Pollution causes cloud-free gaps in clouds

By Martin Setvak and Jan Sulan (CHMI)

The MSG (Meteosat-8) rapid scan loop of the RGB HRV-HRV-IR10.8 product (05:00–16:00 UTC) taken during the daylight hours of 21 January 2011, shows a distinct gap in the stratus clouds spreading from southwest Poland across eastern and central parts of the Czech Republic, down to southwest Slovakia and northeast Austria.

Another loop, the 24-hour microphysical RGB product, (21 Jan 03:00 UTC–22 Jan 11:00 UTC), on the next day shows that the cloud-free gap started to develop around 4am on 21 January, and persisted till about 9am of the next day, weakening and strengthening several times over this period. Besides this large elongated gap, a similar one, but somewhat smaller, formed above western Bohemia, to the lee of Krušné hory (Erzgebirge Mountains). Both of these cloud-free gaps are marked in the NOAA-16 image (21 January 2011, 06:23 UTC, Night Microphysics RGB) by black arrows, and are caused by artificial changes of the cloud microphysics, originating in industrial air pollution.

The increased concentration of pollutants or their specific chemical composition results in the formation of larger droplets, which 'rain-out' the cloud locally, forming partially cloud-free areas downwind of the pollution source, hence the gap in the stratus clouds. Similar mechanisms can occasionally be seen in high-level clouds (e.g. 'punches' in cirrus clouds caused by air traffic, see NASA Earth Observatory: Hole Punch Clouds over West Virginia).

The radar composites (based on radar data from the Czech Republic, Poland and Slovakia) document very nicely the rain-out effect. The radar loop, shows two distinct plumes of rain or large droplets present inside the cloud (arrowed in this 19:00 UTC radar image), matching roughly the elongated gaps in the stratus cloud seen in the MSG imagery.

Please note that the thin spikes appearing randomly in the radar imagery are not of meteorological origin, but are an impact of interference with wireless transmitters of some Internet providers. The rain/snow is documented also by surface observations (15:00 UTC, source: CHMI) at some weather stations, being affected by this effect, and also by the radar 24-hour accumulated precipitation (source: CHMI radars, thus showing only the rain plume in western Bohemia).

In this case there is another gap in the stratus clouds, which can be seen south of the Krkonoše Mountains (Polish/Czech border, see white arrow in NOAA-16 image). Here the dissolution of the stratus clouds above Bohemia results from wave motion on the downwind side of the Krkonoše Mountains. The wave motion is also shown here by a distinct altocumulus wave cloud, composed of super-cooled water, as seen in this NOAA-15 image (21 Jan 2011, 03:27 UTC, Night Microphysics RGB) by greenish colours of the cloud (and indicated by the two arrows).

Besides the gaps, the microphysical images (MSG loop and the NOAA still images) show a number of industrial plumes, as seen in these images by a more yellowish colour (as compared to the darker background). For explanation of these and the microphysical RGB images themselves, see the case study: power station plumes in the Czech Republic and Germany (25 December 2006) .

Tthe stratus cloud gap over Poland poses a bit of a problem as the location of its 'source' drifts back and forth with time. A possible explanation for this is provided in this MODIS Terra image (21 January 2011, 09:55 UTC, True Colour RGB). The red arrow indicates a source of a very faint plume, barely visible here, but merging with the cloud-free gap further downwind. It appears that this might be the real source of the pollutants (located approx. at 51°44'N and 15°52'E), dissolving the clouds (or raining them out) further downwind, where the mechanism has been affecting the clouds for a longer period of time.

Larger Area See also (all images processed at CHMI): Power station plumes in the Czech Republic and Germany (25 December 2006)
NOAA-15 image, 21 Jan 2011, 03:27 UTC (source: NOAA)
NOAA-19 image, 22 Jan 2011, 01:19 UTC (source: NOAA)
NOAA-18 image, 22 Jan 2011, 02:09 UTC (source: NOAA)
NOAA-15 image, 22 Jan 2011, 06:11 UTC (source: NOAA)