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Steven E. Koch and Robert E. Golus

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.

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Steven E. Koch, Robert E. Golus, and Paul B. Dorian

Abstract

This paper presents the results of a very detailed investigation into the effects of preexisting gravity waves upon convective systems, as well as the feedback effects of convection of varying intensity upon the waves. The analysis is based on the synthesis of synoptic surface and barograph data with high-resolution surface mesonetwork, radar, and satellite data collected during a gravity wave event described by Koch and Golus in Part I of this series of papers. Use is also made of the synoptic barograph data and satellite imagery to trace the waves beyond the mesonetwork and thus determine their apparent source region just upstream of the mesonetwork.

It is shown that two of the gravity waves modulated convection within a weak squall line as they propagated across the line. The other six waves remained closely linked with convective systems that they appeared to trigger. However, it is shown that the waves were not excited by convection. Furthermore, the waves retained their signatures in the surface mesonetwork fields in the presence of rainshowers. Two episodes of strongest gravity wave activity are identified, each of which consisted of a packet of four wave troughs and ridges displaying wavelengths of ∼150 km. A Mesoscale Convective Complex (MCC) forms rapidly from very strong or severe thunderstorms apparently triggered by the individual members of the second wave packet. It is suggested that the large size and long duration of this complex were due in part to the periodic renewal and organization provided by this wave packet.

Strong convection appears to substantially affect the gravity waves locally by augmenting the wave amplitude, reducing its wavelength, distorting the wave shape, altering the wave phase velocity, and greatly weakening the in-phase covariance between the perturbation wind and pressure (pu*′) fields. These convective effects upon the gravity waves are explained in terms of hydrostatic and nonhydrostatic pressure forces and gust front processes associated with thunderstorms. Despite the implication from these findings of the loss or obscuration of the original wave signal, the gravity wave signal remained intact just outside of the active storm cores and the entire wave-storm system exhibited outstanding spatial coherence over hundreds of kilometers.

The observations are also compared to the predictions from wave-CISK theory. Although qualitative agreement is found, quantitative comparisons give rather unimpressive agreement, due in large measure to simplifications inherent to the theory.

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