Research contents of plateau meteorology

Basic weather and climate characteristics

The air over the Qinghai-Tibet Plateau is thin, with few impurities, and the density is only half of that over the plain, so the solar radiation is strong. The seasonal and daily changes of the ground are very significant; There are also many dynamic and thermal disturbances in the terrain. Therefore, compared with the same latitude area, the weather and climate of the Qinghai-Tibet Plateau has the following characteristics: ① In terms of surface meteorological elements, the Qinghai-Tibet Plateau has the lowest surface temperature, the lowest air pressure, the lowest humidity and the highest wind force; However, as far as the spatial areas with the same latitude and height are concerned, the Qinghai-Tibet Plateau has the highest temperature (summer), the lowest humidity (summer), the highest air pressure (summer) and the lowest wind force (winter). ② The Qinghai-Tibet Plateau is one of the regions with extremely unstable atmosphere at the same latitude in the world. Compared with other areas, convective clouds are developing all year round, with the most showers, thunderstorms and hail. ③ Mesoscale and mesoscale are the most in plateau area, and Qinghai-Tibet Plateau is the most obvious source of weather system. All the above characteristics are related to the dynamic and thermal effects of the Qinghai-Tibet Plateau.

Plateau dynamic function

Include mechanical action and friction action.

① Mechanical action. In winter, when the westerly airflow passes through the plateau, the windward surface below 6 km is forced to be obviously divided into two branches, north and south, and flows around the topographic contour. After reaching the leeward side of the plateau, the two westerly winds meet again, forming the circulation situation of the south trough of the northern ridge, which is extremely obvious in the middle and lower troposphere of the plateau. In summer, when the easterly airflow passes through the plateau, although there are branches around it, it is not as obvious as in winter. Due to the blocking effect of the Qinghai-Tibet Plateau, when the long-wave trough in the westerly belt moves to the west of the plateau, the middle part of the low trough is blocked and cut into two short-wave troughs in the north and south, bypassing the plateau and moving eastward along the two westerly winds in the north and south of the plateau, affecting the weather in the plateau and its eastern region.

The Qinghai-Tibet Plateau is also very important for the forced climb of atmospheric flow. In winter, the climbing airflow appears on the west and north slopes of the plateau, and the downhill airflow appears on the east and south slopes. Summer is just the opposite. Therefore, there is more precipitation on the western and northern slopes of the plateau in winter than on the eastern and southern slopes, and more precipitation on the eastern and southern slopes in summer. When the pneumatic system is forced to cross the plateau, it will be pressurized due to the shortening of the gas column, weakening or blocking the low-pressure system, making the high-pressure system stronger or developing; When the air pressure system moves out of the plateau, the air column will be decompressed due to elongation, the low pressure system will deepen or develop, and the high pressure system will weaken or die out. This is why the vortex system (or high pressure system) outside the plateau is unlikely to (can) move into the plateau, while the vortex system (or high pressure system) on the plateau can (can't) move out of the plateau and strengthen (weaken) or develop (die out).

The large-scale atmospheric circulation and peristalsis caused by the blocking of the Qinghai-Tibet Plateau and their changes have an extremely important influence on the formation and evolution of long-wave sum, especially the long-wave trough in the westerly belt along the coast of China in winter.

2 friction. The friction of the earth's surface makes the plateau form, and its influence on the side boundary of the plateau is more prominent. It reduces the velocity of the air near the side boundary, while the velocity of the free atmosphere far from the side boundary remains unchanged, thus forming the horizontal shear of the air near the side boundary and generating vorticity. In winter, sex vortices often appear at the westerly boundary of the northern plateau, and they often appear at the westerly boundary of the southern plateau. In summer, things are different. The northern part of the plateau is still the boundary of the west wind, and there are often mesoscale anticyclones. However, due to the eastern boundary of the southern part of the plateau, mesoscale anticyclones often appear.

Plateau thermal action

It can be divided into two types: cold source and heat source of plateau ground and plateau atmosphere. The plateau ground that supplies heat to the atmosphere is called heat source; On the contrary, the ground is called a cold source. Similarly, when the atmosphere above the plateau transfers heat to the surrounding atmosphere, it is called the plateau atmospheric heat source; On the contrary, the atmosphere is called a cold source.

From February to 165438+ 10, the plateau ground is the heat source; From February 65438 to February of the following year, Qiangtang Plateau, as the main part of the plateau, was the cold source, and the rest was still the heat source. The heat source is the strongest in June, and the cold source is the strongest in 65438+ 10. But on an annual average, the Qinghai-Tibet Plateau is a heat source.

From April to September, the plateau atmosphere is the heat source; 1 1 is the cold source until February of the following year; March and 10 are transitional months. The heat source is the strongest in July, and the cold source is the strongest in June, 5438+ 10. In addition to the rainy season, the cold source and heat source centers of the plateau atmosphere are all in the middle of the plateau; In the rainy season, due to the precipitation in Assam, India, a lot of latent heat is released, which makes the center of strong heat source move over Assam.

