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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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Ronald B. Smith

Abstract

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Ronald B. Smith

ABSTRACT

Mountains significantly influence weather and climate on Earth, including disturbed surface winds; altered distribution of precipitation; gravity waves reaching the upper atmosphere; and modified global patterns of storms, fronts, jet streams, and climate. All of these impacts arise because Earth’s mountains penetrate deeply into the atmosphere. This penetration can be quantified by comparing mountain heights to several atmospheric reference heights such as density scale height, water vapor scale height, airflow blocking height, and the height of natural atmospheric layers. The geometry of Earth’s terrain can be analyzed quantitatively using statistical, matrix, and spectral methods. In this review, we summarize how our understanding of orographic effects has progressed over 100 years using the equations for atmospheric dynamics and thermodynamics, numerical modeling, and many clever in situ and remote sensing methods. We explore how mountains disturb the surface winds on our planet, including mountaintop winds, severe downslope winds, barrier jets, gap jets, wakes, thermally generated winds, and cold pools. We consider the variety of physical mechanisms by which mountains modify precipitation patterns in different climate zones. We discuss the vertical propagation of mountain waves through the troposphere into the stratosphere, mesosphere, and thermosphere. Finally, we look at how mountains distort the global-scale westerly winds that circle the poles and how varying ice sheets and mountain uplift and erosion over geologic time may have contributed to climate change.

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Ronald B. Smith

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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

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

Abstract

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

Abstract

Recent observations and numerical experiments indicate that during severe downslope windstorms, a large region of slow turbulent air develops in the middle and upper troposphere while strong winds plunge underneath. A mathematical model of this severe wind state is developed using Long's equation. This theory predicts the altitude of the turbulent air, the strength of the winds, and the mountain drag. In the presence of a wind reversal, the theory indicates which wind reversal altitudes will lead to windstorm conditions.

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Ronald B. Smith

Abstract

This study focuses on the flow over a low, straight section of the Blue Ridge Mountain in the central Appalachians. Aircraft measurements, laboratory simulation, and mathematical analysis using both the linearized equations and the nonlinear hydrostatic equations, are compared. Aircraft observations of a Blue Ridge lee wave indicate that the linear theory correctly predicts the wavelength but badly under-estimates the wave amplitude. This discrepancy is confirmed in the laboratory. It appears that the discrepancy can be accounted for by the effects of a strong nonlinearity in the governing equations. It is suggested that nonlinear effects acting near the mountain can be important for wave generation and that Long's model is restrictive as it includes only the cases for which this aspect of the physics is entirely absent.

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Ronald B. Smith

Abstract

A hurricane beta-drift law is derived by applying dimensional analysis to the numerical simulations of Chan and Williams. The beta-drift speed and direction are given as simple functions of hurricane strength, radius, and the magnitude of beta. The physical significance of the drift law and its relation to previously proposed drift laws are considered.

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Ronald B. Smith

Abstract

No abstract available.

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