The abundant rainfall this weekend in the form of torrential rain and strong winds is a response to the meteorological phenomenon known as DANA. Its arrival can be predicted, but not its consequences in the form of rain or storms.
It is an isolated atmospheric depression at high levels that takes place due to the collision of a mass of cold air at height with the warm air of the surface. This phenomenon, traditionally called “cold drop”, gives rise to intense showers and storms. Thus, the entry of a mass of cold air in the core of the atmosphere that comes into contact with warmer air near the ground causes instability, which favors the formation of clouds that cause strong storms.
The DANA is formed from the polar jet, a current of very intense winds (between 150 and 300 km/h) that circulate in the upper part of the atmosphere (at 9,000 meters of altitude) and whose route revolves around from the North Pole and from west to east (from North America to Europe and Russia). This phenomenon moves causing undulations, meanders that circulate around the planet, with time scales of between 7 and 10 days and which are drivers of squalls, anticyclones and DANA.
Aemet explains that a DANA has two types of causes: dynamic (associated with the phenomenon) and thermodynamic (due to the difference in temperatures). In the northern hemisphere and in mid-latitudes, “what is normal is that storms and anticyclones circulate from west to east”, but sometimes ripples form and “one of the waves forms a kind of air pocket cold and remains isolated with a more erratic movement”.
The erratic movement of the cold air mass “makes it more difficult to predict” its trajectory, according to Aemet. In addition, a DANA “can cause adverse weather, but also high temperatures because, if it is located far to the west, it generates the entry of wind from the south and a rise in thermometers. This means that “storms will not form whenever we have a DANA nearby; they favor it, but not always”.
The State Meteorological Agency recommends using the acronym DANA to refer to this phenomenon, rather than the term “cold drop”, which comes from the German school that named it Kaltlufttropfen. The Fundació de l’ Espanyol Urgent points out that the cold drop takes place due to the entry of “a mass of air that emerges from a very cold current” and that, when descending on hot air, ” it causes great atmospheric disturbances”.
Extreme weather phenomena are increasing due to the impact of climate change. It is not that DANA is produced by climate change, but that it influences an increase in DANA and quite possibly the increase in the virulence of precipitation.
In a country like Spain, the influence of sea temperature on the formation of DANA is fundamental to understanding this process. The Mediterranean Sea offers this necessary ingredient for the formation of strong storms and intense rain. Its warmer waters at the end of summer act as a triggering mechanism for storms and torrential rains when there is the arrival of a DANA. This makes it easier for storm-type clouds to form and rain to be concentrated in a shorter space of time, so they can dump 100 to 200 liters per square meter in as little as an hour. According to climate change projections for the coming decades, the temperature of the Mediterranean will continue to rise. It has already risen 0.8ºC during the last hundred years on the Spanish Mediterranean coast and with an accelerated rise since 1980. / Redacció
