Learning about cut-off lows
DETAILED information to help us better understand the feared weather phenomenon known as a ‘gota fría’ or DANA in Spanish – and a ‘cut-off low’ or ‘closed upper-level low’ in English, has been published by state weather agency AEMET.
In the Mediterranean area a cut-off low can cause violent storms at any time of the year.
This weather system was the cause of the floods in September which devastated large swathes of the Vega Baja and south of Valencia province.
However, on average around 15 cut-off lows or ‘gota frías’ can affect the Iberian Peninsula each year and, contrary to popular opinion, they don’t always bring heavy rain.
Director of production and infrastructure at AEMET, José Antonio Fernández Monistrol has provided an in-depth analysis of a phenomenon which most commonly hits south-east Spain. He explains how high pressure over the British Isles or Northern Europe causes a ‘blockage’ which can lead to adverse weather in Spain. Although his analysis is at times very technical, it is worth persevering with his description to have a better knowledge of something which affects the lives of everyone in this area.
What is it?
A cut-off low forms at levels above five kilometres as ‘it is being cut-off from the circulation of westerlies in mid-latitudes and the polar jet stream’.
Sr Fernández Monistrol explained that the polar jet stream ‘undulates successively northwards and southwards, giving rise to troughs and ridges in latitude’.
“A trough is a breaking southward of air which is colder than that around it,” he explained.
“Within it, low-pressure areas or depressions form with fronts that separate the cold from the warm air.
“The succession of troughs and ridges at high levels usually moves eastwards.”
Sometimes this movement is hindered by the presence of potent masses of warmer air that occupy the entire troposphere and ‘a cut-off low breaks away southwards from a trough in a vortex shape as it tries to pass round the block’.
“Once it is away from the conveyor belt, the cut-off low moves more haphazardly and unpredictably, being subjected to the conditions in the zone which it is passing through – mountain systems, warm seas, etc,” he noted.
The ‘structure of cold air surrounded by warmer air can also gradually spread down to lower levels’ until it reaches the earth's surface in a final state known as a ‘cold cut-off low pressure area’.
“With respect to Spain, the most dangerous situations arise when the cold air at high levels draws closer to the Mediterranean Sea at the end of summer, when the temperature is higher,” he explained.
What happens in a cut-off low?
It can bring several different adverse phenomena – strong winds, heavy downpours of rain or snow, and storms. Severe storms occur ‘when there is a notable difference between the temperature near the surface and that at higher levels’.
“A cut-off low has colder air at its centre, which is why it is predictable that when it passes over zones with hot air convection is unleashed as soon as a trigger mechanism comes into play, such as, for example, rising up the sides of coastal mountain systems,” he said.
“If, besides being warm, the air that sustains the storm is humid, we find that, on the one hand, it supplies more water that can precipitate and in addition to this, where there is enhancement or intensification of the convection due to the effect of the warmth given off into the atmosphere because of the water vapour condensing as it rises, this adds more energy to the storm.”
Therefore the ‘adversity’ of a cut-off low depends on both its own structure and conditions external to it.
He highlighted two zones in a cut-off low – those that originate in its centre due to instability deriving from the core of cold air at upper levels and those that occur in what is known as the ‘baroclinic leaf shield’ in the eastern advance zone of the structure – usually shown on charts as a front.
“The strongest levels are usually associated with the baroclinic leaf shield,” he explained.
“The high degree of instability and the presence of input at lower levels lead to convection becoming organised and gives rise to storm structures that last for several hours and can persist over a single point, causing flooding.
“This contrasts with the less organised, shorter-lived convection that forms below the core.”
He noted that precipitation strengths rely heavily on both humidity input conditions at low levels on account of a large amount of water vapour resulting from the continuous evaporation from a warm sea at the end of summer and the presence of wind maximums that in conjunction drive this highly unstable air towards the coastal mountains systems that force it to rise and act as a trigger for powerful convection.
