Exercises

Dust storm over the Canary Islands and Madeira: 3 March 2004

Early March 2004, a cold air outbreak from Europe to Western Africa caused a major dust storm over large parts of Western Africa. On its way southwards, the cold air fanned out across the Sahara, highly diverging over subtropic regions giving the dust front the form of a (spanish) fan. Have a look at the animation of the Dust-RGB and try to answer the following questions.

1. What can you tell about the visibility near the surface by analysing the sequence?

2. The Dust-RGB does not work at night, because?

In the following days, the dust was blown out across the Atlantic Ocean and affected the Canary Islands and Madeira on the 4th and 5th of March (see forward trajectories). The effects in Madeira were more severe as flights had to be even cancelled due to reduced visibility.

In the set of high resolution visible images the dust can be seen as it moves from Morocco over the Atlantic towards the Canary Islands. Because the mainland consists of desert areas it is hard to pick up the dust using visible channels. Also interesting is the appearance of the lee waves near the island of Fuerteventura.

At 16 UTC on March 3rd it is possible to see how Gran Canaria Island (with a peak of 1949 metres) is making a barrier to the dust flow. This fact shows that, the dust producing a strong signal in the visible channels of MSG is mainly at the lower 2 km of the troposphere. In fact, 24 hours backward trajectories starting at Gran Canaria confirm the eastern flow over the Islands. This indicates that trajectory models with forecasts from NWP could be used in this situation.

With the natural colour RGB (Meteosat 8 - 1.6μm; 0.8μm; 0.6μm), the dust over land can be discriminated quite easy. Interesting in this set of images are the von Karman vortices that develop southwest of the Canary Islands.

3. What remarkable feature in satellite tells you that the visibility was reduced over the Canary Islands during this event?

The appearance of the Dust storm was also picked by Modis-satellite. Two images are provided (source NASA). Click here to see an image of the 4th of March and for four days later on the 8th of March as also dust can be seen in the middle of the Atlantic, here.

Dust storm over the Canary Islands and the Iberian Peninsula: 22 July 2004

On 22nd July 2004 Meteosat-8 observed a large plume of Saharan desert dust blowed over the Canary Islands towards the Iberian Peninsula. The dust plume is presented here with the artificial composite of the Dust-RGB, that makes the tracking very easy.
While with the use of the Dust-RGB the dust cloud can be traced easily as it moves over the Atlantic into Portugal and Spain, it is very difficult to do this with high resolution visible. The difference between the surface and the dust aloft is very narrow.

A third, more unconventional way of showing the dust cloud is by making use of the products from the Nowcasting SAF (NWCSAF) and its Cloud Mask (CMa). . The CMa algorithm includes a sequence of threshold tests, which allows the identification of cloud free areas where other products (total or layer precipitable water, land or sea surface temperatures, snow/ice cover) may be computed. It also allows identifying cloudy areas where other products (cloud type and cloud top temperature/height) may be derived. . In addition, a specific and independent algorithm allows the detection of dust, which is stored in a so-called dust flag. Currently, the dust flag from NWCSAF detects dust over water around Europe, but the detection is not as successful over land. In fact, it is easier to make a clear discrimination of dust over Africa than over Europe, where the density of dust is not as high. More on the detection of dust, the NWCSAF and its products can be also be read here.

Dust storm over the Canary Islands: 25 August 2004

On 25 August 2004, Meteosat-8 observed a large plume of Saharan desert dust blowing over the Canary Islands. The dust plume, which is best seen in the High-Resolution Visible (HRV) image and with the use of the Dust-RGB, was drawn into a low pressure system located to the north-west of the Canary Islands. Advected by southerly winds, the dust also reached Madeira (NW of Canary Islands) the day after.

With the use of high resolution visible the dust is easily detected. Other features seen in the animated HRV images are:

the effects of trade winds blowing from the north-east, forming stratocumulus cloud over the northern parts of the Canary Islands (Gran Canaria, Tenerife, La Gomera, El Hierro, and La Palma).

the dust coming from the south-east is ascending above the inversion, which is lower in the east and higher to the west.

the mixing of dry, warm Saharian air with cold, moist Atlantic air, resulting in the "drying out" of the stratocumulus clouds (warm, dry air + cold saturated air = moderately warm, but unsaturated air). This mixing of air masses largely destroys the low-level inversion.

towards the end of the animation sequence, a very big convective system can be seen in the lower right corner of the image, located over the Western Sahara.

As the dust cloud moved northwest it also started to affect the Island of Madeira (Northwest of the Canary Island) on the 26th of August. For the timeframe between 10:30 and 12:00 UTC again the set of satellite images from the Dust-RGB are introduced and, next to that, the third more unconventional way of showing the dust cloud using the Dust Flag included in the NWCSAF´s CMa product.
The dust cloud was also picked by Modis-satellite. Click here to see the image (source NASA) of the Dust storm.

4. On the satellite images a large convective systems can be seen over Morocco. How come these cloudsystems do not show up in the dust-flag product?

East Mediterranean Dust storm: 24 February 2006

Two days prior to this event (22nd February) the ECMWF model showed a classical cyclogenesis predicted for the next night on the Sahara desert. On (Thursday) the 23rd February 2006 the cyclone moved over the Strait of Sicily causing winds up to 60 knots and surface pressure of 988 hPa at the low centre. As the low moved further to the northeast, it brought ahead of it a large amount of dust from the Sahara. The dust reduced visibility in Greece, especially in Creta, and later that day in Turkey. To the right you can see the animation of this dust event. Below an interactive module allows you to analyse the dust event in more detail. The image presented below is from 24 February 2006 at 04UTC.

5. On the leading edge of the dust cloud small patches of blue are seen. Using the tool called "Probe" analyse these patches and answer what they are?

Mediterranean Dust storm: 25 October 2006

6. On the 25th October a dust storm was monitored over Northern Africa in Algeria. Before opening the loop, take into account the surface analysis of that day. Can you guess in which direction the dust is moving to?

7. If we would not concentrate on the dust over Algeria but on the very dark to black clouds north of the Pyrenees. What can you tell about these clouds in comparison to the red clouds?