Wind Regimes - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 18

The vertical profiles of these boundary layer winds over different surface types
will be analysed in more detail in the upcoming Chaps. 3 - 5 .

2.6 Thunderstorm Gusts and Tornados

There are strong winds which cannot be used for wind energy generation, because
they are short-lived and rare in time and place, such that their occurrence is nearly
unpredictable. Most prominent among these phenomena are thunderstorm gusts
and tornadoes. Offshore tornadoes are called waterspouts. They can be so violent
that they can damage wind turbines. Therefore, the probability of their occurrence
and their possible strength should be nevertheless investigated during the proce-
dure of wind turbine sitting.
While onshore tornadoes mostly form in the afternoon and the early evening at
cold fronts or with large thunderstorms when surface heating is at a maximum,
offshore waterspouts are more frequent in the morning and around noon when the
instability of the marine boundary layer is strongest due to nearly constant sea
surface temperatures (SST) and cooling of the air aloft overnight (Dotzek et al.
2010 ). However, the seasonal cycle is different. Onshore tornadoes most fre-
quently occur in late spring and summer. Offshore waterspouts peak in late
summer and early autumn. In this season, the sea surface temperature of shallow
coastal waters is still high, while the first autumnal rushes of cold air from the
polar regions can lead to an unstable marine boundary layer favourable for
waterspout formation (Dotzek et al. 2010 ).
Although the characteristics of tornado formation are understood in principle
today, the prediction of their actual occurrence remains difficult because a variety
of different favourable conditions have to be met simultaneously. In general,
following Houze ( 1993 ) and Doswell ( 2001 ), tornado formation depends largely
on the following conditions:

• (potential) instability with dry and cold air masses above a boundary layer
capped by a stable layer preventing premature release of the instability;
• a high level of moisture in the boundary layer leading to low cloud bases;
• strong vertical wind shear (in particular for mesocyclonic thunderstorms);
• pre-existing

boundary layer vertical vorticity (in particular for non-

mesocyclonic convection).

A rough estimation how often a tornado could hit a large wind park is given in
Sect. 6.5 .