Ducting and Biases of GPS Radio Occultation Bending Angle and Refractivity in the Moist Lower Troposphere
Xuelei Feng, Feiqin Xie, C. O. Ao, Richard A. Anthes
Abstract
Abstract Radio occultation (RO) can provide high-vertical-resolution thermodynamic soundings of the planetary boundary layer (PBL). However, sharp moisture gradients and strong temperature inversion lead to large gradients in refractivity N and often cause ducting. Ducting results in systematically negative RO N biases resulting from a nonunique Abel inversion problem. Using 8 years (2006–13) of Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) RO soundings and collocated European Centre for Medium-Range Weather Forecasts interim reanalysis (ERA-I) data, we confirm that the large lower-tropospheric negative N biases are mainly located in the subtropical eastern oceans and we quantify the contribution of ducting for the first time. The ducting-contributed N biases in the northeast Pacific Ocean (160°–110°W; 15°–45°N) are isolated from other sources of N biases using a two-step geometric-optics simulation. Negative bending angle biases in this region are also observed in COSMIC RO soundings. Both the negative refractivity and bending angle biases in COSMIC soundings mainly lie below ~2 km. Such bending angle biases introduce N biases that are in addition to those caused by ducting. Following the increasing PBL height from the southern California coast westward to Hawaii, centers of maxima bending angles and N biases tilt southwestward. In areas where ducting conditions prevail, ducting is the major cause of the RO N biases. Ducting-induced N biases with reference to ERA-I compose over 70% of the total negative N biases near the southern California coast, where strongest ducting conditions prevail, and decrease southwestward to less than 20% near Hawaii.