Fog/low clouds in winter high pressure situation (13-14 February 2008)

by Mária Putsay (Hungarian Meteorological Service)

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In winter anticyclones fog or low stratus often form at night. An example is given in the Meteosat-9 24-hour cloud microphysics RGB image below (14 February 2008 at 01:15 UTC, see also surface chart from 00:00 UTC, JPG, 102 KB). For the colour interpretation please click on the link below the image. Light green colour indicates fog or low clouds, pinkish colour clear land. High-level ice clouds appear with a reddish brown colour (thick cloud) or black (thin cloud). We see clear areas in France and Southern Europe, fog/low cloud over UK, Germany, Hungary and Romania. At the northeastern part of the image we see cold front cloudiness. The cold advection due to the cold front intensified the fog/low cloud formation in UK, Denmark and Poland (see SatRep analysis at 06:00 UTC, JPG, 298 KB, source: ZAMG).

The 24-hour cloud microphysics RGB works at night and during the day, so it is particularly useful for creating night-day loops. Moreover in twilight conditions this is the best RGB for fog/low cloud detection. In the animation one can nicely see the spreading valley fog in Germany and eastern part of France. One can also notice the formation of high-level lee clouds over the Southern Carpathian Mountains (persistant for several hours) and later over the mountains of Bulgaria and the northern part of the Czech Republic. By overlaying the ECMWF high-level wind field on the satellite image it can be seen that the strong northerly wind is almost perpendicular to the Southern Carpathian Mountains, creating optimal conditions for high-level lee cloud formation (see RGB image with ECMWF 300 hPa winds, 13 Feb 23:45 UTC, JPG, 363 KB).

During daytime we can see the foggy areas in the high-resolution RGB composite images (at 07:30 UTC (JPG, 248 KB) and 10:30 UTC (JPG, 259 KB)). This RGB has better spatial resolution, better color contrast and it shows the snow covered clear areas as well, but it works only during daytime. The snow is light blue, while the water cloud is light pink. The clear land is reddish brown; the sea is black. In the animation we can see the slowly dissipating valley fog in Germany and in the eastern part of France. One can also see that the fog/low clouds could not cross over the Southern Carpathian mountains. The clouds do not reach the mountain tops, they reach only some saddles where the 'light blue' snow can be perceived next to the 'light pink' water clouds.

The high-resolution daytime images are also very useful for the detection of low-level wave clouds (see 07:30 UTC image with interpretation, JPG, 259 KB), which are even more obvious on the following day (see bottom image). Furthermore, on both days there are convergence lines over the Mediterranean Sea, more or less parallel to the coast.

Meteosat-9 RGB Composite
Met-9, 14 February 2008, 01:15 UTC
RGB Composite (24-hour cloud microphysics)
IR12.0-IR10.8, IR10.8-IR8.7, IR10.8
Full Resolution (JPG, 276 KB)
Interpretation (JPG, 296 KB)
RGB image with ECMWF 300 hPa winds (13 Feb 23:45 UTC, JPG, 363 KB)
Animation (13 Feb 21:00 UTC - 14 Feb 07:15 UTC, AVI, 2404 KB)
See also:

Meteosat-9 High-Resolution Visible RGB Composite
Met-9, 14 February 2008, 10:30 UTC
RGB Composite (HRV Fog)
NIR1.6, HRV, HRV
Full Resolution (JPG, 259 KB)
Interpretation (JPG, 279 KB)
Animation (07:00-15:15 UTC, AVI, 1931 KB)

Meteosat-9 High-Resolution Visible RGB Composite
Met-9, 15 February 2008, 07:15 UTC
RGB Composite (HRV Fog)
NIR1.6, HRV, HRV
Full Resolution (JPG, 311 KB)
Interpretation (JPG, 323 KB)
RGB image with ECMWF 300 hPa winds (JPG, 219 KB)
Animation (05:45-07:45 UTC, AVI, 1295 KB)

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