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SUMMARYThe response of a stratified airstream to combined thermal and orographic forcing is investigated theoretically using the linearized hydrostatic equations of motion. The magnitude of the heating aloft is computed from observed rainfall rates. The elevated heating is shown to produce vertically propagating waves whose amplitude, relative to the mountain waves, is determined by the parameter gQb/cpTUaNh (symbols Appendix I). For typical wind speeds and rainfall rates the thermally generated waves will equal or exceed the orographically generated waves. These waves produce large vertical variations in the wave momentum flux at the levels where heat is being added and alter, or even reverse, the mountain drag. In hydrostatic flow, the phase relationship between the heating rate and the induced vertical displacements is found, unexpectedly, to be negative. In the vicinity of the heating, air parcels are displaced downwards. This result can be explained by wave propagation ideas but not by parcel arguments. This helps to explain why mountain wave amplitudes are sometimes reduced in moist atmospheres. The broad heat-induced descent may act to limit the amount of condensation-precipitation occurring in a stable middle-level orographic cloud. Other types of orographic precipitation involving embedded convection in stratus layers, low level feeder clouds, or deep cumulus convection may not be adversely effected by the heat-induced descent.

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