Saharan dust has been spreading into Europe’s southwestern regions since 14 days ago. It transformed snow and skies into a desert-like landscape. This week’s weather pattern is expected to bring another dust cloud across central Europe, possibly a large one, before the next strong frontal system with lots of rain from the north. From Tuesday to Thursday, large plumes containing dust particles will rise north over most of the southern half Europe.
The weather pattern across Europe has been unusually stable, extremely dry since the end of the cold season. However, there have been strong upper Highs (and ridges) over the continent. The pattern will change as a deep trough from the north emerges, increasing the jet stream and shifting it to a more Southwesterly direction.
This is a great recipe for making a substantial amount of Saharan dustLifted from the deserts in northern Africa, slowly spreading into the Mediterranean, central Europe and the Alps. The frontal system was moving southwards, and the Balkan was crossed into southeastern Europe.
The Saharan Dust Cloud will travel along the increasing southwesterly strong-level winds between the persistent blocking High to its north and the northerly Trough. The establishing weather pattern across Europe suggests a lower geopotential height towards the west and higher pressure across the east- and south-eastern parts of the European continent.
This means that the wind flow will be shifted to bring the air mass directly out of the Sahara desert. The possibility of massive dust cloud particles spreading north into Europe increases with the direction of higher-level winds. We actually saw something very similar on March 15th when a large amount of dust was observed. covered the snow across the Pyrenees mountain range.
This event brought a remarkable amount Saharan dust to the Iberia peninsula as well as parts of the western Mediterranean. As you can see from the tweet above, historical dust concentrations made skies and snow orange-dune-like. Some scenes looked literally apocalyptic from the Pyrenees mountain range.
Saharan dust levels are expected to be extremely high this week and spread across large areas of Europe. The particles will settle to the ground due to the rainfall that will occur after Wednesday. They will cover the snow in the Alps, and accumulate into the soil. Let’s quickly go through the basics of the desert dust clouds formation and their effect on the air quality.
ORIGINAL OF THE SAHARAN DUST COUD
As its name suggests, the Saharan dust cloud is made from the Sahara desert.The general wind direction in our upper atmosphere is from south-southwest to north-northeast. Dust particles can travel north into the southern, southwest, and southeast parts of Europe. Many dust particles originate from the Algerian deserts. Dust particles are sometimes transported even further north into Eastern, Western, or Northern Europe.
Dust will be brought to ground when a large dust cloud event occurs, which coincides with a frontal storm and falling rain. We will be able observe the dust that has been deposited on objects. We can find dusty drops on cars, dusty dust on them, and similar. Visibility will decrease and the skies will become cloudy.
Sometimes, thunderstorm activity over northern Africa can cause a dust cloud to form. The strong winds blow the dust particles away downwind, usually towards Europe from the north-northwest.
Depending on the intensity and speed of the jet stream, large dust clouds could form and travel great distances, even across oceans.
Besides gaseous pollutants, the Earth’s atmosphere is also polluted by different particles. These particles can be in suspension, fluid, or solid-state and have a divergent size and composition. They are sometimes called aerosols. These aerosols are often referred to floating dust, but are more commonly known as PM particles. PM is an abbreviation. Particulate Matter. These PM particles are often classified based upon their aerodynamic diameter.
Aerodynamic diameter of dust particles is, by definition, the diameter of any sphere-shaped particle which exhibits the same behavior as a dust particulate (which does not necessarily need to be sphere-shaped). Particulate matter is crucial in the context of air quality problems it could cause.
Particulate Matter includes coarse particles (PM10) and fine particles (PM2). 5), and ultrafine particles (PM1) is defined as the fraction of particles with an aerodynamic diameter smaller than respectively 10, 2.5, or 1 micrometer (one µm). For you to better imagine how small these particles actually are, 1 µm is 1 millionth of a meter or 1 thousandth of a millimeter.
These are very small particles. And if we compare those with the average diameter of human hair which equals 50-70 µm, we can see dust cloud particles are 5-50x smaller than that.
SAHARAN DUST CLOUD DEVELOPING IN THE MEDITERRANEAN REGION
The weather pattern will start to change in the next 48-hours. The upper-level ridge will slowly weaken and collapsing up until Wednesday, while a large and deep depression develops over Europe’s northern half. A weak upper wave, which is gradually merging with the large, north-facing trough, begins to build to the east of Azores high. This results in a strengthening and expansion of the southwesterly jet stream towards Southern Europe.
The mid/upper-level flow change was the beginning of the upper wave above the southern Mediterranean and to west Morocco from Sunday to Monday. This had brought to our attention the ongoing activity over North Africa, specifically Algerian deserts, where massive amounts Saharan powder have been moving towards Europe.
