Cancer solstice 2020

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The Cancer solstice, with the Sun directly over the Tropic of Cancer, took place in 2020 on 20 June at 21:43 UTC.

Date & Time
20 June 2020
Satellites
Meteosat-11, GOES-16, Himawari-8
Instruments
SEVIRI, ABI, AHI
Channels/Products
Solar channels

By Ivan Smiljanic (CGI) and Jose Prieto (EUMETSAT)

After the solstice, days shorten in the northern hemisphere, and get longer in the southern hemisphere.

Solstices define the change of astronomical season. To avoid the solstice oscillation in date, meteorological seasons start on the first day of the month, for instance 1 June is the start of the northern hemisphere summer.

During solstices sunglints, direct reflections of the Sun on flat Earth surfaces, like calm oceans, take 'northernmost paths' in the geostationary satellite imagery. In June solstice, the Northern Hemisphere imagery displays sun glints even in the Arctic region.


Figure 1: Single channel imagery (Albedo in %) in the 800 nm region, viewed form the north pole, taken by three geostationary satellites: Himawari-8, Meteosat-8 and GOES-16. Images are captured every full hour on 20 June.

The animated gif for 20 June (Figure 1), seen from the north pole and based on three geostationary satellites, shows:

  • Sunglint in yellow hues, turning to red (higher reflectivity) for the high angles (satellite to Sun, for respective satellite).
  • Dust transport over Atlantic in similar hues (similar reflectivity).
  • Straylight for Meteosat-8 satellite, visible during respective night hours (light blue tone over the full disc view).
  • The full disc (FD) view of Meteosat-8, overlaid by the FD view of other two satellites – where there is a lot of continent mass (Africa), not relevant for sunglint tracking.
Figure 2
 
Figure 2: Composite of channels close to 0.8µm in GOES-16, Himawari-8 and Meteosat-8 to show the graphical generation of sunglint areas generation almost in the bisection of the satellite vertical and the sun vertical angles. Images at 05:00 UTC on 20 June 2020 on the left and 19:00 UTC on the right.
 

The location of the sunglint areas is explained in Figure 2. Most of the time it is around 11° N for an intermediate longitude between the vertical sun and the geostationary sub-satellite point. The blue circles indicate the sunglint locations for each of three satellites. At 19:00 UTC, the dust accummulation over the Atlantic nears the Brazilian coast and enhances reflection above an otherwise dark ocean surface, but the sunglint area is offset, actually east of the dust. At 05:00 UTC on 20 June the glint for Meteosat 41.5 E is over the Indian Ocean. For GOES it is Arctic glint, slant and forward, which makes it particularly bright over the cold waters of Baffin Bay.

Figure 3
 
Figure 3: Comparison of GOES-16 ABI 0.8µm at 05:10UTC (left) with the ice concentration product of the Government of Canada (right) for 21 June 2020. Areas D and C show an anomalous liquid condition, whereas B is thick ice, not responsive to the sunglint geometry at that time. Quiet liquid surfaces offer perfect conditions for specular reflection of the mid-night sun at Baffin bay. Area B is thick ice, lacking flatness. E is similarly icy land, not regular for good reflection. Area A is of 10% ice concentration, which keeps it reflecting under both sunglint conditions and under normal illumination.
 
Figure 4
 
Figure 4: Centre of the sunglint location for the 21st of the month at any time (white squares) from the perspective of a geostationary satellite. Here times are UTC for Meteosat-0°. For other satellites, one hour is due to be added for each 15° of subsatellite point longitude.

Solstices can be applied to monitoring Arctic ice. Figure 3 offers a comparison of channel 0.8 µm and the ice concentration product by the Government of Canada. In the sunglint area, waters with thick ice are not reflective, whereas ice-free waters are very reflective. Outside of that area, around 20 minutes after or before sunglint, the reflectivity pattern is the opposite, since ice is not flat and puts light into the satellite. Area A on Figure 3, with only 10% of ice, reflects moderately in both situations. According to Canadian ice products, the amount of ice on Baffin Bay that week was 90% below the 30-year average for areas D and C, as revealed by the strong sunglint at 05:10 UTC. A similar analysis (not shown) explains the reflection pattern in the White Sea, east of Scandinavia, for Meteosat-11 (Figure 6)

Note that spectral channels around 800 nm (0.8 µm) were preferred for tracking the sunglint position over water bodies, instead of the 600 nm region which shows less reflection for land, hence a worse land-sea contrast. Oceans are also more reflective and brighter at shorter solar wavelengths, adding noise to a sunglint detection.

On the geostationary satellite images, the sunglint evolves along an oval during the course of the day (Figure 4). Close to the solstice sun slant reflection occurs in the forward direction on sea portions without waves, close to 70 degrees north latitude.

Examples are provided on Figures 5-7, first for Himawari around 15:00 UTC on 20 June, then for Meteosat-11 around 00:00 UTC on 21 June, then for GOES-16 around 05:00 UTC.

Curiously, the glint moves eastwards inside each satellite field of view, but follows a westward satellite sequence — the satellite and the Sun being on opposite sides of the Earth.

Figure 5
 
Figure 5: Himawari-8 ABI 0.8 µm from 14:10–15:00 UTC on 20 June.
 

 


Figure 6: Meteosat-11 SEVIRI 0.8 µm from 23:30–00:30 UTC on 20-21 June.

 


Figure 7: GOES-16 ABI 0.8 µm from 04:30–06:00 UTC on 21 June.

Also a comparison of two GOES images one year apart (Figure 8) results in no difference, apart from the cloud distribution. Cloud scatters the light, erasing the sunglint.

Figure 8
 
Figure 8: Comparisons of images on 21 June 2019 (top) and 2020 (bottom) at 04:50 UTC for channel 0.8 µm of ABI
 

Note: This year a new Moon joined for the occasion on 21 June and produced an annular eclipse over eastern Africa and India.

 
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