Torrential rains – genesis and occurrence on the territory of Poland

deszcze nawalne

Storm phenomena in Poland are observed throughout the year. Between 2002 and 2013, an average of more than 150 days per year with a thunderstorm was recorded. The number rose to more than 170 days in mountainous areas, and to about 180 in the Baltic Sea region. The occurrence of active thunderstorms is accompanied by a series of violent phenomena such as strong wind gusts, intense rainfall (including torrential rain), lightning, and in some cases the development of tornadoes. Among the most dangerous phenomena resulting from extreme storm precipitation are flash flood surges.

How do torrential rains form?

The immediate cause of flash floods is torrential rains. They are characterized by high intensity and short duration, from a few to several tens of minutes. They are associated with intense storm activity. The measure of a driving rain is its yield, expressed in the volume of rainfall per unit area at a specific time.

In Poland, torrential rains occur only in the warm half of the year, mainly in June, July and August, and occasionally in May and September. Heavy rains reach their maximum in the afternoon and evening, when the strongest ascending currents occur due to the strong heating of the air. Like thunderstorm phenomena, heavy rainfall occurs throughout the country, with greater frequency in mountainous areas and on the coast.

The most important factor, which determines the formation of heavy rainfall, is the unstable equilibrium of the atmosphere, and thus the development of convection and thunderstorms. This is a situation in which a portion of air, due to the influence of various factors, begins to be lighter than the air surrounding it and, under the action of buoyancy force, rises upward.

It occurs in areas of atmospheric fronts (generally cool fronts and occlusions), on lines of wind convergence, as a result of ground heating and under the influence of terrain or thermal contrasts at the interface between sea and land. The air cools as a result of rising, and the water vapor in it condenses – condensing – releasing additional energy. As a result, uplifted storm clouds form.

In order for a storm cloud (known professionally as a storm cell) to bring with it torrential rains, several more factors must be present. One of them is the high water vapor content of the entire atmospheric profile. This happens, among other things, with the influx of warm and humid air masses.

In the case of Poland, these are polar maritime air masses from over the Atlantic and tropical air masses from over the Black or Mediterranean Sea. Thanks to the moisture, energy is supplied all the time, enabling the storm cloud to grow and develop dynamically. At the same time, moist air below the base of the cloud reduces evaporation of precipitation before it reaches the earth’s surface. The more extensive the cloud and the lower its base, the stronger the precipitation will be. The high humidity also means that it will be intense and not get exhausted too quickly.

High humidity in the atmospheric profile promotes continuous and prolonged storm activity. A single storm cell is active for several to several dozen minutes, after which, as a result of rainfall, it disintegrates. It is unlikely to be so extensive as to carry a torrential rainfall. Such a situation is more common when a new storm cell, and another, and another, is created to replace one disintegrating storm cell.

This happens with high humidity and strong vertical movements in the atmosphere. Uninterrupted convection results in organized linear, multicellular or supercellular structures. Their lifespan is even counted in hours (Szewczak, 2014). These types of storms are much more likely to produce heavy rainfall.

In addition to the large moisture reserves and the very nature of the storm cell, the direction and speed of its movement is important. If it moves quickly over an area, and another and another does not form behind it in the same place, the situation will not be conducive to heavy rainfall and the resulting flash floods.

However, if the speed of the storm’s movement is slow or successive cells form in the path of existing ones, there is a cumulative build-up of rainfall totals and duration in a given area. This situation can occur when the wind speed in the atmospheric profile is low. Quasi-stationary storm cells are then formed, which disintegrate at maturity and disappear as driving rain.

Another situation conducive to the accumulation of intense precipitation will be the blocking of the movement of the storm front by an extended high-pressure system, generally by a high located over Scandinavia or western Russia. This can lead to a linear arrangement of storm cells and their successive passage over an area.

Terrain can also contribute to the intensification of storm precipitation. It forces the direction of storms to move, such as along river valleys or mountain ranges, accumulating rainfall totals in a given area. In southern Poland, mountain ranges force moist air to rise up the slopes, causing increased precipitation in these areas.

A separate case in which heavy rainfall is possible is the formation of an extended storm structure called a supercell. It is a well-organized storm structure, reaching a size of several hundred kilometers, which can exist for many hours and pass through the entire country during that time. It is associated with extreme phenomena, including prolonged and heavy rainfall and hail.

Heavy rains, because of their efficiency, which can lead to flash floods, are subject to classification. It was developed by Chomicz and is still the basis for determining rainfall intensity in the form of the so-called “rainfall intensity”. performance factor α. The classification is based on two parameters: the amount of precipitation in millimeters and its duration in minutes.

The coefficient of performance α takes values of <1.4 for normal and heavy rains; 1.41 – 5.65 for torrential rains divided into four grades (I – IV), and above 5.65 for torrential rains divided into grades V to XI. The latter in Poland are generally classified in grades V and VI, with a performance factor α of up to 8.0. With this compilation, individual cases of driving rain can be compared with each other or applied to the design of stormwater drainage networks.

Heavy rains cause flash floods

As a direct result of torrential rainfall and complex environmental conditions, sudden floods, so-called “flash floods,” occur. flash floods (the flash flood ). They can happen in virtually any area of Poland and do not have to be related to a violent overflow on a river. They can also manifest as flooding caused by rainwater accumulation.

If such flooding occurs in urban areas, it is referred to as urban flooding (urban flood). Flash floods are among the greatest threats to human health and property. Direct damage, such as destruction of property, damage to buildings or flooding of crops, and indirect damage – loss of health and life – associated with them are very high.

The increase in the threat of these floods is largely due to: the intensification of precipitation phenomena caused by climate change, the improper management of catchment areas and their increasing sealing, the poor sizing of stormwater drainage networks, and the lack of a warning system for people living in flood-prone areas. But these issues have already been discussed in detail in another article in the current issue.

In the article, I used, among others. From the works:

Burszta-Adamiak E. et al, (2020). Methodology for the development of the Polish rainfall intensity atlas (PANDA), IMGW-PIB
Chomicz K. (1951). Downpours and torrential rains in Poland, “News of the Hydrological and Meteorological Service,” 2, 3, pp. 5-88.
Derek P. et al. (2015). Instructions for meteorological stations, IMGW-PIB
Pietras B., Pyrc R. (2018). Genesis of extreme storm precipitation and flash flooding in Bieliny on May 26, 2018, Proceedings of the Student Scientific Circle of Geographers of the Pedagogical University of Cracow, vol. 7, 2018
Pociask-Karteczka J., Żychowski J. (2014). Flash floods – causes and course [in] “Water in the city Monographs of the Hydrological Commission PTG”, (vol. 2) Institute of Geography, Jan Kochanowski University, Kielce, 2014, pp. 213-226
Szewczak P. (2014). Meteorology for the airplane pilot, Avia-Test Lech Szutowski
Taszarek M., Czarnecki B., Koziol A., (2015). A Cloud-to-Ground Lightning Climatology of Poland,” Monthly Weather Review, (vol. 143): 4285-4304

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