Floods: Vb weather situation: The accumulation of an event of the century

This news will be heard with relief in the east and southeast of Germany: After the prolonged rainfall, the weather will calm down this Tuesday morning, predicts the German Weather Service (DWD). Dry, warm continental air then takes over the regime. However, this causes the snowfall line to rise, which, according to the DWD, leads to “additional runoff due to the thawing snow cover.” This could even worsen the flood situation locally – but the highest level of river levels will also be reached in this country on Wednesday or Thursday.

The reason for the huge amounts of rain that fell over eastern Central Europe in the past few days was a weather condition known as Vb – V as the Roman five. It goes back to the meteorologist Wilhelm Jacob van Bebber, who discovered that the tracks of low-pressure areas over Europe were subject to cyclical fluctuations and, in 1891, carried out a classification in order to improve weather forecasting. What remains to this day is the weather situation Vb: a low that moves from the Mediterranean to northeastern Europe. The DWD speaks of a “low pressure trough”: “The high-altitude current runs in a long, narrow curve (meander) from the North Atlantic, first southwards, then turns sharply to the north over southern Europe and, after a route across the Czech Republic and Poland, usually ends up in Scandinavia. « Enriched with a lot of moisture from the Mediterranean, these “hot and humid” lows on the Vb channel lead to long-lasting and heavy rainfall.

Although the Vb weather situation is rather rare, the consequences are still poorly remembered in parts of eastern Germany and Bavaria. Examples of this were the Oder floods in 1997, the Elbe floods in 2002 and the 2013 floods on the Danube and Elbe.

Geosphere Austria, Austria’s meteorological service, took a closer look at this weather situation for the Alpine republic a few weeks ago. Result: “Although only comparatively few low pressure systems (5%) are Vb lows, they are responsible for 45% of the extreme precipitation events in Austria and the Czech Republic.” Almost all major flood events on the Danube were triggered by Vb lows. The medium-term trend also creates a worrying picture: The 50 strongest Vb events in the period 1961 to 2015 show an increase in the total amount of precipitation on the northern edge of the Alps of around 20 percent with an increase in frequency of 13 percent.

As man-made climate change progresses, Austrian weather experts expect that these events could become less frequent. Reason: As the polar jet shifts towards the north and therefore weakens, especially in summer, air compression becomes less likely, which will make it more difficult for Vb weather patterns to form in the long term. However, such modeling is controversial because the connection between climate change and a weakening jet stream has not yet been really proven.

However, the second forecast from Geosphere Austria is widely shared, especially since there is already evidence of this: even higher amounts of precipitation in Vb events. The experts justify this by saying that, according to the Clausius-Clapeyron equation, the air can absorb around seven percent more moisture in the form of water dissolved in gaseous form per degree of warming.

This still applies to the current flood disaster: Meteorologist Oliver Hantke from wetter.de points out that even the northern Mediterranean around Italy up to the Adriatic, where Vb lows usually form, is still 24 degrees warm. These are “ideal conditions for releasing a lot of moisture into the air.” In other words: The floods in Central-Eastern Europe are a late consequence of the summer heat wave in Southeastern Europe, which brought record temperatures to the Mediterranean this year.

Major events like this used to be called floods of the century, which statistically speaking occurred every 100 years. But because of the accumulation, the term has become worn out. In the Czech Republic, the authorities are now talking about a millennium flood, with 500 liters of rain per square meter in some areas. The German record so far is 312 liters, achieved in Zinnwald-Georgenfeld in the Ore Mountains on the morning of August 13, 2002 – in Vb weather conditions.

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