Refratividade atmosférica em áreas oceânicas baseada em dados de sondagens atmosféricas na região da Ilha de Trindade

Abstract

The coastal region of Brazil has an intense maritime traffic, composed by several types of ships. The safety of navigation is important and therefore vessels equipped with communication and detection equipment, and the performance of such equipment is strongly affected by the conditions of electromagnetic propagation, caused by strong vertical gradients of atmospheric refractivity may favor the communication / detection at a distance higher than expected by the manual of such equipment, the phenomenon responsible for this behavior is called the evaporation duct. Therefore the objective of the present study is to evaluate the atmospheric refractivity conditions, as well as to estimate the heights of the evaporation ducts based on the modified index (M), using atmospheric soundings data collected at Trindade Island. The data correspond to the period from 2005 to 2016, and the presence of 3348 surface ducts was observed, 65% of them up to 220 m height. The highest hight averages were observed in the winter months, with 160 m in June and July. The lowest values were observed in December with 107 m. In the case studies, the evaluation of thermodynamic profiles demonstrated that in days with occurrence of evaporation ducts the presence of unstable layer near the ocean surface were observed, with a stable layer above them, and the synoptic conditions for these days are characterized by the presence of high-pressure system or by the action of Subtropical South Atlantic Anticyclone. In episodes with absence of surface duct, the presence of frontal system, as well as a vertical zero gradient of humidity and the presence of a stable boundary layer were observed, inhibiting the formation of ducts.