Observations of Bora Events over the Adriatic Sea and Black Sea by Spaceborne Synthetic Aperture Radar

Abstract

Bora events over the Adriatic Sea and Black Sea are investigated by using synthetic aperture radar (SAR) images acquired by the advanced SAR (ASAR) on board the European satellite Envisat. It is shown that the sea surface roughness patterns associated with bora events, which are captured by SAR, yield information on the finescale structure of the bora wind field that cannot be obtained by other spaceborne instruments. In particular, SAR is capable of resolving 1) bora-induced wind jets and wakes that are organized in bands normal to the coastline, 2) atmospheric gravity waves, 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 C-band wind scatterometer model CMOD_IFR2 to convert normalized cross sections into wind speeds. It is argued that spaceborne SAR images acquired over the east coasts of the Adriatic Sea and the Black Sea are ideal means to validate and improve mesoscale atmospheric models simulating bora events.

Complex bora flow in the lee of Southern Velebit

“…The general characteristics of bora flow during the 20Dec04 and 25Jan05 episodes, presented here, are in good agreement with the jet and wake structure of the northern Adriatic bora documented well in SAR images of Alpers et al (2009) andSignell et al (2010). The low wind speeds in the lee of Southern Velebit are visible in all their examined episodes, associated with wake formation over Zadar peninsula that is responsible for the 'Zadar calm'.…”

Unique phenomena in Lake Baikal, Russia, imaged and studied with SAR and multi-sensor images

“…All these winds leave footprints on the lake surface, as do similar local winds elsewhere, e.g. the bora in the coastal zones of the Adriatic Sea and the Black Sea (Alpers et al 2009(Alpers et al , 2010. The sea surface roughness patterns associated with these wind events and captured by SAR can yield information on the spatial extent and the spatial structure of these local winds.…”

“…They have been interpreted based on the approach by Alpers et al (1998Alpers et al ( , 2009Alpers et al ( , 2010 as manifestations of the mountain bora-like winds (figure 2), strong easterly winds and the Barguzin (figure 3) and as manifestations of katabatic winds blowing from both sides of the lake (figure 4). (Alpers et al 2009(Alpers et al , 2010. The bow-like feature along the Selenga River delta seems to be a local atmospheric front at the boundary between the opposite blowing winds.…”

Quantitative analysis of Bora events in the Adriatic Sea by means of SAR-based techniques and the ETA model

“…However, spatial averaging of the atmospheric quantities, which is assumed by numerical simulations, could cause the loss of atmospheric features, thus affecting a comprehensive description of jets' structure. In contrast, detailed description of jets over the sea surface can be detected by SAR imaging [1]. In this paper, wind predictions provided by the ETA atmospheric model and relevant to the Bora episodes occurred on 2nd and 5th February 2012 on the Adriatic Sea were compared with the wind fields obtained from ASAR/Envisat observations.…”

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Bora is the north-eastern wind which, blowing in the Adriatic Sea from NE and interacting with the orography of the Croatian Dinaric Alps, is characterized by multiple surface intense wind jets [1]. The exceptional Bora events of the beginning of February 2012 over the Gulf of Trieste have been studied by means of two ASAR Wide Swath (WS) images, acquired on the 2nd and 5th of February.

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