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 is the Earth's radius, and [math]\displaystyle{ \varphi }[/math] is the latitude.
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 [math]\displaystyle{ 2\pi = 360 }[/math] degrees. Wavenumber 2 has two ridges and two troughs around 360 degrees.
Wavenumber 0 corresponds to zonal (symmetric) flow. Wavenumbers 1–3 are called long waves and often synonymous in meteorological literature with the mid-latitude planetary Rossby waves, while wavenumbers 4-10 are often referred to as "synoptic" waves.[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.[5]
Original source: https://en.wikipedia.org/wiki/Zonal wavenumber.
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