Difficult to predict
This combination of elements characteristic of a cut-off low and the environment within which it moves – together with the differentiating fact that the paths of a cut-off low lack any powerful steering element and are more erratic – make forecasts during these episodes particularly difficult.
“To sum up, we can state that not all cut-off lows produce heavy downpours over the Iberian Peninsula and that the fact that they happen depends on other conditions to which the position of the centre of the cut-off low is very sensitive, which in turn determines the position of the baroclinic leaf shield and the intensive transportation of very humid air groundwards,” he added.
“The effects of any natural disaster are related to the power of the phenomenon that provokes them, though also to the vulnerability and exposure of the affected zone.
"For our purposes, the strength of the phenomena that are triggered by a cut-off low will depend on two factors, as has been discussed.”
■ The structure of the cutoff low with respect to how deep it is. In other words, with respect to how much the pressure at its centre diminishes and how unstable it is, which is in turn conditional upon the decrease in temperature at height.
■ External conditions. Essentially on the supply of energy at lower levels associated with the temperature of the air, its water vapour content and the strength of the winds that carry it along, as well as the presence of agents that trigger convection, such as the convergence of winds or orographic rises.
How long do they last?
“The life cycle of a cut-off low usually lasts for about two days and only one third of them reaches, or goes on for more than, three,” he noted.
“In their final phase, cut-off lows are equally divided between those that abate and fade away, and those that are subsumed into a more potent system.”
Are they rare?
“As people associate a cut-off low over our latitudes with very heavy rainfall, this gives the idea that they are not very frequent,” explained the meteorologist. “Yet this is not so.”
A study from 2005 showed there are around 15 a year on average in Western Europe and the eastern Atlantic.
Sr Fernández added: “Another notable fact is that there is a falling trend in the number of cut-off lows that form in spring and a rise in those that occur in summer and autumn.
“This is highly significant, given that it squares with the warm sea and on-shore conditions in the Mediterranean zone, which probably work in favour of instability and are likely to increase the heaviness of downpours.”
Are the effects intensifying due to climate change?
Sr Fernández Monistrol noted that rising temperatures and water evaporation, according to the laws of physics that apply to meteorology, are leading to an increase in the torrential nature of downpours.
This can be difficult to measure due to ‘variability of torrential rain events originating from convection’. For example, one town can record 200 litres per square metre (l/m2) of rainfall while 10 kilometres away another town may register just 10 l/m2.
Catastrophic flooding events
Floods are not always caused by cut-off lows. The record rainfall recorded by a weather station in 24 hours was in Oliva – 817 l/m2 on November 3, 1987. This event was not triggered by a cut-off low but ‘by the interaction of a cut-off low-pressure area that occupied the entire troposphere on the vertical plane over the gulf of Cádiz’.
On September 30, 1997, Alicante and the south of Valencia province were hit by torrential rains that led to the overflowing of several tributaries of the Júcar river. In Alicante 270 l/m2 accumulated in under six hours causing four deaths. This was a cut-off low focusing on the Atlantic coast of Morocco. The low pressures in northern Africa and an anti-cyclone centring on the north of France combined to produce a corridor of wind that moved across the western Mediterranean, becoming laden with moisture and feeding the rainfall on the coastal strip with water and energy.
The Marina Alta in Alicante province suffered torrential rain on October 12-13, 2007. At several points more than 450 l/m2 fell in under 24 hours. The rise in the level of the river Girona caused multiple damage. This time there were two cutoff lows – one over the Iberian Peninsula and another in the central Mediterranean.
The cut-off low in September 2019 which devastated the Vega Baja area ‘can be described as extraordinary, on account of both its life cycle (it went on for five days) and its path as it took a course towards the south and subsequently returned northwards, which meant that in certain zones its impact was felt twice in only a brief time-span’.
“In this case all the factors that make cut-off lows dangerous came together at once – an unusually warm Mediterranean, the positioning of its centre which directed the wind towards land after a long journey across the warm sea, and interaction with highly unstable subtropical air,” noted Sr Fernández Monistrol.