From March to April, the intensity of heat source in Qinghai-Tibet Plateau increased sharply. Since April, a thermal depression has appeared in the eastern part of the plateau, which extends westward and northward. In July, the whole plateau was controlled by heat and low pressure. At this time, there are two obvious strong and low pressure centers, which are located in the west and east of the plateau. Since August, the intensity of atmospheric heat source has weakened rapidly. In September, a cold source was established in the northwest of the plateau. The plateau cold high first appeared in the northwest of the plateau, and then rapidly expanded to the southeast, reaching the strongest in June+10/October, 5438, when the main part of the plateau was occupied by the cold high.

Plateau atmospheric boundary layer

The atmospheric boundary layer on the plateau refers to the layer (including the side boundary layer) which is1~ 2km away from the plateau ground. In this layer, the influence of ground friction and turbulent viscosity is very prominent, which makes the ground wind speed change exponentially with height. At this level, there are power and thermal high and low pressure systems. The atmospheric boundary layer on the Qinghai-Tibet Plateau is the highest in the world. Its horizontal amplitude is small in winter and large in summer, with a large bottom and a small top; The thickness of the boundary layer is thin in winter and thick in summer, which is about 1 km from the ground in winter and about 2 km from the ground in summer. This boundary layer doubles the effective area of the plateau affecting the atmosphere and increases the effective height by 1 ~ 2km. In other words, the effective height of the Qinghai-Tibet Plateau's impact on the atmosphere is not 4-5 kilometers, but 6-7 kilometers, and the effective horizontal range of the plateau is not 2.5 million square kilometers, but more than 5 million square kilometers.

plateau monsoon

Due to the cold and heat sources of the plateau surface and atmosphere, the wind direction prevailing in the Qinghai-Tibet Plateau and its adjacent areas in winter and summer is almost opposite; With the change of wind direction, the weather system and climate characteristics also have obvious seasonal changes, which is called Qinghai-Tibet Plateau. The monsoon has not only interannual and diurnal variations, but also quasi-periodic variations of 14 days. In winter, the Qinghai-Tibet Plateau is a cold source, with cold high pressure in the boundary layer, westerly winds prevailing in the northern part of the plateau and easterly winds prevailing in the southern part. The whole plateau is often dry and cold with little precipitation. In summer, on the contrary, the Qinghai-Tibet Plateau is a heat source, and there are many thermal depressions in the boundary layer. The east wind prevails in the north of the plateau and the west wind prevails in the south. The whole plateau is humid, "warm" and rainy. The existence of winter and summer monsoon in Qinghai-Tibet Plateau makes Hadley circulation in summer change into strong monsoon meridional circulation in the longitude zone where the plateau is located. Moreover, because the meridional circulation of the plateau monsoon in winter is consistent with Hadley circulation, the Hadley circulation in winter in this area is particularly strong.

Plateau weather system

Under the unique topography of the Qinghai-Tibet Plateau and its dynamic and thermal effects, a unique weather system in the plateau area has been formed. For example, the South Asian high in the upper troposphere and the low vortex sum on the isobaric surface of 500 hectopascals. Although the low latitude tropical depression system can affect the plateau atmosphere, this situation is rare.

① South Asia High. It is the strongest, most stable and largest high pressure in the upper troposphere in summer, which was discovered in the late 1950s when surveying and mapping1000 hectopascals. The east-west major axis of South Asia High can reach 180 longitude (tens of thousands of kilometers), and the north-south minor axis can reach 40 latitude. The high pressure is most obvious on the isobaric surface of 150 hPa, and the strongest center of the high pressure is over the Qinghai-Tibet Plateau, which belongs to the warm anticyclone circulation system. Also known as summer tropospheric anticyclone, summer Asian monsoon high, summer Asian-African monsoon high, summer 100 hPa Qinghai-Tibet high, etc. At an altitude of1000 hectopascals, the location of the center of South Asia High has obvious seasonal variation: it is located near the Philippine Islands in winter, spreading westward from April to the South China Sea, moving to Indo-China Peninsula in May, moving to the Qinghai-Tibet Plateau in June, steadily strengthening in the plateau and its adjacent areas in July and August, leaving the plateau in September and gradually moving back to the southeast winter location. The distribution of land and sea is the basic background of the formation of this high pressure, but it is also related to the heating of the Qinghai-Tibet Plateau (including Bangladesh and Assam, India) and the northward movement of the whole belt, which moves with the position change of the tropospheric high temperature area. After entering the spring, the mainland heats up faster than the ocean, the upper atmosphere heats up obviously, and the high pressure moves to the mainland. Because the heating of the Qinghai-Tibet Plateau is the most obvious, the strongest and warmest high pressure center is often formed over the plateau. After autumn, the mainland heats up quickly and the ocean heats up slowly, forming a relatively warm area over the ocean, and the high pressure moves over the ocean.

