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Xie, FQ, Adhikari L, Haase JS, Murphy B, Wang KN, Garrison JL.  2018.  Sensitivity of airborne radio occultation to tropospheric properties over ocean and land. Atmospheric Measurement Techniques. 11:763-780.   10.5194/amt-11-763-2018   AbstractWebsite

Airborne radio occultation (ARO) measurements collected during a ferry flight at the end of the PREDepression 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 (similar to 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 radioholographic 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 (ERAI) profiles shows only a small root-mean-square (RMS) refractivity difference of similar to 0.3% in the drier upper troposphere from similar to 5 to similar 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 similar to 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.

Wang, KN, Garrison JL, Haase JS, Murphy BJ.  2017.  Improvements to GPS Airborne Radio Occultation in the Lower Troposphere Through Implementation of the Phase Matching Method. Journal of Geophysical Research-Atmospheres. 122:10215-10230.   10.1002/2017jd026568   AbstractWebsite

Airborne radio occultation (ARO) is a remote sensing technique for atmospheric sounding using Global Positioning System signals received by an airborne instrument. The atmospheric refractivity profile, which depends on pressure, temperature, and water vapor, can be retrieved by measuring the signal delay due to the refractive medium through which the signal traverses. The ARO system was developed to make repeated observations within an individual meteorological event such as a tropical storm, regardless of the presence of clouds and precipitation, and complements existing observation techniques such as dropsondes and satellite remote sensing. RO systems can suffer multipath ray propagation in the lower troposphere if there are strong refractivity gradients, for example, due to a highly variable moisture distribution or a sharp boundary layer, interfering with continuous carrier phase tracking as well as complicating retrievals. The phase matching method has now been adapted for ARO and is shown to reduce negative biases in the refractivity retrieval by providing robust retrievals of bending angle in the presence of multipath. The retrieval results are presented for a flight campaign in September 2010 for Hurricane Karl in the Caribbean Sea. The accuracy is assessed through comparison with the European Centre for Medium Range Weather Forecasts Interim Reanalysis. The fractional difference in refractivity can be maintained at a standard deviation of 2% from flight level down to a height of 2km. The phase matching method decreases the negative refractivity bias by as much as 4% over the classical geometrical optics retrieval method.