Wind Power at Sea as Observed from Space

With the increasing demand of electric power and the need of reducing greenhouse gas emission, the importance of turning wind energy at sea into electric power has never been more evident. For example, China is vigorously studying and pursuing the potential of wind energy to lessen dependence of coal consumption (McElroy et al., 2009). The White Paper on Energy (DTI, 2007) lays out an ambitious plan to the British Parliament in meeting the Renewables Obligation with offshore wind energy. The paper posted a challenge not only to Denmark, the leader of European offshore wind energy, but also to the world. New technology has also enabled floating wind-farms in the open seas to capture the higher wind energy and reduce the environmental impact on the coastal regions. Detailed distribution of wind power density (E), as defined in Section 4, at sea is needed to optimize the deployment of such wind farms. The distribution is discussed in Section 5. Just a few decades ago, almost all ocean wind measurements came from merchant ships. However, the quality and geographical distribution of these wind reports were uneven. Today, operational numerical weather prediction (NWP) also gives us wind information (Capps & Zender, 2008), but NWP depends on numerical models, which are limited by our knowledge of the physical processes and the availability of data. Recently, spacebased microwave sensors are giving us wind information with sufficient temporal and spatial sampling, night and day, under clear and cloudy conditions. Results from the most advanced passive sensor, which measures only wind speed, and active sensor, which measures both speed and direction, will be discussed. The principles of wind retrievals by active and passive microwave sensors are described in Section 2 and 3 respectively. The dependence of wind speed on height above sea level and on atmospheric stability is discussed in Section 6 and 7.