Some areas of Europe saw a significant drop in temperatures on 1 April 2016 due to a large cloud of Saharan dust.
14 November 2020
01 April 2016
By Jochen Kerkmann (EUMETSAT), HansPeter Roesli (Switzerland), Mike Fromm (US Naval Research Laboratory) and Klaus Sievers (VC — German Airline Pilots' Association)
The dust was raised during previous days over Algeria. It was lifted and ingested in the cloud system over the Mediterranean. This led to the formation of many small ice particles, seen as yellow in Convection RGB, 1 April 12:00 UTC (Figure 1).
These ice particles are small and tend to stay aloft much longer than larger ice particles which gradually fall due to their mass and evaporate. This explains why the 'dusty' cirrus clouds did not dissolve within a few hours but instead stayed aloft for several days.
The same phenomenon has also been observed with high-level lee clouds (orographic wave clouds), which can remain airborne for several days and only dissipate very slowly, see our case study from 25 January 2011.
Finally, it should be noted that (global) NWP models do not model the interaction between dust and clouds and, therefore, are not able to correctly predict cloud coverage, temperatures and precipitation in cases of major dust outbreaks from Northern Africa to Europe.
Indeed, because of the unforeseen clouds there was definitely less sunshine and the temperatures stayed lower than forecast over parts of Europe. For example in Darmstadt, Germany temperatures were forecast to reach 21 °C but only reached around 16 °C.
The progress of the dust cloud, from its formation over Algeria to its final destination over Eastern Libya, with a branch moving across the Mediterranean Sea to France, Switzerland and Germany, can be seen in the 24-hour microphysics animation, 31 March 06:00 UTC–4 April 06:00 UTC.
Figure 5 is from SNPP VIIRS of 2 April at 01:50 UTC, at 750 m spatial resolution. The dusty cirrus cloud appears as the white veil that covers Italy, Switzerland and part of France.
The background in black and white is from the Day-Night band (DNB), on which, because the cirrus is not very thick, the city lights can be seen shining through the cloud.
The DNB is overlaid with the semi-transparent IR temperature field (band M15) that spans the range from yellow/warm to white/cold (see temperature scale along the right-hand border of the image).
German pilot Captain Klaus Sievers saw the dust cloud as he flew, as a passenger, to Frankfurt from Japan.
He had noticed some cruising-level aerosol layers over the Baltic Sea and then later on descent into Frankfurt he saw the dusty cloudy haze. Then again on his next short flight to Munich.
Figure 6 is a collage of four photos which Klaus took from the plane during his flight to Frankfurt (top) and later on his flight to Munich (bottom).
Scientist Mike Fromm of the US Naval Research Laboratory was also tracking the dust cloud, using the NAAPS (Navy Aerosol Analysis and Prediction System) Global Aerosol forecast model.
He explains: "We had some predictive help from the Navy's operational dust forecast. We had a eureka moment earlier this year... that the NAAPS model might give a multi-day lead time on these events when it shows dust AOD spreading meridionally into a telltale cyclone or conveyor belt shape. We saw this dust storm a few days in advance, and of course it (the NAAPS forecast) was verified."
Clouds instead of sunshine - Desert sand thwarted forecasts (Wetter Online, in German)
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