Statistical Analysis On Foehn In Great Xing'an Mountains

Climate is not only the important environment condition of human life , but alsothe indispensably natural resources for the human material production.China is amountainous country and mountainous region occupies two-third of our country landarea.The climate of mountainous region is variety in different elevation, slopeorientation and landform. In mountainous region, there are lots of seriousmountain-climate disasters attribute to the terrain, such as foehn, bora, cold air lakeand so on. The research of these disasters is an important part of mountain-climateresearch.It plays an important role in the agri-climate regionalization in mountainousregion, the exploitation and utilization of mountain-climate resource and the hazardprevention, etc.The foehn caused by north-south orientation mountain —Great Xing'anMountains is prominent in this area. In winter ,the foehn produces a north-southwarm-ridge which spreads about 10 latitudes and a distinct low humidity area whichis called dry-tongue. At the same time, a big gale often goes with the foehn. It is animportant climate factor which might cause the forest fire. More seriously, it will havenegative impacts on the people's life in Mountainous region when the ice and snowmelted..This paper tries to study on the mechanism and rules of the foehn whichoccurred in Great Xing'an Mountains region. The study may provide a reference forpeople to have a right understand of the mountain-climate phenomena and then willbenefit the people and take full advantage of mountain-climate resource.By analysis of the meteorological data and the outcome of the trailblazer's study,the foehn in Great Xing'an Mountains region is certain to be droved by the terrainpressure effect. The escarpment of the east slope is the primary drive factor for foehnand the existence of the gentle slope in east ulteriorly promoted the foehn in GreatXing'an Mountains.The small scale ridge just weakened the intensity of the foehn,actually it doesn't change the characteristics of the over mountain flow.When the foehn occurred in the east slope of Great Xing'an Mountains, there isalways a powerful synoptic system which dominates or strengthens the circulate ofthe west airflow.A prominent foehn process is produced by the conjunction of theascend of the bottom airflow and the adiabatic descent of the upper level air flow.Asfor the function way of the over mountain airflow, the authoritative scholar ingeography and meteorology have put forward several theories, such as "blocking",large amplitude lee waves and water-jumping theory.The foehn in Great Xing'anMountains region is primary related to the downhill flow, and the large amplitude leewaves can promote the development of descend flow.The spatial variation rules of the foehn effect are : in horizontal direction, thetemperature increased remarkably in the middle and the northern Great Xing'anMountains, especially in 47°N which is a belt area of Suolun—Lindian;and in thevertical direction , isotherm reaches the tiptop at the foot of the eastern Great Xing'anMountains where the Mean altitude is about 400m—500m. It shows that the foehnwarming effect is prominent in this altitude.The standards of foehn are defined as: an obvious over mountain flow, the flowin high level must have a vertical ponderance, leeward side have the ground windfrom mountainous direction, temperature increased when the humidity descendsharply at the same time. Then it is defined as a foehn process.According to thestandards, foehn days in Great Xing'an Mountains region are about 76.64 days in ayear with a slight annual fluctuation. Foehn occurred mostly in spring and then inwinter and autumn, summer is the least one. Foehn occurred most frequently inJanuary, May and October, and it is fewer in February, July and November. In a day, itmostly appeared at night and it has a heavier intensity than the daytime.