Climatological Factors Affecting Electromagnetic Surface Ducting in the Aegean Sea Region

Abstract

We have studied the conditions that affect atmospheric electromagnetic surface ducts in the Aegean Sea region and how those conditions are related to regional and global scale climate variations. As a primary source for our calculations, we used radiosonde soundings from three different stations situated around the Aegean Sea, analyzing a 20-year period, from 1991 to 2010. We derived statistics on ducting parameters and correlated them with the seasonal means of selected climate system variables. We focused on seasonal and interannual variations in surface ducting frequency, height, and strength gradient. We found that variations in low level atmospheric moisture were a dominant factor governing variations in surface ducting conditions. The moisture variations were mainly associated with fluctuations in (1) moisture advection associated with mid-latitude cyclones in winter; and (2) local and regional evaporation in summer. The frequency of surface ducts in the summer was twice that for the winter due to larger amounts of atmospheric moisture in the summer. Variations in large-scale subsidence did not seem to significantly affect surface ducting variations. From an interannual perspective, the years that were characterized by increased moisture amounts also tended to exhibit stronger and more frequent ducting conditions for both winter and summer. We found significant correlations between Aegean surface ducting conditions and (1) local and regional moisture(r = 0.85 with significance level p = 99%); and,(2) climate variations that affect local and regional moisture, such as those associated with the Arctic Oscillation (AO), North Atlantic Oscillation (NAO), and south Asian monsoon conditions (r 0.60 with p 95%). As a by-product, we also discovered significant correlations (ranging from r = 0.67 to r = 0.96 with p 95%) between ducting parameters and the vertical resolution of the radiosonde data, indicating that differences in data collection procedures need to be accounted for when conducting ducting analyses based on radiosonde data.