# Scorer parameter

From AMS Glossary

## Scorer parameter

The quantity where

*l*(*z*) arising from the wave equation for atmospheric gravity waves describing flow over a mountain barrier:*N*=*N(z)*is the Brunt–Väisälä frequency and*U*=*U(z)*is the vertical profile of the horizontal wind, both quantities determined from an atmospheric sounding upstream of the barrier.The first term on the right-hand side usually dominates, but occasionally the second term, the velocity-profile curvature term, can be of similar magnitude. When

*l*^{2}is nearly constant with height, conditions are favorable for vertically propagating mountain waves. This parameter is most often used, however, as an indicator of when trapped lee waves (*see*mountain wave) can be expected; they occur when*l*^{2}(*z*) decreases strongly with height. This is especially true if this decrease occurs suddenly in mid troposphere, dividing the troposphere into two regions, a lower layer of large*l*^{2}(*z*) (high stability) and an upper layer of small*l*^{2}(*z*) (low stability).*l*, the square root of the parameter, has units of wavenumber (inverse length), and the wavenumber of the resonant lee wave lies between*l*of the upper layer and*l*of the lower layer—the equivalent wavelength generally lying between 5 and 25 km in the atmosphere. Mountain ranges wide enough to force wavelengths long relative to*l*_{upper}(the*l*in the upper layer) produce vertically propagating waves with wavenumbers less than*l*_{upper}. Small objects (that force wavenumbers greater than*l*_{lower}) produce waves that are evanescent, or vanishing with height.