High‐resolution mapping of Bora winds in the northern Adriatic Sea using synthetic aperture radar

Abstract

[1] The Adriatic Sea is regularly subjected to strong Bora wind events from the northeast during winter. The events have a strong effect on the oceanography in the Adriatic, driving basin-scale gyres that determine the transport of biogeochemical material and extracting large amounts of heat. The Bora is known to have multiple surface wind jets linked to the surrounding orography and have been the focus of many studies, but it has not been possible to describe the detailed spatial structure of these jets by in situ observations. Using high-resolution spaceborne RADARSAT-1 synthetic aperture radar (SAR) images collected during an active Bora period (23 January-16 February 2003), we created a series of high-resolution (300 m) maps of the wind field. The obtained winds show reasonable agreement with several in situ wind observations, with an RMS wind speed error of 3.6 m/s, slightly higher than the 2-3 m/s errors reported in previous studies. These SAR images reveal the spatial structure of the Bora wind in unprecedented detail, showing several new features. In the Senj region of Croatia, several images show rhythmic structure with wavelengths of 2-3 km that may reflect Bora pulsation seen at fixed locations by previous investigators. Along the Italian coast, several images show a wide (20-30 km) band of northwesterly winds that abruptly change to the northeasterly Bora winds further offshore. Meteorological model results suggest that these northwesterly winds are consistent with those of a barrier jet forming along the Italian Apennine mountain chain.

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'.…”

“…The highest bora wind gust ever recorded in Croatia (69 m s −1 ) was measured in the lee of the southern tip of Velebit, under the influence of what could be considered the Velebit tip jet (e.g. Doyle and Shapiro, 1999) or Drage jet as termed in Signell et al (2010).…”

Possible observation of horizontal roll vortices over the Adriatic Sea during bora

“…Note that the large shear above the PBL (Figure 4) is consistent with gravity-wave generation. Furthermore, the spatial bora structure in the along-coast direction is complex with several processes interacting at different, although on occasions overlapping scales (e.g., Signell et al, 2010). These processes result in the typical bora jets and wakes and include gap flows, gravity waves, and nonlinear processes such as wave breaking, hydraulic jumps, and the resulting potential vorticity banners.…”

“…A number of complexities arise because of the terrain inhomogeneity, particularly the mountain gaps and peaks along the Dinaric Alps. The major gaps and peaks result in the formation of bora jets and wakes, respectively (e.g., Jiang and Doyle, 2005;Belušić and Klaić, 2006;Signell et al, 2010), which are then related to potential vorticity banners (e.g., Grubišić, 2004) and varying turbulence structure (Večenaj et al, 2012). A number of other features appear within the bora flow, such as lee rotors of different types (e.g., Gohm et al, 2008;Prtenjak and Belušić, 2009;Stiperski et al, 2012) and quasi-periodic pulsations (e.g., Belušić et al, 2007), but they mostly do not extend far over the sea.…”

On the feasibility of the use of wind SAR to downscale waves on shallow water

“…Atmospherical models do not represent the detailed range of bora wind spatial variability, like the dual-jet nature of the Trieste jet or the several-kilometer-wavelength structures in the Bakar and Senj jet regions. Estimated wind fields at fine scale from SAR satellite allow the observation of morphology, wake patterns, the formation of the barrier jet on the western Adriatic coast and, where present, dual-jet structure of the bora wind (Signell et al, 2010;Adamo et al, 2013).…”