'Formation Mechanisms of the Torrential Rain Episodes at the Leeward Side of the Mountains with Poland as an Example'

Background

To the south of central lowlands of Poland the terrain rises slowly and is dominated by highlands. Mountain ridges of the Carpathians and the Sudetes stretch along the southern border. Annual precipitation totals show great spatial diversity and are dependent not only on the height above sea level, but also on the position relative to the entire block of mountains and on the exposure/screening in the western sector. Occasionally the flow of warm and humid airmass may be forced around an isolated mountain, rather than over it. In this case the leeside convergence of air may result in uplift and possibly in convective rainfall. These phenomena occur frequently during S-SW circulation which is consistent with natural terrain falloff between the Sudetes and the Carpathians – the Moravian Gate. It extends lengthwise in SW-NE direction for a distance of about 50 km.

Aim

The main goal of this paper is to present the development of strong convection at the lee side of the mountains, frequently resulting in torrential rain. Special attention is paid to the variability of the integrated columnar water vapour content (IWV) during such events.

Method

Selected events were studied by analysing synoptic and mesoscale meteorological condition, surface observations, satellite and radar data. To analyze moisture transport and absolute humidity distribution the Advance Research Weather Research and Forecasting Model (WRF) version 3.4.1, in resolution 5 x 5 km was used. The model was run in reanalysis mode in three one-way nested domains (Europe 45 km, Poland 15 km, SW Poland 5 km) and 3 hours temporal resolution. Authors also decided to apply GNSS (Global Navigation Satellite System) tomography model TOMO2 to resolve the integrated water vapour before, during and after the mesoscale convective complexes. The applied technique allows to get full picture of IWV spatial distribution.

Results

The most diversified spatial layout of IWV in Poland is observed for days with opposite type of vorticity at lower and upper isobaric level and days with a very weak air mass advection from SW with a stationary front present. On such summer days mean IWV above 30 kg·m^-2 is typical near the vent of Moravian Gate. Throughout the whole year, in such circumstances, the highest positive deviations of IWV from seasonal and monthly means – about +4 kg·m^-2, are observed in the Moravian Gate compared to the other part of the region. That is the reason for the higher IWV values observed frequently on the mountain lee-side region – close to the Moravian Gate (the Silesian Upland).

Usually south-westerly flow of moistened and heated air prevails especially in the lowest 500 m through the Moravian Gate. Northward, another westerly flow component enters. This leads to the formation of convergence, where not only lifting is induced but also the low-level supply of moist warm air is at a maximum. Orographic effects, veering winds in the lowest part of troposphere, low-level heat and moisture content as well as forced lifting are most pronounced and facilitate the development of supercell storms with hail and extreme precipitation.

Another pathway leading to the severe weather of the convective genesis is the partial blocking of a quasi-stationary cold front by orography. The front moves slowly eastward over the Sudetes and their close foreground, while further to the north and to the south the eastward motion of the front is faster. In such conditions, at the leeward side of the mountains, an isolated portion of the warm and humid air remains surrounded by the cold air which from all sides pushes the mass of warmth up. As a result, the existing inhibition layer breaks down in the region where ascending currents are strongest and vigorous convection reaches the upper troposphere, locally even the level of the tropopause. The result of such a strong and localized convection may be a short-lived torrential rain. Due to the local nature and short duration of the phenomenon and a small thermal contrast of air masses, the formation of the closed cells of warm air is usually not reflected in the course of the front lines in synoptic maps drawn on a continental scale.

Convection-originated extreme rainfalls extend over tens and hundreds of square kilometres. Such precipitation typically occurs during conditional instability with the presence of an inhibition layer at a certain level, where the thermodynamic equilibrium is stable or even there is a weak free temperature inversion. The mechanism of precipitation is such that the inhibition layer is protruded from below only by a few strongest existing convective currents. If such protrusion happens, total volume of the humid air, transferred through a single convective cell from the ABL to high layers of troposphere, is much larger than in case of a very unstable equilibrium with the lack of inhibition layer. In the latter situation a large number of convective cells would develop in close proximity which would compete each other for access to the supply of warm and humid air.

This is confirmed by numerous examples, but four of them were analysed in details: 01 September 2002, 15 August 2008, 28 July 2015 and 7 July 2017.

Conclusions

Typical development leading to the formation of torrential rain of the convective genesis is the deformation of slowly moving front under the influence of the mountain range. The channeling of warm and humid airmasses, coming from the south through some gaps in orography (e.g. the Moravian Gate), contributes to higher likelihood of severe thunderstorms. They are particularly frequent downstream from such passes and gaps. Hence, the mid-south of Poland (Silesian Upland) is particularly exposed to thunderstorms, torrential rainfall, hail, downburst or even tornadoes.