Global generation mode and movement mechanism of wind
Global generation mode of Wind
All renewable energy sources on Earth, including wind resources, come from the sun. Global winds are caused by pressure differences across the Earth’s surface caused by uneven heating of the Earth by solar radiation. For example, the Earth’s surface absorbs far more solar radiation in the equatorial regions than in the polar regions. This change in input energy creates a convection zone in the near-surface (troposphere) layer of the atmosphere. Described using a simple flow model, air rises at the equator and falls at the poles. Atmospheric circulation due to uneven heating is strongly influenced by the effect of Earth’s rotation (about 1670 km/h at the equator, tapering to zero at the poles). In addition, seasonal changes in solar energy distribution amplify the cyclic variability.
Spatial changes in the heat transfer to the Earth’s atmosphere produce changes in the atmospheric pressure field, which cause air to flow from areas of high pressure to areas of low pressure. The pressure gradient force in the vertical direction is usually counteracted by the downward gravitational force. Therefore, the wind blows mainly on the horizontal plane and is related to the horizontal pressure gradient. At the same time, there are various forces that try to mix the air of different temperatures and pressures distributed over the Earth’s surface. In addition to pressure gradients and gravity, the inertia of the air, the rotation of the Earth, and friction with the Earth’s surface (causing turbulence) all affect atmospheric winds. The effect of each force on the atmospheric wind system varies depending on the scale of motion considered. As shown in Figure 1, the worldwide wind cycle contains multiple large-scale wind patterns that cover the entire Earth and have an impact on the near-Earth prevailing winds. It should be noted that this model is too simplistic as it does not reflect the influence of the earth’s mass on the wind distribution.
The mechanism of wind movement
The simplest model to describe the motion of wind in the atmosphere considers four atmospheric forces, including pressure, the Coriolis force due to the Earth’s rotation, the inertial force due to large-scale circular motion, and the frictional force on the Earth’s surface.
The pressure Fp acting on a unit mass of air is given by
where ρ is the air density and n is the normal direction of the isobar. Therefore ∂p/∂n is defined as the pressure gradient in the normal direction of the isobar. The Coriolis force (unit mass) Fc is a virtual force measured relative to a rotating reference frame (the Earth) and can be expressed as:
Fc=fU (1.2)
where U is the wind speed, f is the Korotkoff parameter (f=2wsin(Φ)), Φ represents the latitude, and ω is the rotational angular velocity of the earth. Therefore, the magnitude of the Coriolis force depends on wind speed and latitude. The direction of the Coriolis force is perpendicular to the direction of air movement. The winds created by these two forces are called geostrophic winds and tend to be parallel to the isobars (see Figure 2).
The magnitude of the geostrophic wind, Ug, is a function of the resultant force and is given by:
Geostrophic winds are an idealized situation because the presence of high-pressure and low-pressure regions causes arcuate bending of isobars. This exerts another force on the wind, centrifugal force. The wind formed after considering centrifugal force is called gradient wind Ugr, as shown in Figure 3.
The gradient wind is also parallel to the isobar, which is the result of the resultant force;
In the formula, R is the radius of curvature of the air micro-element path, and Ug is obtained by replacing the formula (1.3):
The last force acting on the wind is the frictional force of the ground. The ground exerts a horizontal force on the moving air, slowing the flow as a result. This force decreases with height above the ground and is negligible above the boundary layer (defined as the atmospheric near-earth region, where viscous forces are important). Above the boundary layer, the wind does not experience friction, and the wind flows along the isobar at the speed of the gradient wind. Ground friction causes winds to flow more towards low pressure areas.