Meteorologists expected 80 mm of rain in Grazalema, which was already a lot. They are already going for 180 mm

This Wednesday the storm Leonardo is showing all its strength in a good part of Andalusia, something that has forced to cancel classes or even to mobilize the UME due to the possible floods that may occur. One of the focuses of this storm is set in the Sierra de Grazalema in Cádizone of the places where it rains the most in Spain. The point here is that a large number of liters per square meter was expected to fall, but reality has surpassed everything previously calculated.

The data. As collected the user in X @Vigorrothe discrepancy between what the model “saw” and what has fallen from the sky is massive: from a forecast of 60 to 80 mm accumulated at 7 in the morning, has moved to a reality of 180 liters per square meter.

And this makes us have many questions in our heads… How is it possible that in the era of Big Data and high-resolution models, we fail by more than double in such a short-term prediction? The answer is in the orography.

Harmonie’s failure. The technological protagonist here is in Harmonie-AROMEthe mesoscale model used by the AEMET to predict local phenomena. Unlike global models such as the European IFS, Hamonie is designed to see detail down to resolutions of a few kilometers to calculate, for example, how many liters will fall in a specific location.

However, today it failed in the Sierra de Grazalema with the differences that we have seen before. And although the AEMET reacted activating the red warningwith an extreme risk of receiving up to 200 liters in one day, the real-time evolution during the early morning hours was much more explosive than the model output indicated. And the worst thing is that there is still a day ahead.

A “wall”. To understand why the software falls short, you have to look at the mountain, and the Sierra de Grazalema acts as a formidable physical barrier against the humid winds of the Atlantic. In this way, when these storms hit the mountains, the air is forced to rise abruptly, cooling and condensing all its moisture in a very small space and gives what is known as orographic enhancement.

In this peculiar storm, two factors have come together that have undoubtedly magnified it. On the one hand, we have had an atmospheric river which acts as gasoline for the clouds, intensifying precipitation much more than models anticipate, especially when they collide with a mountain.

It fails on the scale. On the other hand, we also have the limitations of the numerical models that we use on a daily basis such as collected in the Stormchaser forum. Here they point out that these models continue to have problems resolving short-duration, high-intensity events in complex orographic areas. And they are good at saying that it is going to rain a lot, but they fail when we talk about the magnitude of a specific flood.

It rains in the wet. The problem of this underestimation is not only meteorological, but also hydrological since this torrential rain falls on terrain that no longer supports even one more drop.

The context in this case is critical, since the month of January already broke historical records in this area with accumulated amounts of around 1,300 liters per square meter. That is why the soil, which is composed mainly of clays is completely saturated, which means that the infiltration rate is zero, causing everything that falls to immediately run into the bed of the Guadalete River and others.

Images | Freysteinn G. Jonsson

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