Abstract
This paper is the first in a series of three papers concerning a gravity wave event that occurred over the north-central United States on 11–12 July 1981. The event is analyzed with superb detail resulting from the availability of digitized radar, surface mesonetwork, and other special data from the Cooperative Convective Precipitation Experiment (CCOPE) in Montana. The subject matter of this paper consists of 1) a statistical determination of the wave characteristics, 2) a demonstration that the observed phenomena display a nature consistent with that of gravity waves, and 3) a discussion of the principles and limitations of statistical methods for detecting and tracking mesoscale gravity waves.
Two distinct wave episodes of ∼8 h duration within a longer (33 h) period of wave activity are studied in detail. Both episodes contain strongly coherent, bimodal wave activity. The primary (secondary) wave mode isolated from autospectral and perturbation map analyses displays mean periods of 2.5 (0.9) h and mean horizontal wavelengths of 160 (70) km. The horizontal phase velocities are essentially identical for the two wave modes. Cross-spectral analyses confirm the impression that the wavefronts are not truly planar, but rather are arc- or comma-shaped in appearance.
Perturbation pressure (p′) and wave-normal wind (u*′) are found to be in phase with one another. The importance of this finding is that it strongly supports the interpretation of the wave signals as gravity waves, a conclusion that rests upon the availability of the mesonet wind data. The observation that rainbands were positioned immediately ahead of the wave crests in those situations where the waves did not propagate through the rainbands also agrees with gravity wave theory. Consistency checks between the observed values of p′, u*′, and the wave phase velocity are made using the impedance relationship to further substantiate the gravity wave interpretation of these data. The certainty of these interrelationships between the pressure, wind, and precipitation fields is the direct consequence of statistically analyzing data with unprecedented detail compared to previous case studies of mesoscale gravity waves.
