Convection caused by unstable atmospheric stratification. During the process of convection movement, heat, momentum and mass (including water vapor) are exchanged with the surrounding atmosphere, thus changing the state of the ambient atmosphere.In the atmosphere, the air flow in the high pressure zone and the air flow in the low pressure zone circulates with each other, which promotes the alternation of dry and wet seasons in the earth environment.
Extracumulus downdraft
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The convective circulation of cumulus is composed of the updraft inside the cloud and the downdraft outside the cloud. The updraft obtains energy from unstable stratification, which is not used to strengthen the upward movement, but also used to maintain the downdraft.The supplementary downdraft outside clouds is a resistance of updraft.The appearance of extracloud subsidence of ordinary cumulus convection has adversely affected the development of convection[1]。
In the middle and low latitudes, there is an obvious dry downdraft area in the clear sky near the convective cloud and between the clouds. Generally speaking, in the middle and upper part of the cloud, the downdraft speed outside the cloud is about 20%~50% of the main updraft speed inside the cloud. In the place close to the updraft boundary, the downdraft is the strongest, and it gradually weakens away from the updraft.It is generally believed that the adiabatic compression of sinking air will lead to abnormal temperature increase in the clear sky area around convective clouds.
The circulation of the updraft in the convective cloud and the downdraft outside the convective cloud shows that there can be a larger range of clear sky downdraft around the convective cloud, and the low-level water vapor convergence caused by this process is also a mechanism to further maintain and strengthen the convective cloud group.
Downdraft in strong storms
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The mesoscale circulation of the strong storm weather system has an obvious feature: on the quasi two-dimensional section of the mesoscale circulation, there is a warm and wet airflow with the boundary layer beginning to tilt up to the top of the troposphere, and another downdraft with the central control of dry and cold air.This dry and cold downdraft mixes with the saturated air in the cloud to make the raindrops in the cloud evaporate and cool. The downdraft with evaporative cooling has the function of maintaining the strong storm system and may organize the strong convection. Therefore, it is often observed that new convective cells are formed in front of the downdraft.
The extracloud downdraft of ordinary cumulus convection and the downdraft in strong storms play different roles in the development of convection.
