The Wave Length

Figure 8: Plot of a sine wave showing three pairs of corresponding points between which wavelength (λ) can be measured. ©Richard F. Lyon, CC BY-SA 3.0

Wave length is the distance between two points with same phase (figure 8). The wave length of lee clouds depends on atmospheric stability and wind speed but not on topography. Short wave lengths occur in very stable atmospheric conditions with low wind speeds while high wind speeds and a less stable atmosphere generate longer wave lengths. The relationship between wave length λ [m], wind and stability is given by:

with l(z) being the Scorer parameter. Typically wave lengths are in the order of 5 to 35 km with an average around 10 km. Hence lee waves can be observed easily with today's geostationary satellite image resolutions as long as their wave length does not fall below 5 km in mid-latitudes.

In cases with complex orography (e.g. several mountain chains in parallel), lee waves may be damped or enhanced depending on the relationship between wave length and distance between the mountain chains (figure 9).

Figure 9: Schematic describing the interaction between wave length and complex topography (WMO, 1978).