Negative Wind Shear

Negative wind shear refers to an atmospheric condition in which the wind speed decreases with height above ground. This is a reversal of the typical situation, in which the wind speed increases with height. Negative wind shear can occur during calm conditions in hilly terrain as a result of cold air drainage, a phenomenon whereby cold air flows down a slope because of its higher density relative to the surrounding air.

But in the flatter, windier locations that typically interest wind energy developers, negative wind shear is a rare phenomenon, and apparent negative wind shear often turns out to be an artifact of anemometer icing. For example, if the highest anemometer is the one most severely affected by icing, it may as a result record a wind speed that is artificially low, perhaps lower than the other anemometers record. This apparent negative wind shear can serve as evidence for icing.

You can search for negative wind shear and flag iced data segments using the Flag With Rules window.

In negative wind shear conditions, the power law exponent may drop below zero. But the surface roughness can never be negative, so Windographer often cannot find a valid curve fit for the logarithmic law in negative wind shear conditions.