Water bombs

In Italy, we are being hit by water bombs. I think I need to explain what this term means. "Water bomb" is a literal translation from the Italian term "bomba d'acqua" which started to be used in Italy some 3-4 years ago, and it's increasingly used to indicate an extreme rainfall event. The latter is characterized by unusual rainfall intensity, and therefore there is the feeling that such extreme events did not occur in the past, or occurred with much less frequency than today. For this reason, water bombs are deemed to be related to climate change, that provoked an increase of the frequency of extreme rainfall events.

The term "water bombs" was introduced in the web in Italy and became extremely popular in the media. Actually, the term "water bomb" in English has another meaning, as one can see at https://en.wikipedia.org/wiki/Water_balloon, which would be the English translation of the Italian "gavettone" (see https://it.wikipedia.org/wiki/Gavettone). The correct translation in English of what we Italians mean for water bomb would probably be "rain bomb". These events could be given by microbursts. For an example of such a phenomenon, one may see http://canyouactually.com/this-incredibly-rare-rain-bomb-falling-from-the-sky-looks-insane/. The figure that you can see above is a sketch, taken by the above Wikipedia page, of the mechanisms leading to the genesis of microbursts.

The question of weather these rain bombs are due to climate change is extremely relevant. Indeed, in Italy we observed an increase of flash flood events in the recent times, which calls for immediate action as these events cause the loss of lives and substantial economic damage. In order to plan effective actions for reducing the risk we need to identify the causes for such events. If the main cause is climate change, then actions need to be taken at large scale. If it is the increased urbanization and river training, which may imply a significant increase of the runoff coefficient (namely, the portion of rains that is effectively converted to river flow), then actions need to be taken at local scale.

The fact is that I could not see up to now any data-supported proof that the extreme rainfall intensity is increased in Italy in recent years. The analysis of the daily data, which is extremely interesting in Italy because we have the longest daily rainfall series of the world, does not reveal any evident change in rainfall intensity. The question is what the results would be if one analyzed rainfall data observed at very fine time scale (subdaily).

Given that we need to immediately take effective actions for reducing the risk, I think a suitable data analysis is needed to get more information to support the above identification of causes. For this reason, together with Simon Papalexiou (Technical University of Athens) I am starting to study historical rainfall data observed at fine time scale all over the world and will try to perform a simple analysis of the frequency of extremes along the observation period. We will present the results at the next EGU General Assembly, to be held in Vienna in April. I am curious to know more about "water bombs"!

Bologna, January 18, 2016