A ducting climatology derived from the European centre for medium-range weather forecasts global analysis fields

Abstract

A global ducting climatology based on 6 years of European Centre for Medium-Range Weather Forecasts (ECMWF) data is presented. The ECMWF data have a resolution of 1.5°, 60 vertical levels, and a 6-hour daily frequency. Ducting probability, altitude, layer thickness, and magnitude are calculated for different seasons and universal times by analyzing the refractivity gradient with respect to altitude. Owing to the limited ECMWF vertical resolution, presented results are restricted to the lowest 2-2.5 km of the atmosphere. The climatology has mainly been generated for radio occultation data analysis, where ducting events found at higher altitudes are generally not affecting the signal acquisition. Since ducting is so much associated with boundary layer inversions and in the subtropics with cloud-topped boundary layers, this study can also be used as an inversion climatology. High ducting probabilities (≈100%) are found off the west coasts of the Americas, Africa, and Australia in typical stratocumulus conditions. High probability is also observed over the Arabian Sea, with large seasonal variations. Polar nights lead to high ducting probabilities, especially visible over Antarctica but also over Greenland and Siberia. The daily cycle of the probabilities shows the impact of radiative cooling during the night over desert and tundra areas. Maximum mean ducting altitudes are found further off the west coasts. Land-based ducting events are generally near the surface. The mean ducting layer thickness is higher over the sea, with maximum thicknesses of up to 250 m. The mean magnitude of ducting is usually just below the critical gradient. Copyright 2004 by the American Geophysical Union.