A Climatology of Gravity Waves and Other Coherent Disturbances at the Boulder Atmospheric Observatory during March–April 1984

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  • 1 Laboratory for Atmospheres, NASA/Goddard Space Flight Center, Greenbelt, Maryland
  • | 2 Wave Propagation Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado
  • | 3 CSIRO, Division of Environmental Mechanics, Canberra City, Australia
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Abstract

We present a climatological study of gravity waves and other coherent disturbances at the Boulder Atmospheric Observatory, during the period mid-March-mid-April 1984. The data were collected by a network of microbarographs, and by sensors on the 300 m tower. The total observational period was divided into 522 time segments of 5120 s each. Coherent and incoherent motions were identified on the basis of a cross-correlation coefficient, calculated from the microbarograph network for each time segment and frequency band analyzed, using the assumption that the atmospheric state can be described by an equivalent plane wave. Five passbands were considered in the period range 1–20 min.

The analysis indicates that the atmospheric state at these passbands displays highly coherent structure, most of the time. During the interval from 0800 to 1800 LST, coherent motions with cross-correlation coefficient larger than 0.5 are present about 25% of the time for periods between 1 and 5 min and more than 80% of the time for periods between 10 and 20 min. In the remaining hours of the day, the percentages rise to more than 40% and 95% of the time, respectively.

A relationship is illustrated between the turbulent kinetic energy measured on the tower and the amplitude of the rms pressure field at the ground for disturbances having up to 5 min periods. For longer periods, such a relationship appears to be absent, indicating that the longer the scales, the deeper the atmospheric zone important to the dynamics of the pressure fluctuations.

Abstract

We present a climatological study of gravity waves and other coherent disturbances at the Boulder Atmospheric Observatory, during the period mid-March-mid-April 1984. The data were collected by a network of microbarographs, and by sensors on the 300 m tower. The total observational period was divided into 522 time segments of 5120 s each. Coherent and incoherent motions were identified on the basis of a cross-correlation coefficient, calculated from the microbarograph network for each time segment and frequency band analyzed, using the assumption that the atmospheric state can be described by an equivalent plane wave. Five passbands were considered in the period range 1–20 min.

The analysis indicates that the atmospheric state at these passbands displays highly coherent structure, most of the time. During the interval from 0800 to 1800 LST, coherent motions with cross-correlation coefficient larger than 0.5 are present about 25% of the time for periods between 1 and 5 min and more than 80% of the time for periods between 10 and 20 min. In the remaining hours of the day, the percentages rise to more than 40% and 95% of the time, respectively.

A relationship is illustrated between the turbulent kinetic energy measured on the tower and the amplitude of the rms pressure field at the ground for disturbances having up to 5 min periods. For longer periods, such a relationship appears to be absent, indicating that the longer the scales, the deeper the atmospheric zone important to the dynamics of the pressure fluctuations.

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