Investigation of surface duct conditions over Istanbul, Turkey

A comprehensive examination of 2 yr of radiosonde data to determine the surface duct conditions over Istanbul (4°N, 29°E), Turkey, was made. The refractivity of the atmosphere is a function of air temperature and water vapor pressure. Any negative gradient in the modified refractivity results in the presence of a duct in the atmosphere. Therefore, the occurrence of ducts strongly depends upon both the synoptic and the local meteorological conditions that prevail over the region. The characteristics of surface ducts occurring over Istanbul were examined statistically. It was found that most of the ducts occur in May and July. The highest occurrence rate of surface ducts was observed in the summer season, and the lowest rate was observed in the winter season. The median duct thickness and duct strength are found to be the highest and the strongest in summer, whereas they are the lowest and the weakest in winter. When the data are separated into stable and unstable atmospheric subgroups, it is seen that surface duct characteristics show clear seasonal differences. Surface ducts in a stable atmosphere are found to be stronger than those in an unstable atmosphere. Also, daytime (1200 UTC) surface ducts occur more frequently than nighttime (0000 UTC) surface ducts in Istanbul. These statistics are discussed in association with local meteorological conditions and weather systems affecting the Istanbul region, and comments are made on the importance of their possible consequences in the region.