Clockwise rotating cirrus clouds over the Atlantic

By Jochen Kerkmann (EUMETSAT)

On 25–26 January several amateur meteorologists in Europe, referring to a large cirrus cloud shield over the North Atlantic, noted a strange clockwise rotation of the clouds, calling it a 'clockwise rotating cyclone' (Figure 1). In reality, the cirrus clouds actually rotated around a strong anticyclone located south of Iceland, hence the clockwise rotation.

The strange thing, and this is what probably confused the amateur meteorologists, was that there were so many cirrus clouds in an area of high pressure where you would normally expect low coverage of high clouds (except for some contrails or isolated cirrus patches, but certainly not a large cirrus shield). So the question is: why were there so many cirrus clouds over the North Atlantic within an area of high pressure and why did they persist for several days, instead of dissolving?

The origin of these clouds was tracked back to high-level lee clouds to the lee of Greenland (which developed on 24 January) and later again over Iceland. These clouds are composed of very small ice particles, generally staying aloft much longer than larger ice particles which are weighed down by their mass.

To find the answer to these questions we have to back-track these cirrus clouds to their origin and to find out the conceptual model that best fits them, i.e. the physical mechanism that led to their formation. This can be done by looking at their appearance in satellite images (texture), their physical properties (e.g. thickness, phase, particle size, top temperature) as given by satellite observations, the physical background as given by NWP models and certain key parameters to achieve a 3D or even 4D view of the state of the atmosphere.

In summary, with the help of the hourly image sequence in Figure and the six-hourly archive of ePort , the origin of these clouds was back-tracked to high-level lee clouds (orographic wave clouds) to the lee of Greenland which developed on 24 January (see bottom image) and again over Iceland on 25 January (see top image). These clouds are composed of very small, quasispherical ice particles with effective diameters less than 25 microns, much smaller than the typical ice crystals found in non-orographic clouds. On the other hand, small ice particles have a higher reflectance in the so-called microphysical channels (NIR1.6 and IR3.9) than large ice particles, and this can be seen in animated satellite images and derived RGB products.

To illustrate this, Figure 3 shows the Convection RGB product from 24 January, 12:00 UTC, which is composed of the WV6.2–WV7.3 difference (on red), the IR3.9–IR10.8 difference (on green) and the NIR1.6–VIS0.6 difference (on blue).

Cold, high-level ice clouds appear with a red/magenta colour in this RGB product. However, as small ice particles have a higher reflectance in the IR3.9 channel, the wave cloud to the lee of Greenland can be seen in a strong yellow colour because of a much larger IR3.9–IR10.8 brightness temperature difference (which adds more green, thus generating the yellow colour). The image below also shows that the high-level lee clouds developed along the right (anticyclonic) side of a jet stream (see isotachs at 300hPa), as described in the Manual of Synoptic Satellite Meteorology.

Finally, it is important to know that small ice particles tend to stay aloft much longer than larger ice particles which gradually fall due to their mass and evaporate. This explains why the cirrus clouds did not dissolve within a few hours but instead stayed aloft for several days. The same phenomenon has also been observed with so-called 'dusty' cirrus clouds, which can remain airborne for several days and only dissipate very slowly (see our case study from 11 February 2010 ).