### The Scorer Parameter

The Scorer parameter l [m^{-1}] is used to describe whether gravity waves will develop or not. It combines the Brunt-Väisälä frequency with characteristics of the vertical wind profile and is given by the following equation:

where N = N(z) is the Brunt-Väisälä frequency and U = U(z) is the vertical profile of the horizontal wind. Both quantities are determined from an atmospheric sounding upstream of the barrier. The first term on the right-hand side usually dominates.

The typical profile of the Scorer parameter shows a high gradient in low levels due to increasing wind speeds. When **l ^{2}(z)** decreases strongly with height, conditions are favorable for

**trapped lee waves**. This is especially true if the decrease of the Scorer parameter occurs suddenly in the mid-troposphere, dividing the troposphere into two regions, a lower layer with large values (high stability) and an upper layer with small values of the Scorer parameter (low stability). At higher levels the Scorer parameter often shows values of about 0.5/km, rarely exceeding 1/km.

However, when **l ^{2}(z)** is nearly constant with height, conditions are favorable for

**vertically propagating mountain waves**.

In the third case, when **l ^{2}(z)** increases with height, the conditions are not favorable for the development of gravity waves.

__Note:__

*In practice, the Scorer parameter is derived from vertical sounding profiles of wind and temperature upstream of a mountain chain to forecast the probability of trapped lee wave development. In cases where the wind data has high vertical resolution, the second term on the right-hand side of the above equation (velocity-profile curvature term) can show values of similar magnitude as the first term. This again leads to a noisy profile of the Scorer parameter. Therefore, either the wind profile is smoothed before calculating the Scorer parameter or the second term is omitted. This is acceptable since Richard S. Scorer postulated an undisturbed air flow upstream of the mountain chain as prerequisite for his parameter.*