NASA MODIS satellite imagery shows that both waves are responsible for the Saharan cloud formation from Algeria towards the northeast.
A thick dust cloud is slowly spreading north-northeast. It has reached Iberia and large parts of the west-central Mediterranean by Monday night. From Spain, Portugal and Italy, dust particles have been reported.
You can see the layers of dust clouds in the satellite image by looking at the colors. The slowly changing weather pattern and the strong jet stream (marked yellow arrow) will push the dust further through these regions. It also helps to push it further northeast towards central Europe, Alps, and then on to the Balkan peninsula, from late Tuesday to early Thursday.
You can also see how mostly cloudless and stable the skies are across large areas of western, central and eastern Europe. This is due to extremely stable weather conditions under strong surface high pressure systems. Allowing air mass to drop inside them, and drying out. It is also warming a lot, since we have been experiencing very warm weather these days.
DUST CLOUD SPREADS TOWARDS THE CENTRAL EUROPE MID-WEEK
Between the decaying European continental Ridge and the north European depression, south-southwesterly upper-level winds have developed. The latter will slowly move south in the coming days. This will allow the Saharan Dust Cloud to move further northeast, reaching central Europe first and then spreading towards western and eastern Europe.
The video animation shows how the dust cloud is moving across Europe’s southern regions this week.
Below is the 500mbar (jet stream wind chart), which shows strong winds ahead the upper wave in southwestern Europe. This is a typical setup for southern Europe when a cloud containing Saharan Dust moves north towards Europe.
On Tuesday, the Saharan dust cloud at its highest concentration will first be seen in the Mediterranean’s northern part. It will then spread north-northeast to reach the Alps and France on Tuesday. You can also see dust particles spreading further west towards Benelux and England.
Due to Monday’s decaying wave, a large concentration of dust is also found over Turkey and Greece. Through Tuesday, southwesterly winds will push desert dust into southern Balkans.
As you can see, the dust cloud that is accumulating over the western Mediterranean is part of a larger and more dense one. This dust cloud will move into central Europe on Wednesday, with a high level of particles predicted. These aerosols will also be present in the ground due to the forecasted rainfall.
The massive amount of dust expected to reach the Alpine Region and the northern Balkans countries Wednesday night is due to the thickness of the dust clouds still hovering over the west Mediterranean. Some areas will see significant dust deposits.
The Copernicus Atmosphere Monitoring Service (or CAMS) warns that high dust levels can have adverse effects on the health of those living in the affected areas. It also adds to the air pollution from nearby sources.
The Saharan dust cloud advection also contains a significant amount of dust particles. This could cause skies to turn orange again, with limited visibility and dust deposits on cars and other objects. Atop the snow in higher Alps.
Thursday morning will see a large trough from the north push the dust cloud further south-southeast. As a result, dust concentrations in the central and western regions of Europe will slowly disappear.
SAHARAN DUST TURNS TOWARDS SOUTHERN BALKANS, AS A DEEP TOUGH EMERGES NORTH
From Thursday through Sunday, the northern trough, which will dominate most Europe, will grow quite large. This will create a lot more frontal systems and rain in the central and southern regions of Europe. We can see the geopotential/pressure lines will be tight on its front end, so the upper winds will be pretty strong from north Africa to southeast Europe.
This will allow dust particles to accumulate more over the Balkan peninsula in the south and the southern Mediterranean on Thursday, and Friday.
This is because the upper High will disappear in the Mediterranean. Therefore, the strong southwesterly winds from northern Africa to southeastern Europe will continue for several days. It is favorable for maintaining the advection of the Saharan desert.
Further north, closer to where the upper low is located, precipitation will bring cloud particles to ground. This will clear the skies completely. The mountain ranges are expected to see a lot of rain, especially in the mountains. Apennines, Italy, and Dynaric mountain range on the western Balkan peninsula.
Part of the Apennines is expected to receive 60-120mm of rainfall through Saturday. Similar conditions are forecast across western Slovenia, northwestern Croatia, and parts of western Slovenia. Further south, 200-300mm of rain is possible, depending on where the rainfall maximum occurs, e.g. Montenegro.
After being extremely dry in many parts of Europe, this will finally bring rain. It will be especially noticeable in Italy, central Europe, and the Balkans.
Note: We will have more updates on the development of a large upper trough soon – stay tuned! A large portion of Europe will see a return to a huge cold pool over the weekend, increasing the risk of frost in many places.
Also, see a similar event in south-central Europe on May 14th 2020.
***The images used in this article were provided by Windy, PivotalWeather, and Wxcharts.
Source: Severe Weather