Meteosat-8 and Himawari-8 observe night-time fog formation in northern India in early December 2016.
14 November 2020
05 December 2016
By Jochen Kerkmann (EUMETSAT) and HansPeter Roesli (Switzerland)
In 2016, the winter (night) fog in the Ganges Valley started around 23 November when the first larger fog patches developed just south of the Himalayas.
In the following days, the fog area expanded gradually every night to cover large areas of the Ganges Valley (Uttar Pradesh) and of Madhya in the period 3–17 December.
Finally, on 18–20 December, despite cloud free conditions, the fog did not form, probably because of changes in the moisture/temperature conditions.
The best geostationary satellite to observe the nightly fog formation in northern India is currently Japan's Himawari-8 satellite, which has 10 IR bands to observe the atmosphere and clouds.
The standard RGB product to observe night-time low clouds and fog is the Night Microphysics RGB developed by Prof. Rosenfeld of the Hebrew University of Jerusalem.
The standard SEVIRI ranges for the Night Microphysics RGB are: Red IR12.0–IR10.8 (-4 to +2 K), Green IR10.8–IR3.9 (0 to +10 K) and Blue IR10.8 (243 to 293 K). For the AHI instrument on Himawari-8, the SEVIRI ranges have been tuned to the following values (based on JMA recommendations): Red IR12.4–IR10.4 (-6.7 to +2.6 K), Green IR10.4–IR3.9 (-3.1 to +5.2 K) and Blue IR10.4 (243.6 to 292.6 K).
Figure 1 shows the Himawari-8 Night Microphysic RGB for 7 December 2016, 00:00 UTC; the synop observations have been overlaid to see the 2-m temperatures and dewpoints.
Figure 2 shows the comparison of the Himawari-8 AHI and Meteosat-8 SEVIRI Night Microphysics RGBs. Note that, in this case, the fog in the Ganges Valley is much clearer (strong green) in the AHI image, probably because of the more narrow range used for the green component. Furthermore, note the higher spatial resolution of the AHI image, in the central and eastern part of the image, and the better detection of moisture boundaries because of using the difference 12.4–10.4 instead of 12.0–10.8 on the red beam.
Low clouds and fog appear as a green colour, clear land is pink (or red if the land is very cold), thick high clouds are dark red (mixed with yellow if the clouds are very cold), and thin high clouds are dark blue.
The animation of this RGB product, 00:00 UTC images from 10 November to 21 December, shows the night-time extension of the fog in the Ganges Valley, as discussed above. It also shows cyclonic storm Nada and severe tropical cyclone Vardah hitting the southern parts of India.
Furthermore, the boundary between dry continental air (more red) and moist tropical air (no or little red) can be seen in southern or central India; the position varies and depends on the strength of the push of the tropical air against the continental dry air.
In the NASA Earth Observatory case titled "Fleeting fog in India" the peculiar aspect of 7 December is that is that the high-resolution Terra MODIS image showed a number of “holes” in the low clouds (most likely fog) that happened to coincide with cities, including India’s capital, New Delhi.
The holes looked similar to punch holes created by aircrafts when flying through a layer of supercooled clouds, a phenomenon known as a 'distrail' (see the Distrails over Central Europe case from November 2013).
The Earth Observatory case authors speculated that the holes could not be distrails (the clouds were not supercooled), but could be due to either additional aerosols or urban heat islands, or both, having an effect on the clouds.
They believed that: "It could be that the fog was burning off faster over the cities, which tend to be warmer than their surroundings.".
In order to check this hypothesis, we have looked at animations of multi-spectral imagery from Meteosat-8 (which was recently moved to the new position at 41.5 degrees East over the Indian Ocean) and at Himawari-8 imagery.
For looking at cloud phase and cloud particle size, the Night Microphysics, Day Microphysics and Natural Colour RGB products were generated. The High Resolution Visible (HRV) imagery was also checked to see more details of the holes.
Figure 3 is a comparison of Meteosat-8 HRV with Himawari-8 VIS0.64. For both satellites the area is close to the edge, but slightly less in the case of Meteosat-8. This is why, in this case, Meteosat-8 HRV is superior to Himawari-8 VIS0.64, not withstanding the higher performance and higher spatial resolution of of Himawari-8 VIS0.64.
The animated gif of the Meteoat-8 HRV images shows the slow dissolution of the fog in the Ganges Valley during the morning and midday hours of 7 December (04:30 to 09:30 UTC). The fog was gradually burning off from the sides and the holes over the cities, confirming the hypothesis of a city temperature effect.
The fog dissolved the most rapidly in the area around New Delhi and to the north-west of the city, where the fog was thinner (shown by the lower visible reflectance, and by the colours in the Day and Night Microphysics RGB), as compared to the Ganges Valley to the south-east of New Delhi, see colour interpretation example .
Figure 4 compares the Natural Colour RGBs of both satellites for 7 December, 06:00 UTC. While in the western part of the image (the area around New Delhi with the cloud holes) Meteosat-8 has a better viewing angle, in the eastern part the better viewing and the higher resolution of Himawari-8 becomes visible. Note the shift of the Himalayas and the Tibetan plateau area, which is due to parallax effects.
By combining the Night Microphysics and the Day Microphysics RGBs from Meteosat-8 it is possible to get an overview of the fog expansion during the night (6–7 December) and its dissolution during the following day. This animation shows the low cloud/fog development from 6 December 16:00 UTC to 7 December 10:00 UTC, first the Night Microphysics RGB (Night) and then the Day Microphysics RGB (Day).
The images around dawn (from 02:00 to 02:45 UTC) display both RGBs (overlaid) with a transparency that changes from highlighting the Night RGB (at 02:00 UTC) to highlighting the Day RGB (at 03:00 UTC).
As mentioned above, in the western Ganges Valley, where the fog is thinner, the fog dissolves more quickly than in the eastern Ganges Valley, see also the animated gif of the area around New Delhi (zoomed in).
The yellow/orange colour of the fog in the Day Microphysics RGB indicates that the cloud droplets of the fog clouds were very small. By 10:00 UTC, the fog in the western Ganges Valley was completely dissolved, but in the central and eastern Ganges Valley, despite the strong diurnal heating (sunshine), the fog did not dissolve completely — there were three major fog patches that remained for the whole day.
Focusing on the area around New Delhi (western Ganges Valley) where the holes in the cloud deck appeared on 7 December, the diurnal cycle of the fog forming during the night and (partly) dissolving during the day can be monitored in the long animation from 6 December 00:00 UTC to 9 December 23:45 UTC . The animation is composed of the Night Microphysics RGB (night, from 11:00 UTC to 01:45 UTC) and the Day Natural Colour RGB (day, from 02:00 UTC to 10:45 UTC), with no transition effects.
The development is more or less the same every day, with some changes in the extension of the fog. The holes in the cloud deck (linked to major urbanised areas like New Delhi) are best seen on 7 and 8 December, less on 9 December when the fog dissolved more quickly. Note also the swirls in the cloud deck, which can be observed during the night.
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