stay Fluid motion In, an attribute quantity flowing through a unit area in a unit time is a physical quantity that represents the transmission intensity of an attribute quantity. according to Newton's second law When there is no external force acting on the object, the speed of the object will not change, and the momentum of the object will not change, which is momentum conservation. The force acting on the fluid is mainly inertial force, including external force, pressure and adhesion force. The physical relationship equation between fluid momentum and external force, pressure and adhesion force is called equation of motion, which is the mathematical basis of fluid momentum change. stay atmospheric boundary layer Under the action of adhesive force, the air movement will form a velocity gradient between the surface and the atmosphere, resulting in shear stress. The process of momentum transmission by this shear stress is called momentum transmission. The momentum transmitted through the unit section in unit time is called momentum flux. （2） Sensible and latent heat flux
Sensible heat refers to the energy transported through heat conduction and convection (turbulence) without the phase change of organisms and media. When two objects with different temperatures come into contact, heat will be transmitted from the side with higher temperature to the side with lower temperature. The heat flow transmitted is called Sensible heat flux 。 Sensible heat flux is proportional to the temperature difference, and this proportional coefficient is called sensible heat transfer coefficient or sensible heat exchange coefficient. The heat energy absorbed or released by the phase change of substances is called latent heat. Water vapor transmission represents potential transmission. The latent heat flow through an area per unit time is called Latent heat flux 。 The latent heat flux is proportional to the concentration difference of water vapor on both sides of the section, and this proportional coefficient is called the latent heat transfer coefficient. （3） Material flux
Diffusion is one of the main mechanisms for the transport of physical quantities such as matter, heat and momentum. The diffusion of static fluid is mainly molecular diffusion driven by concentration gradient, while in the fluid with intense motion, it is mainly turbulent diffusion. The diffusion phenomenon in the atmosphere is dominated by turbulent diffusion in most cases. In the process of diffusion, the amount of physical quantity passing through the plane perpendicular to the diffusion direction is called material flux.
（4） H two O flux
H two O flux is an important characteristic parameter of ecosystem water cycle process. The water vapor transport of the land/atmosphere system is a link of the water cycle, a carrier of the latent heat transport, and an important influence factor of the energy balance. Evaporation is the main form of water vapor transport in vegetation/atmosphere system. In agrometeorology, it usually refers to evaporation from the ground surface. In order to distinguish from soil evaporation, evaporation from plants (mainly leaves) is called transpiration. The evaporation of vegetation underlying surface can be divided into three parts: evaporation of ground or water surface, evaporation of rainfall intercepted by vegetation canopy, and plant transpiration. The sum of the three is called evapotranspiration or evapotranspiration. The evaporation intensity or speed is actually the water vapor flux. The integral value of time is called evaporation capacity.
By obtaining sensible heat, latent heat, momentum flux and CO between the earth and atmosphere of typical ecosystems two The long-term observed data of fluxes provide basic data for meteorological departments to carry out research on climate system models; To provide basic data for comprehensively and systematically carrying out research on the characteristics of ecological processes and microclimate, earth atmosphere interaction, water and heat balance, atmosphere ecology microclimate hydrology soil interaction and impact mechanism in typical ecosystems; Provide basic data for short-term climate trend prediction, climate change impact assessment and other work.
