Cold drop

A stationary cold low, in which a cold drop is found.

A cold drop, also known as a cut-off low (COL), weatherman's woe, a closed low or a cold pool, is defined by the National Weather Service as "a closed upper-level low which has become completely displaced (or cut off) from basic westerly current, and moves independently of that current". Cut-off lows may remain nearly stationary for days, or on occasion may move westward opposite to the prevailing flow aloft (i.e., retrogression). Cut-off lows form in mid-latitudes (usually in the subtropics or between 20° and 45°) and would remain nearly stationary for days.

The meteorological term "gota fría" ("cold drop") is used in Spain which has commonly come to refer to any high impact rainfall events occurring in the autumn along the Spanish Mediterranean coast. In Europe, cold drops belongs to the characteristics of the Mediterranean climate.

Formation

A cut-off low is a cold low where polar air is cut off from the primary subpolar belt of westerly trough of low pressure and cold air, which is the standard track of depressions. It would first arise as a trough in the upper-air flow that close in circulation and then would reach down to the earth. It is defined by concentric isotherms around the core of the low. They are more common in times when there is low index circulation. It can also be at any platform in the atmosphere, and as such, they may not be visible on surface charts. At times, a cut-off low would occur with a cut-off high over higher latitudes, which are typically in blocking circumstances and they can last for a week.

Furthermore, they may also form in an unstable easterly flow on the northern wing of a blocking high. In southeastern Australia, cut-off lows can be associated with Australian east coast lows, which are subtropical cyclones or extratropical cyclones that originate from the east. Whilst cut-off lows can form at any time of the year, they are more common in autumn, winter and spring in much of the areas affected, particularly in Australia and the Mediterranean Basin, when a mass of polar air is brought towards more southern regions (or northern, in the southern hemisphere) by the jet stream moving between 5 and 9 km altitude. In Spain, it appears when a front of very cold polar air, a jet stream, advances slowly over Western Europe, at high altitude (normally 5–9 km or 3–5.5 mi).

The air mass that generates a thunderstorm or downpour is characterized by hot and humid air at low level, while that at altitude is drier and cooler. This unstable contrast causes the lower air to rise in an updraft; the humidity condenses from the altitude where the air temperature reaches that of the dew point, giving saturation, to form the cloud and later generate precipitation. The precipitation core in the cloud ends up being too heavy for the updraft to support. It then begins to descend and when it leaves the cloud. The droplets begin to evaporate because the air there is not saturated.

This partial or total evaporation removes energy from the air surrounding the precipitation, which will therefore be colder than the environment. This air, despite the adiabatic heating, will always be cooler than the surroundings once it reaches the ground and forms a cold drop under the cloud. On the other hand, cool, dry mid-level air can be absorbed into the cloud. The latter is at a higher temperature because it has undergone a wet adiabatic transformation. The injected air is therefore denser and must descend. It always remains colder than the environment during this descent and adds to the cold air dome created by the precipitation.

Characteristics

A cut-off low situated over Italy.

The atmospheric circulation conducive to the formation of a cut-off low is an upper barometric trough with a strong north-south component. The cold air carried towards the southern regions ends up creating a closed depression at all levels on the margins of the general circulation, and surmounted by a dome of cold air. The effects of the cold drop will be more important the higher the temperature of the marine waters, since the water vapor rises suddenly due to the lower density of the warm air and condenses, quickly forming high-altitude clouds (generally, more than 10 km) that are almost always of the cumulonimbus type.

The diameter of a cut-off low can vary from a few hundred to a thousand kilometers. The air there is homogeneous and without a front line separating it from the surrounding masses, while having a decisive influence on the weather. It then most often leads to an atmospheric blocking circulation where we witness the formation of an upper level low. High cold drops, between 1,000 and 10,000 m, are regions of low stability, while low cold drops are regions of relatively stable air. Composed of very cold air of polar origin, it typically has a horizontal extent of 300 to 1000 km and is 5,000 to 10,000 meters above sea level.

