Devastating floods in western Europe

By Silvia Puca (H SAF, Protezione Civile/Italian Civil Protection Department, Presidency of the Council of Ministers), Luca Brocca (CNR-IRPI/ National Research Council of Italy, Research Institute for Geo-Hydrological Protection), Giulia Panegrossi and Alessandra Mascitelli (CNR-ISAC/National Research Council of Italy, Institute of Atmospheric Sciences and Climate (CNR-ISAC)), Peter Krahe (Bundesanstalt für Gewässerkunde, Federal Institute of Hydrology, Germany), David Fairbairn (ECMWF), Pierre Baguis (RMI), Francesco Zauli (HSAF/Centro Operativo per la Meteorologia, Operational Center for Meteorology (Italian Air Force)), Natasa Strelec Mahovic, Christine Traeger Chatterjee (EUMETSAT) and Ivan Smiljanic (CGI)

A slow-moving upper-level low, that resided over western Europe for several days, caused very heavy, long lasting rainfall on the already moist soil, resulting in catastrophic floods in western parts of Germany and eastern parts of Belgium, and to a lesser extent, parts of the Netherlands, France, Luxembourg and Switzerland.

First, a series of precipitation events from 6-12 July led to a large accumulation of rainfall in Switzerland, eastern parts of France and south west Germany. Subsequently, over the following two days, 13-15 July, a slow moving low pressure system, passing over the south sast France towards central and southern parts of Germany, led to very heavy, long lasting rainfall events in western parts of Germany and in the border regions of France, Luxembourg, Belgium and the Netherlands (Figure 1). This resulted in flooding of urban areas next to small and medium rivers, with catastrophic consequences. The floods destroyed towns and villages in Germany and Belgium, causing more than 180 casualties and large material damage. Some large cities, such as Cologne and Düsseldorf, were also affected by the flooding.

Storms that cause floods occur in western Europe relatively frequently, however, the storm on 14-15 July was particularly devastating. The reason was the slow-moving nature of the storm, that caused a much more concentrated accumulation of rainfall over the affected area than it would have if the storm had been moving faster. Such intensive storms and prolonged rain episodes, with excessive precipitation, are becoming more likely with changing climate.

At EUMETSAT it is part of our mission to support the weather services, who observe and forecast such storms, and the emergency services who support the response and recovery.

Synoptic overview

The animation of the Meteosat-11 Airmass RGB overlaid with Absolute Topography at 500 hPa level isolines, shows the movement of an upper-level low that formed within the large low-pressure centre over the North Atlantic (Figure 2). In the days preceding the heavy precipitation episode there was already an upper-level low passing over the same area bringing precipitation (10-11 July). At that time the ULL that formed over the Atlantic was passing south of Ireland, still moving rather fast.

The appearance in the Aimass RGB on 11-12 July indicates strong forcing from the upper troposphere, which can be seen in red shades in the centre of the cut-off low. The red colour in the Airmass RGB comes from dry, potential vorticity-rich air, descending from the upper troposphere or lower stratosphere and supporting cyclogenesis, thus deepening the system.

At the leading side of the trough, within the frontal zone, warm and moist air was advected with strong south west stream, first from the Atlantic and later from the warm Mediterranean. Once the cut-off low reached France it was 'blocked' by a high-pressure area over Eastern Europe on one side and a large high pressure area over the central Atlantic on the other (Figure 3). This blocking situation slowed down the progression of the storm, resulting in intense rainfall falling persistently over the same area.

How much rain fell?

As can be seen from the ground-based data analysis (rain radar measurements), the largest precipitation amounts during the three days from 13 to 15 July were recorded between Germany, Belgium and Luxembourg (western Germany: 100-150 mm; Belgium: 100-200 mm and Poland: > 50 mm (Fig 4-6).

The precipitation estimated from the satellite measurements can be seen in the H SAF precipitation products in Figures 7 and 8. Figure 7 shows a loop of six-hourly rainfall for the period from 12 to 15 July, and Figure 8 shows the accumulated precipitation in the same period.

Microwave-derived satellite precipitation product from H SAF has been able to correctly identify the areas most affected with amounts very close to ground observations. Figure 9 shows the comparison of the accumulated rainfall from satellite data (H SAF H67 product) and accumulate rain gauge measurements for the period 13-15 July 2021.

Comparison of measured and satellite-based daily precipitation

CPC Unified Gauge-based Daily Precipitation Analysis H SAF MW-based total accumulated rainfall

 

The impact of precipitation in terms of flooding have been catastrophic, with flooding over all the rivers at the border between Germany, Luxembourg and Belgium, e.g. the river Ourthe at the border between Belgium and Luxembourg) and the river Sauer on the border between Luxembourg and Germany.

Hydrological summary

Prior to the period of rainfall connected to this storm, there were several episodes of heavy rain in the previous days, so the soil was already very moist, near saturation in many areas. The top metre of soil was completely saturated, or well above field capacity,after the intense rain occurred from the beginning of July in Belgium, Netherlands, Luxembourg and Germany, as shown in root-zone soil moisture products - Soil Wetness Index (Figure 10) and the anomalies of Soil Wetness Index in the root zone (Figure 11). In the flooding areas the soil wetness index shows value between 0.5 and close to 1 (fully saturated), the red dots are the flooded areas in Germany.

The anomalies of the H SAF Soil Wetness Index in the root zone H14 (SM-DAS-2) for the integrated root-zone layer (0-100 cm depth) showed very wet conditions in the flooded regions (10-30% above average) for 14 July. The anomalies are calculated based on the multi-year July average of H141/H142 (1992-2020) as the reference period.

Looking at the soil moisture a few days before the flooding and on the day after the heaviest rainfall, it can be seen that in the flooded areas the soil wetness index increased towards saturation between 10 and 15 July (Figure 12).

The soil layers at 28-100 cm depth and the integrated root zone soil moisture (RZSM) of 0-100 cm depth normally have a memory of a few weeks, whereas the top layers have a short memory and will saturate quickly after intense rain.

From 13 July the surface soil moisture estimate from the surface layer (0-2 cm) for the H08 product, depicts wet conditions indicating the presence of rainfall events before the flooding incident. In the flooded regions the surface soil moisture increased (SSM) towards saturation between the 13 and 15 July. (Figure 13).

The surface soil moisture estimate in the topmost soil layer (<5 cm) is given in degrees of saturation, ranging from 0% (dry) to 100% (wet). On 15 July in the flooded regions, which are displayed as the wettest soil conditions, the SSM increases with value close to 100% (fully saturated). The very wet antecedent soil moisture conditions and the significant precipitation amounts were key drivers of the catastrophic flood occurred on 14-15 July. As shown in Figure 14, flooding occurred in areas in which significant precipitation fell in areas that were already significantly saturated. Satellite products from H SAF have been able to monitor soil moisture and precipitation during the event, thus being an important tool for performing hydrological analysis during flood events.

Flood assessment

For detailed post event rapid mapping products and airborne overflight campaigns you may like to consult the following resources:

• DLR Sit­u­a­tion in­for­ma­tion for dis­as­ter man­age­ment
• Copernicus Emergency Management Service - Mapping

Additional content

Satellites map floods in western Europe (ESA)
Deadly Floods Surprise Europe (NASA Earth Observatory)
Hydro-climatological classification of heavy and continuous precipitation in parts of Germany in connection with the low pressure area "Bernd" from 12 to 19 July 2021 (DWD, in German)
The Extent of Flooding in the Hardest-Hit Areas of Europe (New York Times)
Germany floods: Where are the worst-hit areas? (BBC News)