The dissipation of trapped lee waves . Part II : The relative importance of the 1 boundary layer and the stratosphere

Decaying trapped waves exert a drag on the large-scale flow. The two most studied mechanisms for such decay are boundary-layer dissipation and leakage into the stratosphere. If the waves dissipate in the boundary layer, they exert a drag near the surface, whereas if they leak into the stratosphere, the drag is exerted at the level where the waves dissipate aloft. Although each of these decay mechanisms has been studied in isolation, their relative importance has not been previously assessed. Here we conduct numerical simulations showing that the relative strength of these two mechanisms depends on the details of the environment supporting the waves. During actual trapped wave events, the environment often includes elevated inversions and strong winds aloft. Such conditions tend to favor leakage into the stratosphere, although boundary-layer dissipation becomes nonnegligible in cases with shorter resonant wavelengths and higher tropopause heights. In contrast, idealized two-layer profiles with constant wind speeds and high static stability beneath a less stable upper-troposphere support lee waves that are much more susceptible to boundary dissipation and relatively unaffected by the presence of a stratosphere. One reason that trapped waves in the two-layer case do not leak much energy upwards is that the resonant wavelength is greatly reduced in the presence of surface friction. This reduction in wavelength is well predicted by the linear inviscid equations if the basic state profile is modified a posteriori to include the shallow ground-based shear layer generated by surface friction. 10