Synoptic Observations and Theory of Orographically Disturbed Wind and Pressure

Abstract

A survey of existing synoptic data from the vicinity of major mountain ranges indicates two common aspects of orographic influence on the atmosphere?a hydrostatically generated pressure difference across the mountains and a leftward (in the Northern Hemisphere) deflection of the air as it approaches the mountain. To explain these features, the linear theory of Queney is extended to include isolated mountains, and the force balances implied by the model are clarified by using an expansion in inverse powers of the Rossby number. The validity of this expansion in the far field and the generation of inertial waves are discussed. The results of the theory show that for typical values of the Rossby number, the pressure field and vertical motion field are unaffected by the Coriolis force while the horizontal trajectories of air parcels are altered, in agreement with observation. The ability of an isolated range to block and deform a passing cold front is shown to depend on having a narrow enough range so that the orographic disturbance is strongly ageostrophic.