Remote Sensing of Bora Events over the North East Black Sea with Spaceborne Synthetic Aperture Radar

Bora winds are local strong downslope winds, where cold air is pushed over a coastal mountain range due to the presence of a high pressure gradient or by the passage of a cold front over a mountain ridge. They are encountered in mountainous coastal regions, where the mountain range is not too high, typically below 600 m. In Europe, strong bora winds are encountered 1) at the east coast of the Adriatic Sea, where they are called Adriatic bora or bura, and 2) at the north-east coast of the Black Sea, where they are called Novorossiyskaya bora. Bora winds can attain speeds of more than 30-35 m/s and can be quite hazardous, especially for coastal ship traffic and harbor operations. Definitely bora has impact on marine environment and coastal oceanography invoking different responses of the upper marine layer. In this study bora events over the east coast of the Black Sea, occurred in 2006-2008, are investigated by using synthetic aperture radar (SAR) images acquired by the ASAR instrument onboard the European Envisat satellite. It is shown that the sea surface roughness patterns associated with bora events, which are captured by SAR, yield information on the fine-scale structure of the bora wind field. In particular, SAR is capable of resolving 1) bora-induced wind jets and wakes, which are organized in bands practically orthogonal the coastline, 2) atmospheric gravity waves and wakes of other nature, and 3) boundaries between the bora wind fields and ambient wind fields. Quantitative information on the sea surface wind field is extracted from the Envisat ASAR images by inferring the wind direction from wind induced streaks visible on SAR images and by using the CMOD4 wind scatterometer model. It is concluded that spaceborne SAR imagery acquired during bora events is a valuable source of information about bora for risk monitoring in coastal zones and an ideal mean to validate and improve models simulating the bora impact on the marine environment.
bora, bora winds, synthetic aperture radar, SAR images, impact on marine environment.