A MESOSCALE GRAVITY-WAVE EVENT OBSERVED DURING CCOPE .4. STABILITY ANALYSIS AND DOPPLER-DERIVED WAVE VERTICAL STRUCTURE

This paper summarizes the results of a detailed study from the Coopera tive Convective Precipitation Experiment (CCOPE) of the vertical struc ture of mesoscale gravity waves that disturbed a sizable part of the t roposphere and that played a significant role in the generation of a m esoscale convective complex. These bimodal waves displayed periods of 148 (50) min, wavelengths of 135 (60) km, and phase speeds of 15.2 (19 .8) m s-1. A comparison is made between wave-induced pressure perturba tion fields derived from triple-Doppler wind fields within regions of essentially nonconvective precipitation, pressure perturbation fields obtained by bandpass filtering of surface mesonetwork data, and the ve rtical structure of the pressure eigenfunctions as predicted from a li near stability analysis. It is believed that this represents the first such application of the Doppler radar pressure retrieval technique to the study of gravity waves. In addition, an analysis of the potential for shear instability was performed on all of the special CCOPE sound ings taken on this day to determine the representativeness of the chos en soundings for the theoretical analysis and the likelihood that a wa ve maintenance mechanism endured throughout the 33-h wave event. The a nalysis of the potential for shear instability and the eigenfunctions both indicate that the bimodal waves were able to efficiently extract energy from the mean flow near several closely spaced critical levels in the 4.0-6.5-km layer to maintain their coherence for many wave cycl es. This result serves as the explanation for the observed ability of the waves to organize precipitation into long convective bands whose a xes were along and just ahead of the wave crests. The eigenvalue analy sis predicts unstable modes that are hydrostatic, nondispersive, ducte d gravity waves characterized by half of a vertical wavelength contain ed between the ground and the lowest critical level (at z = 4 km). Eig enfunctions of pressure and other variables all display negligible til t below 2.3-3.3 km, above which a sudden reversal in phase occurs. The vertical structure of the Doppler-derived fields associated with one of these gravity waves agrees in terms of the following respects with the eigenfunction predictions and/or the surface mesoanalyses: (a) the vertical wavelength, horizontal structure, and amplitude of the pertu rbation horizontal wind and pressure fields, and (b) the in-phase cova riance between the pressure and horizontal wind fields at levels below 2.5 km. On the other hand, the theory predicted a much more abrupt ve rtical transition in phase in the pressure fields and weaker amplitude s aloft than were evident in the Doppler analyses. In addition, the si ze of the multiple-Doppler analysis domain was too small to capture an entire horizontal wavelength of the 135-km-scale gravity wave, which made direct comparisons difficult. Furthermore, the linear theory pred icts much smaller amplitudes and somewhat longer horizontal wavelength s for the vertical motions characterizing both wave modes than those s een in the Doppler winds, which likely also contain nonwave effects. T hese discrepancies are largely due to the combined effects of weak con vection, turbulence, and data sampling problems. Despite these drawbac ks, the findings from this and other recent studies using Doppler rada rs and ground-based radiometers suggest that remote sensing of mesosca le gravity waves that occupy a significant fraction of the troposphere should be exploited further.