm and moist Caribbean Sea to the much drier and cooler continental conditions provides a unique opportunity to address the sensitivity of ARO measurements to the tropospheric temperature and moisture changes. This long flight at nearly constant altitude (∼ 13 km) provided an optimal configuration for simultaneous high-quality ARO measurements from two high-gain side-looking antennas, as well as one relatively lower gain zenith (top) antenna. The omnidirectional top antenna has the advantage of tracking robustly more occulting satellites in all direction as compared to the limited-azimuth tracking of the side-looking antennas. Two well-adapted radio-holographic bending angle retrieval methods, full-spectrum inversion (FSI) and phase matching (PM), were compared with the standard geometric-optics (GO) retrieval method. Comparison of the ARO retrievals from the top antenna with the near-coincident ECMWF reanalysis-interim (ERA-I) profiles shows only a small root-mean-square (RMS) refractivity difference of ∼ 0.3 % in the drier upper troposphere from ∼ 5 to ∼ 11.5 km over both land and ocean. Both the FSI and PM methods improve the ARO retrievals in the moist lower troposphere and reduce the negative bias found in the GO retrieval due to atmospheric multipath. In the lowest layer of the troposphere, the ARO refractivity derived using FSI shows a negative bias of about −2 %. The increase of the refractivity bias occurs below 5 km over the ocean and below 3.5 km over land, corresponding to the approximate altitude of large vertical moisture gradients above the ocean and land surface, respectively. In comparisons to radiosondes, the FSI ARO soundings capture well the height of layers with sharp refractivity gradients but display a negative refractivity bias inside the boundary layer. The unique opportunity to make simultaneous independent recordings of occultation events from multiple antennas establishes that high-precision ARO measurements can be achieved corresponding to an RMS difference better than 0.2 % in refractivity (or ∼ 0.4 K). The surprisingly good quality of recordings from a very simple zenith antenna increases the feasibility of developing an operational tropospheric sounding system onboard commercial aircraft in the future, which could provide a large number of data for direct assimilation in numerical weather prediction models.

Citation: Xie, F., Adhikari, L., Haase, J. S., Murphy, B., Wang, K.-N., and Garrison, J. L.: Sensitivity of airborne radio occultation to tropospheric properties over ocean and land, Atmos. Meas. Tech., 11, 763-780,, 2018.
Publications Copernicus
Short summary
The GPS signal going through the atmosphere will experience refraction or bending, which can be precisely measured and used to infer the atmospheric properties. This paper demonstrates that high-quality atmospheric measurement with less than ~ 0.4 K is achievable from a GPS recording system with a simple antenna mounted on top of an aircraft cruising at ~ 13 km. Such a simple airborne GPS system can be implemented on commercial aircraft to provide valuable data for weather models in the future.
The GPS signal going through the atmosphere will experience refraction or bending, which can be...