In summer, the center of South Asia High often moves to the latitudinal direction. It belongs to the eastern high type when it is east of east longitude 100. When it is west of east longitude 100, it is a western high pressure type. Whenever the eastern high-pressure type appears, the altitude in the eastern part of China is above 500 hectopascals, and downward flow prevails, with little rain and drought in the middle and lower reaches of the Yangtze River, Yunnan and Guizhou. There are many rains and floods in North China, Northwest China and West Sichuan. When the western high pressure type appears, on the contrary, the middle and lower reaches of the Yangtze River and Yunnan-Guizhou area are rainy and waterlogged; There is little rain and drought in North China, Northwest China and West Sichuan. When the troposphere over the Qinghai-Tibet Plateau is the South Asia High, if it is also a high pressure area of 500 hectopascals, this area will be less rainy and dry. If the height of 500 hectopascals is a low pressure area, it will be rainy in this area. The transition period between East-West South Asia High is about 13 ~ 15 days.

② Plateau vortex. Vortex or cyclone circulation with closed profile at 500 HPA. Most of these eddies were formed on the Qinghai-Tibet Plateau, and only a few of them moved from outside the plateau. Most of them are plateau atmospheric boundary layer phenomena, the height rarely exceeds 400 hectopascals, and the horizontal range is about 500 kilometers. The main source of vortex is Qiangtang area near Shenzha and the west of Bango Lake. Other places, such as Naqu, Qaidam, Songpan and Jiulong, often produce eddies, but the frequency is far less than that in Qiangtang area. Vortex always forms in front of the trough of low pressure in the westerlies. According to the temperature field structure of the vortex center, the plateau vortex can be divided into three types: (1) temperature ridge warm vortex; (2) Baroclinic vortex in the temperature trough or low pressure trough front of westerly belt; (3) Cold vortex whose center coincides with the low temperature center. According to the statistical results of weather chart data for many years, the vortex frequency is the highest in May, with an average of 30.7, 25.9 in June, 28. 1 in July, 23.2 in August and 10.3 in September. From 10 to March of the following year, vortex rarely appeared on the plateau.

In summer, when the western Pacific subtropical high and the Iranian high on the 500 hPa isobaric surface are far away from the Qinghai-Tibet Plateau, and the South Asian high on the 100 hPa isobaric surface is of a western type, and its ridge line is located north of 30 north latitude, the vortex in the plateau area is very active and there is more precipitation. On the contrary, when the two high pressures are close to the plateau and the South Asian high is of the eastern type, there is less vortex activity and less precipitation in the plateau area. In the initial stage of vortex formation, it is mostly hot and low pressure. At this time, the corresponding clouds are mostly composed of cumulonimbus clouds or low-middle clouds, with sparse structure and small precipitation, and the precipitation center coincides with the warm vortex center. When there is no cold air invasion, the warm vortex moves eastward along the shear line and reaches the vicinity of Naqu. The cloud structure becomes compact, the color is bright and white, and the precipitation increases, but it mostly disappears in the eastern plateau and rarely moves out of the plateau. When cold air invades, warm vortex turns into baroclinic vortex, and clouds and rain develop. The rain area is mostly in the southeast of the vortex center, which can move eastward to the eastern part of China, often causing heavy rains and floods.

③ Plateau shear line. In summer, the discontinuous line with small temperature gradient and opposite wind direction on the 500 HPA isobaric surface of Qinghai-Tibet Plateau is called shear line. It can be divided into two types: quasi-east-west transverse shear line and quasi-north-south vertical shear line. According to the different structure of shear line, it can be divided into: (1) short warm shear line generated between two small high pressures. Short duration, small thickness and light weather change. (2) A vertical shear line is formed between the two subtropical highs. Its northern end is cold front structure, and its southern end is warm structure, with a large thickness, which often brings strong cooling and heavy precipitation to plateau areas. (3) The shear line formed between the easterly airflow on the south side of the small high pressure in the northern plateau and the southwest airflow in the southern plateau. There is obvious cold front structure in the east and warm in the west, and the height can only reach 400 hectopascals, which often causes a lot of precipitation in the central and eastern parts of the plateau. These shear lines on the plateau can often cross the whole plateau. Shear line is one of the most common weather systems in the Qinghai-Tibet Plateau from May to September in summer, and it is also an atmospheric boundary layer phenomenon on the plateau. According to the statistics of 500 hectopascal weather map data, the monthly average frequency is: May 37. 1, June 4 1.6, July 42.4, August 39.9 and September 25.8. Among them, it is the most in June and July, and rarely appears in winter. Except in September, the existence of shear line can be observed at least once on the weather map at 08: 00 or 20: 00 every day from May to August.

The formation of plateau shear line and its weather conditions are similar to those of vortex.

Since the 1960s and 1970s, a series of comprehensive scientific investigations have been carried out, and precious data such as temperature, pressure, humidity, wind, clouds, precipitation and radiation have been obtained, thus some new discoveries have been made on the role of the Qinghai-Tibet Plateau in atmospheric movement and the weather and climate characteristics of Mount Everest. With the implementation of the large-scale plateau atmospheric experiment plan, more new clues and achievements will be obtained in these studies, especially in the study of weather and climate anomalies and climate formation in East Asia or the northern hemisphere.