Ducting and biases of GPS radio occultation bending angle and refractivity in the moist lower troposphere

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 (1608-1108W; 158-458N) are isolated from other sources of N biases using a two-step geometric-optics simulation.