Local meteorology and its effect on electromagnetic wave propagation over the southern coast of the Arabian Gulf

Abstract

The propagation of electromagnetic (EM) waves of frequencies above 100MHz is affected by the existence and properties of the atmospheric duct, i.e. a horizontal layer in the lower atmosphere in which radio signals propagate more efficiently. Atmospheric ducts can be found in many parts of the world ocean including the Arabian Gulf. Ambient winds blowing from different directions bring air masses of different properties into the area and hence have a significant impact on the formation and strength of the atmospheric duct. However, little information is available on the long-term and intra-annual variability of wind and its effect on the ducting phenomenon in the Arabian Gulf region. This study addresses this gap by characterising the local meteorology, with a special emphasis on its effect on electromagnetic wave propagation. This study uses a new methodology to measure the persistence of Shamal wind, by considering the number of days associated with the specific wind pattern in addition to commonly used parameters such as the wind speed. In this study, thirty years (1981-2010) of observations and NCEP/NCAR reanalysis data have been analyzed to identify a long-term trend and the intra-annual variability of various wind systems. Results clearly indicate that the Shamal (the northwesterly wind) is the most frequent meteorological feature over the region; therefore it has been investigated in greater detail. The Suhaili (southerly wind) is the second important wind which can occur any time of the year but it is less frequent than the Shamal. The Al-Nashi (cold and dry northeasterly wind) wind occurs only in December, January and February. The analysis shows that the wind strength and the frequency of Shamal days over the region have decreased over the last thirty years. Variations in the occurrence of summer and winter Shamal days were studied in relation to global atmospheric phenomena, and relationships have been established, synoptically and statistically between the frequency of Shamal days and large-scale atmospheric fluctuations. These links include atmospheric fluctuations over the Caspian Sea (a correlation coefficient of 0.66) and Siberia (0.69) in summer and Greenland (0.51) and Western Europe (0.65) in winter. The frequency of winter Shamal days during December, January and February are shown to be statistically related (a correlation coefficient of 0.41) to the North Atlantic Oscillation (NAO) and (0.49) the Arctic Oscillation (AO), as they influence the pathway of the westerly depression over the north Atlantic Ocean during the winter season. It is also shown that the decline in the number of Shamal days is linked to a decrease in the number of westerly depressions. The EM wave propagation has been examined using the Advanced Refraction Effects Prediction System (AREPS) model for different representative air masses. The radiosonde data from Abu Dhabi airport used in AREPS provided evidence of the general influence of each air mass. It was found that atmospheric ducting conditions and characteristics (height, thickness, and type) were variable in the lower part of the atmosphere (surface to 6000m) as a result of changing air masses. The influence of the Shamal conditions develop an elevated duct at approximately 850mb level. The Suhaili increases the thickness of the evaporation duct. In regards to the surface based and elevated duct, Suhaili and Al-Nashi provide standard atmospheric conditions. Land and sea breezes were mostly associated with the surface based duct and sometimes elevated the duct. Atmospheric ducting could extend the range of electromagnetic wave propagation above the usual range. Good knowledge of atmospheric duct characteristics enables the efficient assessment of the range of EM propagation, which is important for a number of practical applications, for example air traffic control and rescue operations. This could include the selection of the appropriate frequency and altitude of the electromagnetic wave device (e.g. radar and/or communication systems) operating with a frequency above 100MHz to be trapped in the duct to cover long distances.