Surface wind remote sensing

Surface wind measurement by remote sensors is accomplished using satellite microwave scatterometers and land-based lidar, radar, and sodar. Satellite scatterometers provide wind speed and direction over the oceans, under clear or cloudy skies, and both day and night. Scatterometers send microwave pulses to the Earth's surface and measure the backscattered power related to surface roughness. Roughness over land is due to terrain and vegetation differences, but over the oceans the backscatter is due largely to small waves assumed to be in equilibrium with the local wind stress. Backscatter over water increases with wind speed, and the magnitude is influenced further by the wind direction relative to the direction of the radar beam. This gives the scatterometer the capability of measuring both wind speed and direction over the ocean (Liu, 2003).

Scatterometers have been launched on ESA and NASA polar-orbiting satellites since 1978. These instruments provide data on local and regional winds and moisture advection, and they provide a nearly synoptic-scale view of global surface winds.

Lidar, radar, and sodar are employed to remotely measure wind speed for research and experimental purposes more commonly than for routine wind observations. Each of these active remote sensors is based on measuring the Doppler shift of the light, radio waves, or sound waves emitted by the instrument.

Lidar transmits radiation in the UV, visible, and IR wavelengths, and modern lidar systems use a pulsed laser to generate the radiation (Angevine et al., 2003). The transmitted energy interacts with air molecules which have thermal motion or motion due to wind. The energy is changed by its encounter with air molecules, and some energy is reflected or scattered back to the instrument. A Doppler lidar measures the shift in wavelength frequency of the backscattered energy and this information is converted into remotely measured wind velocity (Argall and Sica, 2003).

Radar measurement of wind speed usually requires the presence of an atmospheric target to produce an echo, but modern systems have sufficient sensitivity to sense clear-air returns (Angevine et al., 2003). Doppler radars provide radial velocity or wind speed toward or away from the radar when monitoring precipitation events. Doppler radar radial wind data are used in numerous analytical formulations to produce three-dimensional wind fields at various scales and serving a variety of applications (e.g. Caillault and Lemaitre, 1999; Nissen et al., 2001; Liou, 2002). Specialized wind profiling radars are employed in research and experimental work studying winds within the atmospheric boundary layer (Angevine et al., 2003).

Sodar operates on the principle of acoustic backscattering. It transmits a short pulse of sound which is refracted by the small-scale atmospheric turbulence structure of temperature and velocity in the boundary layer (Angevine et al., 2003). The radial velocity of air is determined by measuring the intensity and the Doppler shift of the sound refracted from the turbulence. Sodar has a maximum range of several hundred meters. It primarily provides measurement of mean wind speed and direction because it samples atmospheric volume, and it samples at multiple points in space and time (Crescenti, 1997). However, expanded applications of sodar are being tested. Contini et al. (2004) propose a method for using a sodar system to measure mean vertical wind velocities averaged over three hours. Sodar systems are employed with other meteorological instruments studying local winds at an expanding number of monitoring sites around the world (e.g. Peters et al., 1998; Sturman et al., 2003; Perez et al., 2004).

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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