Winter Storms over the San Juan Mountains. Part I: Dynamical Processes

John D. Marwitz University of Wyoming, Laramie 82071

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Abstract

The Colorado River Basin Pilot Project was conducted over the San Juan Mountains in southwestern Colorado and ran for five winter seasons, terminating in 1974–75. The objective of the project was to demonstrate the feasibility of increasing the amount of snowpack and, therefore, the amount of available runoff. The Bureau of Reclamation, through its contractors, conducted the project. A number of statistical evaluations of the program have been made. This series of papers represents the principal physical evaluation of the seeding potential of San Juan storms.

The synthesis of several well-documented San Juan storms indicates that most storms evolve through four distinct stages which are related to thermodynamic stability. The stages in sequence are stable, neutral, unstable and dissipation. During the stable stage, much of the flow below mountain top level is blocked and diverted toward the west. During the neutral stage, the storm is deep; it typically extends throughout much of the troposphere. During the unstable stage, a zone of horizontal convergence appears to form near the surface at the base of the mountain on the upwind side and a convective cloud line is often present over this convergence zone. Subsidence at mountain top height causes dissipation. Rare but well-organized storms containing a baroclinic zone that extends throughout the troposphere also pass over the San Juans. Blocked flow does not appear to occur in the well-organized storms.

Abstract

The Colorado River Basin Pilot Project was conducted over the San Juan Mountains in southwestern Colorado and ran for five winter seasons, terminating in 1974–75. The objective of the project was to demonstrate the feasibility of increasing the amount of snowpack and, therefore, the amount of available runoff. The Bureau of Reclamation, through its contractors, conducted the project. A number of statistical evaluations of the program have been made. This series of papers represents the principal physical evaluation of the seeding potential of San Juan storms.

The synthesis of several well-documented San Juan storms indicates that most storms evolve through four distinct stages which are related to thermodynamic stability. The stages in sequence are stable, neutral, unstable and dissipation. During the stable stage, much of the flow below mountain top level is blocked and diverted toward the west. During the neutral stage, the storm is deep; it typically extends throughout much of the troposphere. During the unstable stage, a zone of horizontal convergence appears to form near the surface at the base of the mountain on the upwind side and a convective cloud line is often present over this convergence zone. Subsidence at mountain top height causes dissipation. Rare but well-organized storms containing a baroclinic zone that extends throughout the troposphere also pass over the San Juans. Blocked flow does not appear to occur in the well-organized storms.

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