Seminar Ozean, Eis, Atmosphäre

Date: 03.02.2004

Speaker: Dr. A. von Engeln (IUP)

The real part of the atmospheric refractive index changes with pressure, temperature, and water vapor. The main change is caused by pressure and thus by altitude. This can lead to atmospheric phenomena such as the ability to see behind the horizon, e.g. we can still see the sun at the horizon even though it would not be visible along a straight line. Strong variations in temperature or water vapor can also change the refractive index significantly, leading to phenomena such as mirages.

The refractive index is frequency dependent in the visible range but non dispersive for low frequencies (up to 30 GHz). The radio occultation technique observes GPS signals from a LEO satellite and measures the ability to look behind the horizon. This ability is influenced at the low GPS frequencies by atmospheric temperature, pressure, and water vapor, hence radio occultation is able to predict these parameters. A requirement for this observation is that the ray emitted by the GPS satellite exits the atmosphere on the other end for detection. Ducting describes a large variation of the refractive index which can lead to rays being bended into the Earth or channeled along a waveguide and thus lost for satellite observation.

Ducting frequently happens at the top of the planetary boundary layer but can also happen near the ground. Radio occultation measurements encountering ducting will be distorted or completely lost. The distortion leads to a bias in the measurement, while the lost signal determines the lowest altitude that can be sampled. Thus it is highly desirable for the radio occultation technique to determine where and when ducting might occur.

The occurrence, altitude, and thickness of ducting has been derived from ECMWF global fields and will be presented. Results have been compared to the radio occultation instrument CHAMP. In addition, the military has a high interest in ducting, reasons for this will be outlined.