The Cancer solstice, with the Sun directly over the Tropic of Cancer, took place in 2020 on 20 June at 21:43 UTC.
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.
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.
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 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.
Note: This year a new Moon joined for the occasion on 21 June and produced an annular eclipse over eastern Africa and India.