Globally, we are increasingly seeing floods like last week in Texas with much damage and casualties. On July 13 and 14, 2021, Limburg was hit by floods after extremely heavy rainfall. In a new study published today in Communication Earth and Environment, we investigated with a climate model what could have happened if the weather had developed just a little differently. We call these alternative storylines. And what turns out? The rainfall could have fallen over a different and much larger area and could have lasted for more days, but the amount of precipitation in 24 hours could hardly have been higher. These alternative storylines help us better prepare for such events.
The floods
In some places in Limburg, more than 150 millimeters of rain fell in 48 hours (July 13 and 14) more than twice the normal amount of precipitation that typically falls in the entire month of July (1991-2020 average). The heavy rainfall caused a lot of damage in South Limburg, but also in Belgium, Germany, Luxembourg, and Switzerland. Houses were flooded, roads were closed, rivers overflowed, and in Germany, as many as 200 people lost their lives. The event was one of the five most costly disasters in Europe in the past half-century, with an estimated total damage of 32 billion euros.
Alternative storylines
The climate system is complex; a small change in the current weather can lead to significant differences weeks later. This is known as the butterfly effect – the flap of a butterflys wings can theoretically cause a storm elsewhere within a few weeks.
In our research we make small changes to the weather situation shortly before the heavy rainfall begins using a climate model. We then calculate how the weather develops in that case. By repeatedly doing this for slightly different adjustments to the initial situation, we create a whole collection of alternative possible weather developments. These differences in the amount of rainfall at different times and places result in changes in the nature and severity of the floods. In this way, we create alternative storylines for the floods that occurred.
