Effect of ducting on radio occultation measurements: An assessment based on high-resolution radiosonde soundings
Article first published online: 28 MAR 2007
Copyright 2007 by the American Geophysical Union.
Volume 42, Issue 2, April 2007
How to Cite
2007), Effect of ducting on radio occultation measurements: An assessment based on high-resolution radiosonde soundings, Radio Sci., 42, RS2008, doi:10.1029/2006RS003485.(
- Issue published online: 28 MAR 2007
- Article first published online: 28 MAR 2007
- Manuscript Accepted: 1 DEC 2006
- Manuscript Revised: 27 NOV 2006
- Manuscript Received: 21 FEB 2006
- radio occultation;
- refractivity bias;
- Abel inversion;
 Recent studies have shown that the presence of elevated ducts in the lower atmosphere has an adverse effect on the inversion of GPS radio occultation data. The problem arises because the microwave refractivity within and below an elevated duct is no longer uniquely determined by the bending angle profile. Applying Abel inversion without a priori knowledge of the duct will introduce a negative bias in the retrieved refractivity profile within and below the duct. In this work, high vertical resolution radiosonde data are used to give a quantitative assessment of the characteristics and effects of ducts, including their frequency of occurrences, heights, and thicknesses at different latitudes and seasons. The negative bias from the Abel-retrieved refractivity profiles resulting from these ducts is also computed. The results give a strong indication that ducting in the lower troposphere is a frequent phenomenon over the tropics and midlatitudes. The ducts are shown to be predominantly caused by sharp changes in the vertical structure of water vapor. The majority of the ducts are found to be below 2 km, with a median duct layer thickness of about 100 m. The negative refractivity bias is shown to be largest below 2 km, with a median value of about 0.5–1% in the tropics and 0.2–0.5% in midlatitudes. The bias is about a factor of 2–3 smaller between 2 to 3 km and is negligible above 4 km.