Zonal wavenumber

In meteorological applications, a zonal wavenumber or hemispheric wavenumber is the dimensionless number of wavelengths fitting within a full circle around the globe at a given latitude:[1] where λ is the wavelength, r = 6378 km is the Earth's radius, and

Zonal wavenumbers are typically counted on the upper level (say 500-millibar) geopotential maps by identifying troughs and ridges of the waves.

Wavenumber 1 has one trough and one ridge, i.e. one wavelength fits 2π = 360°.

Wavenumber 0 corresponds to zonal (symmetric) flow.

[2] In the Northern Hemisphere, wavenumbers 1 and 2 are important for the time-mean circulation due to topography (Tibetan Plateau and Rocky Mountains),[3][4] whereas in the Southern Hemisphere, tropical convection is responsible for the presence of mainly zonal wavenumber 3.