The moon

Spotting Etesian winds in solar imagery using moonglint

23 February 14:00 UTC-24 February 00:00 UTC

The moon
The moon

Etesian winds were spotted in the solar spectrum during a moonlit night in February 2021, through specular glint from the Moon.

Last Updated

05 March 2021

Published on

04 March 2021

By Maria Putsay (Hungarian Meteorological Service), Hans Peter Roesli (Switzerland), Ivan Smiljanic (CGI), Jose Prieto (EUMETSAT)

Images from the VIIRS instrument, DNB channel, occasionally point to the presence of reflecting quiet waters, even near midnight. Known radiation sources at night for this band are the Moon, human electricity (towns), fires, lightning from thunderstorms and, in the absence of Moon, starlight, polar auroras or airglow, frequently marking gravity wave patterns induced by severe tropospheric convection. See Suomi satellite brings to light a unique frontier of nighttime environmental sensing capabilities for details and Figure 1 for a visual summary.

Visible light
Figure 1: Illustration of some phenomena producing visible light and able to be spotted by the DNB channel. Credit: Image Science and Analysis Laboratory, NASA Johnson Space Center.

In the DNB image from the night of 23 Feb at 23:25 UTC for that band (Figure 2) the vertical Moon is on the Eastern Atlantic (Figure 3).

DNB channel of VIIRS, around 0.7µm
Figure 2: DNB channel of VIIRS, around 0.7µm, 23 Feb 23:25 UTC, showing calm and rough waters south of Crete, as a result of Etesian winds from the north.
Moon and sun position
Figure 3: Moon position at the time of the VIIRS pass over Crete, 23:25 UTC. Credit: timeanddate.com

The sea south of Crete shows bright reflection on calm surfaces. Full moon was four days later, so the Moon is waxing gibbous at the time of the image, providing 88% of its full illumination. In fact the reflection is not precisely specular from the Moon, since the scan line forms an angle of ca. 30 degrees to it, but a relative illumination maximum is still reached on the west side of the swath, some 3000 km wide. The better illumination on the west of the swath at this Moon phase is clear in the global Mediterranean swaths of Figure 4.

Night time imagery composite
Figure 4: Night time imagery composite showing the mentioned image and the next pass towards West, with a much darker eastern side.

The wind pattern around Crete stems from the Etesians (northerly winds), common in that region. A Meteosat-11 image a few hours before shows the absence of cloud in the area, and already similar features of reflectivity caused by wind on channel 0.8µm (Figure 5) as in the VIIRS image.

For the DNB channel, calm seas provide the brightest signal, and grey streaks mark rough sea surfaces due to wind gaps in the island orographies, both in Crete and the more northern islands of Greece.

Met-11 enhanced channel 2 & HRV
Figure 5: Meteosat enhanced channel 2 (0.8µm) on the left and HRV (0.4-1.0µm) on the right on 23 Feb 2021 14:00 UTC.

As a cross comparison, Figure 6 shows winds from scatterometer that point at two facts. First, the general direction of the wind, in the observed domain predominantly from the north. Second, the effect of the islands and orography, with wind blocking and channelling. In the case of Crete, channelling occurs at the eastern flank of the island, with calm winds in the wake area south of that region. This is also reflected in the DNB imagery, as calm waters mean more reflection.

Metop-A and -B ASCAT 24 Feb 2021
Figure 6: Metop-A and -B ASCAT wind barbs on 24 Feb 00:00 UTC, overlaid on Meteosat-11 channel 10.8µm. Credit: EUMeTrain.