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Lower Turbulent Zones Associated with Mountain Lee Waves

Peter F. LesterDept. of Meteorology, San Jose State University, San Jose, Calif. 95192

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William A. FingerhutDept. of Meteorology, San Jose State University, San Jose, Calif. 95192

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Abstract

The characteristics of the lower turbulent zone (LTZ) which is associated with mountain lee waves have been investigated through the analysis of aircraft observations made near Boulder, Colo. Numerical filters and statistical analysis techniques have been applied to the data from six cases to yield vertical sections of potential temperature, horizontal wind, and turbulence intensity along the aircraft paths. One case study is presented to illustrate the structure of the LTZ and its changes during a frontal passage. In addition, the main features of all the analyses are summarized in the form of a schematic vertical section.

The horizontal dimension of the LTZ varied between 25 and more than 65 km downstream of the first lee wave trough. The vertical dimensions ranged from a few hundred meters AGL at the lee wave troughs to 3 km AGL at the wave crests. Turbulence levels were light, moderate or severe over more than 90% of the total distance flown in the LTZ (nearly 1100 km). Severe turbulence was commonly encountered near the upstream side of the rotor, where the largest horizontal and vertical wind and temperature gradients were also found.

The kinetic energy dissipation rate for a large, long-lived LTZ, such as occurs under hydraulic jump conditions, was estimated to be 20–100 W m−2; thus, the LTZ may play an important role in the large-scale energetics of the atmosphere.

Abstract

The characteristics of the lower turbulent zone (LTZ) which is associated with mountain lee waves have been investigated through the analysis of aircraft observations made near Boulder, Colo. Numerical filters and statistical analysis techniques have been applied to the data from six cases to yield vertical sections of potential temperature, horizontal wind, and turbulence intensity along the aircraft paths. One case study is presented to illustrate the structure of the LTZ and its changes during a frontal passage. In addition, the main features of all the analyses are summarized in the form of a schematic vertical section.

The horizontal dimension of the LTZ varied between 25 and more than 65 km downstream of the first lee wave trough. The vertical dimensions ranged from a few hundred meters AGL at the lee wave troughs to 3 km AGL at the wave crests. Turbulence levels were light, moderate or severe over more than 90% of the total distance flown in the LTZ (nearly 1100 km). Severe turbulence was commonly encountered near the upstream side of the rotor, where the largest horizontal and vertical wind and temperature gradients were also found.

The kinetic energy dissipation rate for a large, long-lived LTZ, such as occurs under hydraulic jump conditions, was estimated to be 20–100 W m−2; thus, the LTZ may play an important role in the large-scale energetics of the atmosphere.

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