Other atmospheric circulation models and the spatial characteristics of wind
Other atmospheric circulation models
The aforementioned general atmospheric circulation model is a good model for smooth spherical surfaces. But in reality, the earth’s surface varies greatly, with large areas of ocean and land. These different surfaces affect air flow due to differences in pressure fields, solar radiation absorption and moisture content.
The ocean is like a great source of energy. Therefore, the movement of air is often influenced by ocean circulation. All of these effects lead to different surface pressures, which affect global winds and many long-term regional winds, such as the appearance of monsoons. In addition, localized heating or cooling may also generate persistent endemic winds, which can be seasonal or intraday, including sea and mountain breezes.
Smaller-scale atmospheric circulation can be divided into secondary and tertiary circulations (see Rohatgi and Nelson, 1994). Secondary circulation occurs when a central region of high or low pressure is formed by the heating or cooling of the lower atmosphere. The secondary circulation includes:
· Hurricane;
· Monsoon;
· Extratropical cyclone.
Tertiary circulations are small-scale, localized circulations characterized by endemic winds. They include:
· breezes from land and sea;
· Valley wind and mountain wind;
· Cyclone-like flows (such as airflow through California Pass);
· Foehn (dry, high temperature wind on the downwind side of a mountain range);
· Thunderstorms;
· Tornado.
Examples of tertiary circulations, valley winds and mountain winds, are shown in Figure 1. During the day, the warmer air on the hillside rises, displacing the heavier, cooler upper air. At night, the flow direction is reversed, and the cold air flows down the slope and settles at the bottom of the valley.
An understanding of these wind patterns and other local effects is important for evaluating potential wind energy sites.
Changes due to location and wind direction
Changes due to location
Wind speeds are largely dependent on changes in local topography and ground cover. As shown in Figure 2 (Hiester and Pennell, 1981), the differences in wind speeds at two sites adjacent to each other can be large. The graph shows monthly and five-year average wind speeds for two sites 21 kilometers (13 miles) apart. The five-year average wind speed differs by approximately 12% (annual averages of 4.75m/s and 4.25m/s).
Wind direction change
Wind direction also varies on the time scale of wind speed variation. Seasonal changes can be smaller, on the order of 30 degrees, while average monthly wind direction changes can reach 180 degrees throughout the year. The short-term directional changes are caused by the turbulent nature of the wind. Short-term changes in wind direction need to be considered in wind turbine design and siting. Horizontal axis wind turbines must rotate (yaw) with the direction of the wind. The yaw causes the entire wind turbine structure to generate rotational loads, testing all the mechanisms involved in the yaw motion. Lateral winds resulting from changes in wind direction will affect blade loads, which will be described later. Short-term changes in wind direction and associated motion can affect the fatigue life of components such as blades and yaw drive mechanisms.