The lowest layer of the atmosphere, located near the Earth's surface and affected by ground friction and resistance.When the atmosphere flows over the ground, various rough objects on the ground, such as grass, sand, crops, trees, houses, etc., will hinder the flow of the atmosphere. This frictional resistance is transmitted upwards due to turbulence in the atmosphere and gradually weakens with increasing height, and can be ignored when reaching a certain height. This height is called the thickness of the Planetary boundary layer, and it varies with meteorological conditions, terrain, and ground roughness, about 300~1000 meters.

　　In the Planetary boundary layer, wind speed, temperature and humidity have obvious diurnal changes, which weaken with height. From the perspective of Fluid mechanics, the Planetary boundary layer has the following characteristics:

　　① The wind speed gradually increases with height:The wind speed is equal to zero at the ground surface, but equal to the Geostrophic wind speed at the outer edge of the Planetary boundary layer.

　　② Turbulent structure:In the Planetary boundary layer, the atmospheric flow has great randomness, which is basically turbulent flow, and the turbulence intensity of the air flow can reach 20%.

　　③ Wind direction deviation:In the northern hemisphere, due to the Coriolis force generated by the Earth's rotation, the wind direction gradually deflects to the right with the increase of height in the direction of the wind near the ground, while at the outer edge of the Planetary boundary layer, it is consistent with the direction of the Geostrophic wind, and the angle of wind direction deflection varies from time to place, generally up to dozens of degrees.

　　④ Temperature stratification (i.e. temperature changes with height):The variation of atmospheric temperature with altitude is complex and variable, and its rate of change directly affects the stability of the atmosphere.

　　⑤ High aerosol concentration:The ground is the main source of aerosols, and this layer is adjacent to the ground. The concentration of aerosols (such as dust, smoke, fog, etc.) is high in this layer.

Ecological environment of Planetary boundary layer top of Chinese Academy of Sciences

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