If the drop is near the ground, a temperature inversion and stratiform clouds of low vertical extension occur. When this air passes over warmer air from a more southerly anticyclone or a local heat source, such as the sea whose water is warmer than the temperature of the drop, the air is then particularly unstable and gives convective clouds. Another kind of a cut-off low is the one arising on a mesoscale under a thunderstorm or downpour when the core of precipitation and cool air from the mid-levels of the atmosphere descends to the ground, being colder than the environment, they form a very stable cold air dome that spreads out under the cloud.

Movement

A cut-off low has a slow movement, typically over a confined region, where it produces heavy rainfall. They are volatile, baroclinic systems that meander to the west with strong convergence and an ascending motion, especially when they are deepening. A cut-off low can persist from a few days to more than a week. It may be absorbed into the general circulation while another forms in the same place a few days later. The evolution and movement of a cold low, like any weather blockage, is therefore uncertain.

To move or absorb such a system, the altitude flow descending towards it must be very powerful and well synchronized and/or the stormy/rainy activity within it mixes the air mass vertically. Improved model equations and resolution have greatly improved the prediction of their behavior, with errors still often occurring. The position of a cut-off low can therefore be incorrectly estimated and the regions of atmospheric convection which also accompany it. The meteorological consequences vary according to the size of the drop, its altitude and its lifespan. In the case of a “low altitude drop”, the depression created is generally weak and fills up spontaneously.

Effects

Effects of a cut-off low of September 1997 in Alicante.

Cut off lows typically create unsettled weather and, in the warm season, they may produce a lot of thunderstorms. In eastern Australia, a cut-off low can bring accumulating and widespread snowfall at low-level areas and as well as elevated regions in the subtropics. If the low extends at altitude, the phenomenon may persist for several days or even more than a week, as the depression above which the cold drop is located moves slowly. The rain/snow can therefore last for days and locally give very large accumulations causing flooding, as well as damage to houses and crops.

Soil saturation with water can also induce solifluction and landslide phenomena. According to the depth of the surface depression and the strength of the thunderstorms, the winds can be strong to violent. In this case, the phenomenon can affect a vast territory. For example, a cut-off low was responsible for the July 2021 floods in Europe, which were catastrophic. The severed depression produced by an unusual accumulation of rain over Germany, Belgium, the Netherlands, Luxembourg, France and Switzerland, resulting in more than 100 deaths in a few days.

The mesoscale cold drop spreads out under the cloud and forms a more or less intense gust front. On average, the low propagates at 10 m/s and last 2 to 3 hours. In numerical simulations, they reach 10 km in radius, whereas in reality they can extend over 50 to 100 km. The meeting of the edge of the drop with a flow of warm and humid surface air can lead to the formation of new convective clouds by lifting of this air and of a cloud line called an arcus under a cumulonimbus. The collision between several drops can also trigger the formation of new storms and thus lead to the formation of new drops of complex shapes.

Occurrence

Spain

If a sudden cut off in the stream takes place (particularly in the Atlantic Ocean), a pocket of cold air detaches from the main jet stream, penetrating to the south over the Pyrenees into the warm air in Spain, causing its most dramatic effects in the Southeast of Spain, particularly along the Spanish Mediterranean coast, especially in the Valencian Community. The torrential rain caused by cold drop can result in devastation caused by torrents and flash floods. For instance, the great Valencia flood of 1957 was the result of a 3-day-long cold drop.

This phenomenon is associated with extremely violent downpours and storms, with wind speeds of 100–200 km (60–120 mi)/hour, but not always accompanied by significant rainfall. For this it is necessary that the high atmospheric torrential rain instability in the lower air layers to combine with a significant amount of water vapors. Such a combination causes the masses of cold air to quickly discharge up to 500 liters per square meter in extremely rapid rain episodes. This phenomenon usually lasts a very short time, as it exhausts its water reserves without receiving a new supply.

Other areas

Cut-off lows are apparent near the Sierra Nevada de Santa Marta in the Colombian Caribbean, with peaks surpassing 5 km in altitude in close proximity to a warm sea. They can also occur elsewhere in the southern hemisphere, such as in South Africa, Namibia, South America and southern Australia. In the northern hemisphere, besides Southern Europe, they can occur in China and Siberia, North Pacific, Northeastern United States and the northeast Atlantic.

See also

• Cold-core low
• Cold pool
• Polar vortex