During the last week of November 2020, a very stable anticyclone, often called blocking anticyclone or a blocking high, caused widespread fog over central Europe.
03, December 2020
By Natasa Strelec Mahovic and Jochen Kerkmann (EUMETSAT) and Ivan Smiljanic (CGI)
Pressure in the centre exceeded 1025 hPa and a very pronounced upper-level pressure ridge formed, with an anticyclone centre. This was apparent even at 300 hPa, see Figure 1.
The ridge structure almost resembles a 'cut-off ' system, often seen in pressure troughs, forming the upper-level lows, only this was a high pressure area.
Winter anticyclones over Europe are not unusual. What was unusual this time was the appearance in the satellite images. The Water Vapour (WV) 6.2 micron loop in Figure 2 shows a circular structure with very dry upper-tropospheric air above Central Europe, a feature that would be more typical for an upper-level low pressure centre.
Usually, the circulation in such dark WV eddies is cyclonic, whereas here, as seen in the loop in Figure 2 it is clearly anticyclonic. Anticyclonic circulation causes descending motion or subsidence, which in this case caused sinking of very dry air into lower layers of the troposphere. This is confirmed by the Skew-T log-p diagram of the radiosounding in Zagreb (Figure 3), where it can be seen how the air was very dry (large difference between the temperature profile on the right and dew-point temperature profile on the left) all the way down to about 900 hPa pressure level (~900 m height in this case).
The weather in anticyclones, due to this sinking of air, is usually very stable, often clear, with no clouds and calm or with light winds. But for the forecasters, the challenge in such synoptic situations, especially during the cold times of the year, is the fog. The air close to the ground can cool down considerably in such stable conditions, so the humidity present condenses and forms fog or low clouds. This was the case here.
The temperature at 950 hPa was around 0 °C, as seen in Figure 4, and the Skew-T log-p diagram in Figure 3 shows that the air below that level was very humid (temperature and dew-point temperature curves are very close together). This is a clear sign of fog in the layers close to the ground. There was also a strong temperature inversion at about 900 hPa, with the difference of temperature below and above the inversion being nearly 15 °C. Such a strong inversion usually causes persistent fog or low cloud shield, that can last for many days, as long as the inversion is present.
In the 24h Microphysics RGB loop (Figure 5) the fog area spread across the large part of Central Europe and persisted in the image for the whole 36 hours of the loop, and it was present in the same area from 24 to 28 November. (Note: the satellite imagery cannot distinguish fog from low clouds, since the satellite sees the top of the cloud only).
The Airmass RGB in Figure 6 shows green colours above the area, forming an apparently cloud-free ridge within the surrounding cloudy air, and the air in more blue and purple colours. The clouds were obviously moving around the high pressure centre. A closer look even reveals red shades in the image, suggesting dry upper troposphere. The green colour in the Airmass RGB is related to warmer air, which was indeed the case in the upper troposphere.
Meteosat-11 Airmass comparison
It is interesting to note that when comparing daytime and night-time imagery that fog is not distinguishable during the night, but it becomes apparent during the day. The reason for this lies in the fact that in the night the cloud-free ground gets colder and the temperature becomes similar to the temperature of the fog layer. However, during daytime, the ground not covered by cloud or fog gets warmer, and the difference between the fog layer and the cloud-free ground becomes clearly visible.
In cases of a stable anticyclone with fog in the lowland, higher mountains are usually cloud free, as was the case here with, for instance, the Alps. Clear skies enable clear views of the snow on the Alpine mountain tops, shown as cyan in the Natural Colours RGB (Figure 7). However, the extent of the snow cover is much smaller than last year at the same time. The fog/low cloud layer is seen in white or light pink colours north and east of the Alps.
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