Classification of rapids

Also known as subtropical westerly jet. It appears in the transition zone between tropical tropopause and mid-latitude tropopause, about 200 hectopascals above the subtropical front. The horizontal shear of wind speed in the upper troposphere is the strongest, and it is weak in the middle and lower troposphere. On the isobaric surface of 500 hectopascals, the jet almost disappears and is difficult to identify. The wind speed on the jet axis changes greatly, and the wind speed is the highest over southern Japan. As can be seen from the daily weather chart, the position and wind direction of subtropical jet are relatively stable, which is different from polar front jet. The subtropical jet has obvious seasonal differences, especially in the northern hemisphere, which is strong and stable in winter and weak in summer, and even loses its characteristics of surrounding the hemisphere; The location is south in winter and north in summer, and the transition season changes greatly from north to south, with a certain mutation.

In winter in the northern hemisphere, the subtropical jet often surrounds the hemisphere with three waves: the ridge is in southern Japan, eastern North America and eastern Mediterranean, and the valley is in the southern foot of the Qinghai-Tibet Plateau, the northeast Pacific Ocean and the northeast Atlantic Ocean. This three-wave structure often forms a branch and confluence with the polar front jet (Figure 2), and the confluence is mostly at the ridge of the three-wave structure of the subtropical jet. The phenomenon of divergence and convergence is most obvious in East Asia. Branches over the Qinghai-Tibet Plateau converge in southern Japan and branch in the Pacific Ocean. The strongest jet over southern Japan is related to the confluence of these two jets there. Short for extreme night jet. In the stratosphere of the winter hemisphere, because the polar regions are in darkness for a long time, the atmosphere is cooled by radiation, but the ozone in the stratosphere in the middle and low latitudes directly absorbs the ultraviolet radiation of the sun and warms up, forming a strong temperature gradient and producing a direct meridional circulation. Under the action of Coriolis force, the air flowing to the polar region gradually turns to the right, forming a westerly jet over the polar region 50 ~ 60km, which can extend down to the height of 20 ~ 30km (Figure 1), so this jet can be seen on the isobaric surface map of 50HPA.

The extreme night jet surrounds the polar regions, with obvious seasonal variation: it is gradually established after autumn, maintained in winter, and does not collapse until the last explosive warming of the stratosphere in spring (see stratosphere and mesosphere circulation). This is an important feature of stratospheric circulation in winter. In some years, the stratosphere will also erupt in the middle of winter, which will lead to the collapse of the extreme night jet, but it can still be restored because it is still in winter. During the collapse of the extreme night jet, the sinking movement prevailed in the mid-latitude, and the upper ozone was transported downward. The relationship between the fluctuation of extreme night jet and factors such as tropospheric circulation and weather is very complicated and needs further study. E.R. Reiter, Meteorology of Strasbourg by springer Publishing House, 196 1.