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Ronald B. Smith

The Special Observing Period of ALPEX (Alpine Experiment) occurred in March and April 1982. Since that time, several types of ALPEX research have been pursued in Europe and the United States, including data analysis, theory, numerical modeling, and laboratory modeling. The current status of ALPEX research in the United States was reviewed recently in a three-day ALPEX workshop held at Yale University in New Haven, Connecticut. This workshop was sponsored by the ALPEX Panel of the National Research Council's U.S. Committee for the Global Atmospheric Research Program. Attending the workshop were 16 United States scientists and agency representatives and two scientists representing active groups in Europe. The purpose of this article is to report the workshop findings to the meteorological community. In addition, the nature of the ALPEX data set will be described.

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Ronald B. Smith

Abstract

Previous models of quasi-geostrophic airflow over mountains have used one or more of the following assumptions: 1) the Boussinesq approximation, 2) a rigid lid upper boundary condition, and 3) the idealization of an infinitely long ridge and two-dimensional flow. To investigate the physical nature of these assumptions, the flow of an unbounded, slightly compressible fluid over an isolated mountain is considered. In this case, the mountain feels a strong “lift” force acting to the left of the stream (in the Northern Hemisphere) due to the cross-stream pressure gradient, while the airstream receives an equally strong impulse to the right. Using the Kutta-Joukowski formula, the rightward impulse given to the airstream can be represented in terms of a far-field barotropic circulation which in turn is associated with the production of vorticity by volume expansion as the air parcels rise over the mountain.

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Ronald B. Smith

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No abstract available.

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Ronald B. Smith

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.

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Ronald B. Smith

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During the ERICA project in 1989, ice crystals were collected from the tops bf two winter storms and one broad cirrus cloud. Deuterium concentration in the storm ice samples, together with a model of isotope fractionation, are used to determine the temperature where the ice was formed. Knowledge of the ice formation temperature allows us to determine whether the ice has fallen or been lofted to the altitude of collection. In both storms, the estimated fall distance decreases upward. In the 21 January storm, the fall distance decreases to zero at the cloud top. In the 23 January storm, the fall distance decreases to zero at a point 2 km below the cloud top and appears to become negative above, indicating lofted ice.

Cloud particle data from the cloud tops show an ice-to-vapor ratio greater than one and indicate the presence of particles with small terminal velocities; both observations support the idea of ice lofting. The satellite-derived cloud tops lie well below the actual cloud top (e.g., 2.5 km below on 23 January), indicating that the lofted ice in winter storms may not be detectable from space using IR radiance techniques.

A comparison of deuterium in cloud-top ice and clear-air vapor suggests that even in winter, when vertical air motions are relatively weak, lofted ice crystals are the dominant source of water vapor in the upper troposphere.

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Ronald B. Smith

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The purpose of this paper is to further develop, and then apply, the “lee wave” theory of lee cyclogenesis described by Smith. The two-dimensional time-dependent quasi-geostrophic theory is shown to give closed form asymptotic solutions which clarify the cyclone growth mechanism and the catalytic influence of orography. Ageostrophic effects are considered and found to have little influence on the Ice cyclone unless a low lying inertial wave critical layer is present. When a rigid lid is added to the model thus allowing baroclinic instability, a rapid two-phase cyclone growth is found. Without a resonant orographic phase, the unstable growth is small. The theory is applied to three mountain ranges: the Alps, the Colorado Rockies, and the southern Appalachians. The results suggest that the “iee wave” model may be partially correct in the first two regions but Problematic in the third.

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Ronald B. Smith

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No abstract available.

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Ronald B. Smith

Abstract

A theory of lee cyclogenesis is proposed, based on a linearized model of baroclinic wave generation by mountains in the presence of a background shear. The theory predicts that lee cyclogenesis will occur when the criterion for the existence of standing baroclinic lee waves is satisfied in the environment. For an infinite ridge, this requires that the component of wind across the ridge must reverse with height. The time scale for cyclone development, and the meaning of the ambiguous term “lee” are clarified by examining the group velocity of the baroclinic waves. Time dependent three-dimensional solutions are discussed along with their application to Alpine Ice cyclogenesis.

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Ronald B. Smith

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To test the idea that observed oscillations in severe downslope winds are due to Kelvin–Helmholtz instability, the eigenvalues for the Taylor–Goldstein equation are found for a family of shear flows arising from local hydraulic theory. These two theories, local hydraulic theory and linear Kelvin–Helmholtz theory, provide a reasonable prediction of the period and speed of movement of the wind oscillations but underestimate their growth. The rule-of-thumb critical Richardson number of 0.25 agrees better than the linear theory value found here, 0.1, possibly indicating a nonlinear subcritical instability.

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Ronald B. Smith

Abstract

The history of the theory of orographic rain, and recent evidence against the “stable upglide” model, are briefly reviewed. A new model is proposed in which the blocking of low level air by a mountain causes approaching cold air to override the warm air, producing an unstable layer upstream of the mountain. This model is compared with recent observations in the Cascades and San Juan Mountains. The suggestion is that under some conditions it is the blocking action of the mountain, rather than forced ascent, which causes enhanced precipitation.

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