20.14 Local Winds – New Zealand and the South Pacific

20.14.2 Describe the general characteristics of a mountain wave set-up with reference to:

Mountain waves, also known as standing waves or lee waves, may form when strong, stable airflow crosses a mountain range.

The airflow is displaced vertically by the terrain and, after crossing the ridge, oscillates above and below its normal level, producing a series of wave crests and troughs downwind of the range.

In New Zealand, well-developed mountain waves are particularly common with strong northwesterly flow across the main mountain ranges.

(a) wave-lengths;

The wave-length is the horizontal distance between:

• one wave crest and the next; or

• one wave trough and the next.

Typical mountain wave-lengths in New Zealand are approximately 10–40 km.

The wave-length is not fixed and depends mainly on the wind speed and the stability of the atmosphere.

(b) position and rotation of any possible rotor zones;

Rotor zones may develop beneath the mountain wave, particularly beneath the crests of the first few waves on the lee side of the range.

The rotor:

• rotates around a horizontal axis approximately parallel to the mountain range;