Radio-Refractivity Changes Associated with the Sea and Land Breezes Along the Arabian Sea and Bay of Bengal Coasts and their Influence on Microwave Propagation

Abstract

A detailed study of the hourly variations of refractivity has been made from the meteorological data for a number of years at stations along the coasts of the Arabian Sea and the Bay of Bengal, during the clear months October to May. The prevailing winds and sea surface temperatures have also been taken into consideration when drawing inferences on the occurrence of anomalous propagation associated with the land and sea breezes.Ducting conditions over the coastal land areas will be absent in the sea breeze during the day when convection is active, but will be experienced from about the evening when the land cools and the temperature inversion begins to form and prevents convection. The ducting increases in intensity till the land breeze begins to flow. With the remaining sea-breeze moisture and the strong inversion, while there may be a fall in refractivity near the surface, ducting conditions will prevail. After sunrise, during the land breeze, there will be an increase in the intensity of the ducting due to the evaporation of moisture in the ground. However, with the increased convectional mixing with the upper layers, the ducting conditions will disappear. Later the warm land breeze which flows over the adjacent sea will cause conditions suitable for anomalous propagation over the sea. Thus, a shifting of the ducting conditions from the land areas to the sea takes place during the warm forenoon till the sea breeze sets in. it is expected that normal propagation conditions will prevail from the time of onset of the sea breeze till about sunset.Along the Arabian sea coast, from October to February, the pattern of ducting conditions changes wi h latitude. The times of onset and duration of the sea breeze undergo changes. However, from March to May, there is an appreciable change due to the altered conditions over the Arabian sea. An anticyclone develops near about the middle regions of the sea and air from the warmer areas flows over colder areas to the west of the anticyclone and later strike the west coast as a westerly over Bombay and to its northern latitudes. As a result anomalous propagation and ducting extends over a large area of the Arabian sea with little diurnal variation, particularly over the latitudes to the north of Bombay. During these months, ducting will be absent only over the land when convection is largest. Normal propagation will prevail all along the coasts of the Arabian sea during the monsoon months.Along the coasts of the Bay of Bengal, the conditions during October to February are different from those on the Arabian Sea. There is no common feature among Madras, Visakhapatnam and Calcutta. The sea breeze is practically absent at Calcutta, it is observed to occur at Visakhapatnam, and the northeast monsoon influences the land and sea breezes at Madras during October and November. However, from March to May, there is a large change in the prevailing conditions due to the variation in the distribution of the sea surface temperature and wind circulation over the Bay of Bengal, and are similar to those on the Arabian Sea. Ducting conditions prevail most of the time along the coasts in March-April and extend over the whole Bay in May when warmer air from the land flows over the cooler sea. During this minth, the large-scale ducting condition over the sea is accentuated near the coasts by the prevailing land breeze which is warmer than the sea most of the day and even night.An interesting feature over the Bay of Bengal coast is observed during the monsoon. While normal propagation prevails over most of the country, the land and sea breezes are strongest and prominent at Madras in this season and ducting conditions are experienced to the south of Madras.