ALBERT

Description: Airborne radio occultation (ARO) is a remote sensing technique for atmospheric sounding using Global Positioning System (GPS) 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 Center for Medium Range Weather Forecasting (ECMWF) Interim Reanalysis (ERA-I). The fractional difference in refractivity can be maintained at a standard deviation of 2% from flight level down to a height of 2 km. The PM method decreases the negative refractivity bias by as much as 4% over the classical geometrical optics retrieval method.