The role of orographic barriers in the origin of extreme rainfalls as exemplified by the front of high Eastern Himalaya and the low northern slope of Carpathians

The paper discusses the role of orographic barriers in generating torrential precipitation in mountainous regions in different climatic zones, the Eastern Himalayas (tropical zone with well-developed monsoon activity) and the northern slope of the Carpathians (temperate zone with transitional climate). Attention has been paid to the different altitudes and courses of the orographic ridges as well as their location relative to the prevailing directions of influx of moist air masses. The cases analysed included torrential rains with monsoon circulation from the S–SW direction at the 2–3 km high edge of the Himalayas, with special consideration to the distance from the margin of the mountains and the exposure of the slopes. They generate frequent flood waves, landslides, debris flows and upbuilding of the alluvial cones in the foreland of the mountain barriers. The impact of the orographic barrier is significantly less marked in the Polish Carpathians. In the western part, the compact edge of the Western Beskids with an altitude of 0.5–1 km and the WSW–NEE course, exposed to moist air masses inflowing from the northern sector, is fragmented eastward into smaller mountain groups, which facilitates the penetration of moist masses of air with occurrence of prolonged precipitation into the mountains. At times, the storm cloud moves along the mountain edge (the margin of the Western Bieszczady Mts.). The marginal scarp of the Foothills has a northern exposure and a height of 150–200 m, and promotes frequent convective precipitation causing local flash floods in small streams. The cases of downpours and high discharges selected for the analysis were those for which there was available a dense network of measuring stations. An insufficient number of stations in constructing precipitation maps based on interpolation would lead to distorting the spatial image. If this were the case, then the role of slope exposure, which has an essential impact on the distribution of precipitation in mountainous regions, would be completely neglected