Satellite images of the Mediterranean sea are informative even on clear days, as is shown in this case from March 2017.
22 October 2020
18 March 2017
By Fausto Polvorinos (AEMET) and José Prieto (EUMETSAT)
On 18 March midday the water temperatures were around 15 °C on the Western Mediterranean coasts, almost two degrees higher than the Meteosat channel 10.8 µm reading from the same time, as a result of humidity.
The wind was gentle, with the highest values, between 20–30 knots, around Corsica and Sardinia, the Gulf of Leon, and close to the Gibraltar Strait (Alboran sea).
ASCAT at 09:30 UTC (Figure 1) shows the winds over the sea surface. Magenta indicates weak winds in clear skies over areas of high surface temperatures, typical of areas where there is little swell. Warm surface anomalies can be the result of weak winds for several days, in particular during windy seasons. Green areas point to cold anomalies.
Additionally, Meteosat provides indicators for sea surface temperature (SST), for humidity close to the surface, and even for wind from solar channels’ reflectivities. So it is enticing to create an RGB which separates anticyclonic areas, with associated higher SST (in magenta), from windy areas with cooler SST and higher humidity inside the boundary layer (in cyan).
As the basis for this RGB, we insert on red the values from channel at 10.8 µm, a proxy for the SST. On green the values at 0.6 µm, an indication of wind in the absence of cloud, since the wind increases the reflectivity through foam and waves dispersing light in all directions. In blue we put the difference 10.8 µm–12.0 µm, which is an indicator of humidity at low tropospheric level. Magenta pixels appear on warm surfaces almost calm, and humid at low levels. On the western Mediterranean, cyan pixels are for colder sea surfaces and stronger wind, still humid. The low-level humidity is smaller for the green pixels probably due to wind from the colder land, with colder sea surfaces than more to the south.
In the example provided for days 18 and 19 March, calm and warm areas slightly change location, but the main difference is around the Strait of Gibraltar, windy on the 18th, and calm on the 19th. The drier air reaches the Balearic and Sardinia on 19 March.
An cyclonic gyre can be seen on Figure 3 close to Malaga, as a result of river water rich in sediment discharging in the sea, following rains on 14 March.
The Copernicus Sentinel-3 instrument OLCI captured the situation with better resolution at 10:30 UTC on 18 March. Figure 3 is a composite of channels at 510, 440 and 410 nm, depicting the stronger winds close to the Strait of Gibraltar.
The stronger winds towards the west of Gibraltar give the wrong impression of a direct sunglint on the west, which does not correspond to the easterly sun position. The reason is the winds are increasing the surface reflectance.
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Etesian winds seen in MSG High-Resolution Visible images (13 July 2007)
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