Global distributions and temporal variations of low-level wind speed maxima for Airborne Wind Energy applications

Although the greatest wind power densities on Earth are found near the jet streams, reaching such high altitudes to extract wind power and generate electricity is a challenging task for Airborne Wind Energy (AWE) systems. Near the Earth's surface, the greatest increase in wind speed with height occurs between ~50 m and ~500 m of altitude. The altitude range between 500 and 2000 m has been so far relatively un-interesting for AWE applications because it is characterized, on average, by uniform wind speed profiles. However, after analyzing 21 years of global, high-resolution, hourly wind data from the National Center for Atmospheric Research's (NCAR) CFDDA (Climate Four Dimensional Data Assimilation) re-analyses, we found that several locations on Earth exhibit low-level wind speed maxima located between 500 and 2000 m. These wind speed maxima coincide in some cases with the well-known nocturnal low-level jets, such as those in the US Great Plains or in Ethiopia. Low-level jets are distinctly different from the jet streams because they are formed by the decoupling of the atmospheric boundary layer after sunset, or spatial contrasts in insolation, or from horizontal variations in the physiography. Other semi-persistent low-level wind speed maxima are found between 600 and 1600 m of altitude near the Tropics, where the descending branches of the Hadley Cells cause a subsidence inversion. The strongest summer-time wind speed maxima are found near-shore of the horn of Africa at an altitude of 600-900m. We present here preliminary global distributions of low-level wind speed maxima in January and July and details of their diurnal evolution at selected locations worldwide. We conclude that low-level wind speed maxima are more ubiquitous than previously thought and have high wind power densities (up to 15 kW/m2). Therefore they represent a potential ideal target for AWE systems well below the jet streams.