Resonant gravity-wave drag enhancement in linear stratified flow over mountains

• Autor(a) principal: Teixeira, M. A. C.
• Data de Publicação: 2005
• Outros Autores: Miranda, P. M. A., Argaín, José Luís Almaguer, Valente, M. A.
• Tipo de documento: Artigo
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10400.1/11483
• topic: Severe downslope windstorms; 3 spatial dimensions; 2 dimensional mountain; Hydrostatic flow; Surface pressure; Shear flow; Topography; Stability; States; High-drag states; Linear theory; Mountain waves

High-drag states produced in stratified flow over a 2D ridge and an axisymmetric mountain are investigated using a linear, hydrostatic, analytical model. A wind profile is assumed where the background velocity is constant up to a height z(1) and then decreases linearly, and the internal gravity-wave solutions are calculated exactly. In flow over a 2D ridge, the normalized surface drag is given by a closed-form analytical expression, while in flow over an axisymmetric mountain it is given by an expression involving a simple 1D integral. The drag is found to depend on two dimensionless parameters: a dimensionless height formed with z(1), and the Richardson number, Ri, in the shear layer. The drag oscillates as z(1) increases, with a period of half the hydrostatic vertical wavelength of the gravity waves. The amplitude of this modulation increases as Ri decreases. This behaviour is due to wave reflection at z(1). Drag maxima correspond to constructive interference of the upward- and downward-propagating waves in the region z < z(1), while drag minima correspond to destructive interference. The reflection coefficient at the interface z = z(1) increases as Ri decreases. The critical level, z(c), plays no role in the drag amplification. A preliminary numerical treatment of nonlinear effects is presented, where z(c) appears to become more relevant, and flow over a 2D ridge qualitatively changes its character. But these effects, and their connection with linear theory, still need to be better understood.