Ducting and Biases of GPS Radio Occultation Bending Angle and Refractivity in the Moist Lower Troposphere

Radio occultation (RO) can provide high vertical resolution thermodynamic17 soundings of the planetary boundary layer (PBL). However, sharp moisture gradients and18 strong temperature inversion lead to large refractivity (N) gradients, and often cause19 ducting. Ducting results in systematically negative RO N-biases due to a non-unique Abel20 inversion problem. Using 8-year (2006-2013) Constellation Observing System for21 Meteorology Ionosphere and Climate (COSMIC) RO soundings and collocated European22 Centre for Medium-Range Weather Forecasts (ECMWF) interim reanalysis (ERA-I) data,23 we confirm that the large lower tropospheric negative N-biases are mainly located in the24 subtropical eastern oceans, and quantify the contribution of ducting for the first time. The25 ducting-contributed N-biases in the northeast Pacific (160°W~110°W; 15°N~45°N) are26 isolated from other sources of N-biases using a two-step geometric-optics simulation.27 Negative bending angle biases in this region are also observed in COSMIC RO28 soundings. Both the negative refractivity and bending angle biases from COSMIC29 soundings mainly lie below ~2-km. Such bending angle biases introduce additional N30biases to those caused by ducting. Following the increasing PBL height from the southern31 California coast westward to Hawaii, centers of maxima bending angles and N-biases tilt32 southwestward. In areas where ducting conditions prevail, ducting is the major cause of33 the RO N-biases. Ducting-induced N-biases with reference to ERA-I comprise over 70%34 of the total negative N-biases near the California coast where strongest ducting conditions35 prevail, and decrease southwestward to less than 20% near Hawaii.