WKB approximation in application to acoustic–gravity waves

The WKB approximation often provides a useful tool for the study of wave propagation through varying media. The utility of this tool in the case of acoustic–gravity waves, propagating through regions of varying background temperature, has been somewhat questionable because of an ambiguity in the assignment of an appropriate expression for the vertical wavenumber. The ambiguity is examined here, and shown to be inconsequential except in circumstances such that the WKB approximation is itself inapplicable; a unique choice of expression for the vertical wavenumber is recommended on the basis of simplicity and. utility. Conclusions reached in a number of recent papers are reexamined in the light of the insight that has been gained.