A long term (1999–2008) study of radar anomalous propagation conditions in the Western Mediterranean

In this paper a study of the radio propagation environment of electromagnetic waves prevailing in the lower troposphere of the Western Mediterranean basin is presented. Deviations from atmospheric average or standard radio propagation conditions (anomalous propagation or AP) can affect significantly the quality of weather radar observations and other telecommunication systems. This is particularly important when ducting or superrefraction is present and spurious echoes resulting from the interaction of the beam with the ground or sea surface may appear. These AP conditions occur mainly when temperature inversions or strong moisture gradients are present. The period covered in this study ranges from 1999 to 2008 and conditions were derived from the Weather Research and Forecasting (WRF) modeling system, using the Japanese 25-year Reanalysis (JRA25) dataset as initial and boundary data. From the WRF model, we use the temperature, moisture, and pressure fields with a grid length of 30-km horizontal resolution and 250 m vertical resolution to compute several indices such as the Vertical Refractivity Gradient, Vertical Modified Refractivity Gradient and a Ducting Index. Results obtained show that on the Western Mediterranean coast the most favorable conditions for superrefraction are found in summer, while the most affected areas are the Gulf of Valencia, the Strait of Gibraltar and the Northern Gulf of Lion. Additionally, a comparison with radiosonde data recorded in Barcelona (NE Spain) is also performed indicating an overall agreement between model and observational data despite a tendency to decrease subrefractive events by the WRF model.