Satellite-Based Measurement Systems
Wind speed and direction can be measured from satellites over water surfaces by satellites using LiDARs, microwave scatterometers, and synthetic aperture radars. Satellite lidar systems are similar to the systems that are used for wind speed and direction measurements on the ground or from turbines. They typically leverage the movement of the satellite to obtain multiple viewing angles for use in wind field reconstruction, rather than having any ability to adjust their beam’s trajectory. An ultraviolet Doppler wind lidar system was launched by the European Space Agency as the Aladin instrument on the Aeolus satellite in late 2018. This instrument has the capability to measure wind speeds in the atmosphere at altitudes up to 30 km with an approximate 500-m vertical resolution and 3-km horizontal resolution, with errors less than 1 m s-1 in the lower 2 km of the atmosphere. Some horizontal and vertical aggregation should be expected.
Microwave scatterometers measure the amount of microwave energy reflected by the ocean surface and relate that to surface wind speed using empirical relationships. Wind direction is derived from the wind speed vectors using a fitting method. Scatterometers deliver wind speeds at the surface over pixels that are several tens of kilometers on a side, therefore they cannot be used near the shore. Wind speeds are measured at or near the surface. As a result, wind speeds can only be estimated at wind turbine hub heights by extrapolation.
The European Space Agency, National Aeronautics and Space Administration (NASA), and the National Space Development Agency of Japan have all flown microwave scatterometers on several satellites. NASA operated a scanning scatterometer called SeaWinds on the QuikSCAT satellite from 1999 to 2009, and then operated an almost-identical scatterometer on the international space station from 2014 on. Data from the QuikSCAT satellite have been used to estimate wind resources in offshore regions (Karagali et al. 2014). Biases compared to surface stations are around zero but the standard deviation is around 1 m s-1 or more. Together with coarse temporal and spatial resolution, this large standard deviation renders data from the current generation of scatterometers unsuitable for use for validating wind estimates from models in coastal regions. Despite their limitations, the data provided by scatterometers can be used as boundary conditions to global and mesoscale weather models.
Synthetic aperture radars use two radar beams with identical or different polarization to detect the wind speed near the surface. Wind directions are retrieved by analyzing features, such as streaks, and are not directly observed. The spatial resolution is around 1 km or less, and although it is a better resolution than what can be obtained using microwave scatterometers, it is still relatively coarse. A summary of the use of synthetic aperture radars for wind measurements can be found in Dagestad et al. (2013) and a recent application at the Anholt wind farm is discussed by Ahsbahs et al. (2018).
Satellite measurements are also employed for measuring solar radiation. The amount of energy received from the sun at the ground cannot be measured by satellites directly. Instead, a satellite can observe the distribution, structure, and opacity of clouds, which are the primary way in which the top-of-atmosphere irradiance from the sun is modified. Combining simple models of the top-of-atmosphere irradiance with cloud masks, aerosol data, and radiative transfer models such as FARMS (Xie et al. 2016) allows the global horizontal irradiance and direct normal irradiance to be estimated to within a few percent of the values obtained by high-quality surface stations. Such single-column methods have been commercialized and are the basis of some solar energy resource forecasting products.