Why do typhoons bring heavy rain and cool down?

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The cause of typhoon is uncertain, but it is known that it developed from the disturbance of tropical atmosphere. In the tropical ocean, the sea surface is exposed to direct sunlight, which makes the temperature of seawater rise, and the seawater is easy to evaporate into water vapor and spread in the air. Therefore, the air in the tropical ocean has high temperature and high humidity, and this air expands due to high temperature, resulting in the decrease of density and weight. The wind near the equator is weak, easy to rise, and convection occurs. At the same time, the surrounding cold air flows in to replenish and then rises again. This cycle will eventually make the temperature of the whole air column higher and the mass lower. Air flows from high pressure to low pressure, just as water flows from high pressure to low pressure, air with higher ambient pressure will flow to low pressure, thus forming "wind". In summer, due to the direct sunlight moving from the equator to the north, the southeast trade winds in the southern hemisphere cross the equator and become southwest monsoon to invade the northern hemisphere. When they meet the northeast trade wind in the northern hemisphere, the air is forced to rise, thus increasing the convection. Due to the different directions of southwest monsoon and northeast trade wind, when they meet, they often cause fluctuations and vortices. The convergence caused by the southwest monsoon and the northeast trade wind and the continuation of the original convection make the vortex with low pressure continue to deepen, that is, the air around it accelerates to flow to the center of the vortex, and the faster the flow, the greater the wind speed; When the maximum wind speed near the ground reaches or exceeds17.2m per second, we call it a typhoon.

Judging from the typhoon structure, such a behemoth must have unique conditions.

First of all, there must be a vast atmosphere with high temperature and high humidity. The temperature and humidity of the bottom atmosphere on the tropical ocean surface are mainly determined by the sea surface temperature, and typhoons can only form on the warm ocean surface where the sea surface temperature is higher than 26℃-27℃ and the sea surface temperature is higher than 26℃-27℃ within 60 meters.

Second, there should be an initial disturbance in which the lower atmosphere converges to the center and the upper atmosphere spreads outward. Moreover, the divergence of the upper layer must exceed the convergence of the lower layer in order to maintain enough updraft and continuously strengthen the disturbance of the lower layer;

3. The difference of vertical wind speed should not be too big, and the relative motion of the upper and lower air is very small, so that the latent heat released by water vapor condensation in the initial disturbance can be stored in the air column in the typhoon eye area, forming and strengthening the typhoon warm center structure;

Fourth, there must be enough geostrophic deflection, and the earth's rotation is conducive to the formation of cyclonic vortices. The geostrophic deflection force is close to zero near the equator and increases at the north and south poles. Typhoons basically occur on the ocean surface about 5 latitudes from the equator. Structure and energy of typhoon

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At the lower level, typhoons mainly flow into low-pressure inflow. Because of the angular momentum balance, the inner region can produce strong wind speed, and the outflow airflow of anticyclone is in the upper layer. Strong upward movement connects the upper and lower circulation, which is the main feature of typhoon circulation. The hottest temperature in a typhoon is caused by the sinking movement, which occurs at the inner edge of the eye wall, where there is the strongest sinking movement. Convergence is strongest at the maximum wind speed radius in the lower layer of typhoon, and the maximum wind speed radius does not change much with height, which is located in the eye wall. In addition, the asymmetry of typhoon structure is also a feature that people pay attention to this year. The analysis shows that there is obvious asymmetry in the inner and outer areas of the typhoon, which plays an important role in the development of the typhoon and the transportation of momentum and kinetic energy. Weather-scale typhoon is a powerful dynamic energy source in the atmosphere, so it should have an important impact on the change and maintenance of atmospheric circulation in terms of energy, which has attracted people's attention. On the issue of energy, it was also pointed out this year that the horizontal vortex transport of angular momentum is very important outside typhoons; In addition, the generation and transport of momentum in the outer region is also very important, which should not be ignored in the typhoon energy budget. These are all related to the asymmetry of the typhoon.