Etesian winds seen in MSG High-Resolution Visible images.
14 May 2021
13 July 2007
by Jochen Kerkmann (EUMETSAT)
As described in the paper from Kotroni et al (2001), the Etesian winds (northerly winds), which blow over the Aegean Sea during summer, strongly affect human activities in that region. The numerous islands of the Aegean and especially Crete (a mountainous island oriented perpendicular to the Etesian wind) seem to play an important role in the modification of this wind field. The Crete mountain ranges are an excellent example of a major isolated topographic feature which significantly modifies the regional airflow and pressure. On the upstream side, the wind flow is decelerated and partly deflected (to the left, facing downstream).
On the downstream side, the wind flow is blocked to the lee of the high mountain ranges and accelerated through the north-south oriented valleys that lead to the south coast. On a day with Etesian winds it is not uncommon to find a perfectly calm sea to the lee of the White Mountains (e.g. in Agia Roumeli at the mouth of the famous Samaria Gorge) while further east and further west, east of Hora Sfakion and west of Sougia, strong downslope winds turn the sea into a region of white, rough water. The strongest and most dangerous winds can be found in the narrow gorges that lead to the south coast, e.g. in the Kourtaliotiko Gorge (the name Kourtalia (rattle) comes from noises (like handclapping) caused by the strong north wind).
The Meteosat-8 image below shows how the Etesian winds can modify the ocean surface reflectance in the HRV channel (broadband visible channel). Usually, ocean surfaces appear black in this channel, i.e. the reflectance is very low (only a few percent), but in areas with strong surface winds (rough sea, foam, white caps) the HRV reflectance can increase significantly (see paper from Koepke , 1984). If one enhances the range between 0 and 10 % reflectance, as done in the image below, areas with rough sea surfaces (strong surface winds) may become greyish instead of black.
Consequently, the interpretation of the image below is: the grey colour over the Aegean Sea and around Crete indicates the presence of strong Etesian winds; the black colour south of Crete indicates areas of weak winds (calm sea) to the lee of the high mountain ranges of Crete; white areas indicate clouds. The areas of weak winds (black areas south of Crete) coincide perfectly with areas of wind blocking from model simulations (see Figure 6 of the paper of Kotroni et al).
It should be noted that the reflectance in the HRV channel is also influenced by other factors like aerosols (dust, smoke), phytoplankton, ocean salinity and the pigment concentration. While the latter factors can be excluded in this case, it is likely that aerosols (smoke) have contributed to the greyish colour of the Aegean Sea, but to which extent is not known.
Meteosat-8 HRV Image
Metop-A, AVHRR, Channel 2 (VIS0.8) (08:12 UTC)
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