Anelastic semigeostrophic flow over a mountain ridge

P. R. Bannon, Chu Pe-Cheng Chu

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Scale analysis indicates that five nondimensional parameters (R0 2, ε, μ, λ and kλ) characterize the disturbance generated by the steady flow of a uniform wind (U0, V0) incident on a mountain ridge of width a. Here μ = h0/HR is the ratio of the mountain height h0 to the deformation depth HR = fa/N where f is the Coriolis parameter and N is the static buoyancy frequency. The parameters λ = HR/H and kλ are the ratios of HR to the density scale height H and the potential temperature scale height H/k respectively. There are two Rossby numbers; ε = V0/fa, and R0 = U0/fa. If R0 2 ≤ 1, then the mountain-parallel flow is in approximate geostrophic balance and the flow is semigeostrophic. If the flow is anelastic (λ ≃ 1), no direct correspondence between the two approximations was found. However the anelastic effects are qualitatively similar for the two and lead to: i) an increase in the strength of the mountain anticyclone, ii) a reduction in the extent (and possible elimination) of the zone of blocked, cyclonic flow, iii) a permanent turning of the flow proportional to the mass of air displaced by the mountain, and iv) an increase in the ageostrophic cross-mountain flow. The last result implies an earlier breakdown of semigeostrophic theory for anelastic flow over topography. Apart from a strengthening of the cold potential temperature anomaly over the mountain, the presence of a finite potential temperature scale height (ie k nonzero) does not significantly alter the flow solution. -from Authors

Original languageEnglish (US)
Pages (from-to)1020-1029
Number of pages10
JournalJournal of the Atmospheric Sciences
Issue number6
StatePublished - 1988

All Science Journal Classification (ASJC) codes

  • Atmospheric Science


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