Wind Regimes - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 9

Chapter 2
Wind Regimes
The principle origin of the winds in the Earth's atmosphere and the potentially
available power from these winds have been qualitatively described in Sect. 1.4 .
This general description of the driving forces for the wind has to be brought into a
mathematical formulation for precise turbine load and energy yield calculations
and predictions. Therefore, this chapter will present the basic wind laws in the free
atmosphere. Vertical wind profiles in atmospheric boundary layers over different
surface types will be presented in the subsequent Chaps. 3 - 5 .
2.1 Global Circulation
Flow patterns and winds emerge from horizontal surface and atmospheric
temperature contrasts on all spatial scales from global to local size. Globally, the
tropical belt and the lower latitudes of the Earth are the main input region for solar
energy, while the higher latitudes and the poles are the regions with a negative
energy balance, i.e. the Earth here loses energy through thermal radiation. Ocean
currents and atmospheric heat conduction are not sufficient to compensate for this
differential heatingof the globe. The global atmospheric circulation has to take
over as well. Main features of this global atmospheric circulation are the Hadley
cell, the Ferrel cell and the polar cell which become visible from a latitude-height
plot showing an average over all longitudes of the winds in the troposphere and
stratosphere. The Hadley cell exhibits a direct thermal circulation. Warm air rises
near the equator, moves towards the poles aloft and descends in the subtropics. The
region of sinking motion is characterised by large anticyclones in the surface
pressure field and deserts. Likewise, the polar cell exhibits a direct thermal
circulation as well. Here, cold air sinks over the poles and rises at higher latitudes.
This is the reason for generally high pressure over the poles. In between the
Hadley cell and the polar cell lies the thermally indirect Ferrel cell. This cell is
characterised by rising colder air at higher latitudes and sinking warmer air in the