Wave breaking signatures in noctilucent clouds

Abstract

Results of a recent modeling study of gravity wave breaking in three dimensions by Andreassen et al. and Fritts et al. showed wave saturation to occur via a three‐dimensional instability oriented normal to the direction of wave propagation. The instability was found to occur at horizontal scales comparable to the depth of unstable regions within the wave field and to lead to substantial vertical displacements and tilting of isentropic surfaces. Because of strong similarities between the wave and instability structures in the simulation and the structure observed in noctilucent cloud layers near the summer mesopause, we have used these model results to compute the advective effects on cloud visibility and structure for a range of viewing angles and cloud layer widths. Our results show the gravity wave breaking signature to provide a plausible explanation of the observed structures and suggest that noctilucent cloud structures may be used in turn to infer qualitative properties of gravity wave scales, energy and momentum transports, and turbulence scales near the summer mesopause.

Wave breaking signatures in sodium densities and OH nightglow: 2. Simulation of wave and instability structures

“…We showed previously that instabi]ity structures arising due to breaking of a high-frequency gravity wave could account for the spatial and temporal scales of rippled features observed in images of noctilucent clouds [Fritts et al, 1993]. Similar observations in nightglow images accompanying evidence of large-scale overturning in the motion field were described by Hecht et al…”

“…The majority of these studies, however, were limited either by data sets inadequate to resolve the dominant wave and instability structures or by artificial constraints (two-dimensional and/or monochromatic) in modeling or theoretical studies. As a result, the nature of the wave breaking process remained a mystery until more recent three-dimensional numerical studies were performed [Fritts et al, 1993Andreassen et al, 1994b;Winters and D'Asaro, 1994]. These showed the primary instability of a breaking wave to comprise counterrotating streamwise vortices (elongated along the direction of wave propagation) due to convective instability within the wave field.…”

All‐sky measurements of short period waves imaged in the OI(557.7 nm), Na(589.2 nm) and near infrared OH and O2(0,1) nightglow emissions during the ALOHA‐93 Campaign

“…These patterns tended to occur on nights when there was marked gravity wave activity in the form of extensive bands and they often appeared to be aligned orthogonal to the larger scale waves. However, it is not thought that they are the signature of large-scale gravity waves breaking as their horizontal wavelengths (typically 10-20 kin) are too large and their lifetimes (>1 hour) too long to result from such an instability [Fritts et al, 1993]. One possible explanation of this type of wave pattern, based on the interference of two ducted short-period band motions exhibiting similar characteristics, but slightly different propagation headings, is discussed in Taylor et al [1995].…”

Two-dimensional spectral analysis of mesospheric airglow image data

“…Fritts et al have investigated the properties of convective instabilities associated with the breakdown of large-scale waves. 31 Their model indicates that short-lived, small-scale waves oriented approximately orthogonal to the large-scale wave can be produced by gravity wave breakdown. This mechanism could possibly explain these data; however the model predicts scale sizes that are significantly smaller ͑ϳ5 km͒ than those observed on this night ͑see Table 2͒.…”