Kelvin-Helmholtz billows and their effects on mean state during gravity wave propagation

G. Chen,J. Xu,H. Gao,X. Liu

doi:10.5194/angeo-27-2789-2009

Abstract

Abstract. The Kelvin-Helmholtz (KH) billows which appear in the process of gravity wave (GW) propagation are simulated directly by using a compressible nonlinear two-dimensional gravity wave model. The differences between our model and others include: the background field has no special initial configuration and there is no initial triggering mechanism needed in the mesosphere and lower thermosphere (MLT) region to excite the KH billows. However, the initial triggering mechanism is performed in the lower atmosphere through GW, which then propagate into the MLT region and form billows. The braid structures and overturning of KH billows, caused by nonlinear interactions between GWs and mean flow, can be resolved precisely by the model. These results support the findings in airglow studies that GWs propagating from below into the MLT region are important sources of KH billows. The onset of small scale waves and the wave energy transfer induce the shallower vertical wave number power spectral densities (PSD). However, most of the slopes are steeper than the expected kz−3 power law, which indicates that GWs with 10 km vertical wavelength are still a dominant mode. The results also show that the evolution of mean wind vary substantially between the different processes of GWs propagation. Before the KH billows evolve, the mean wind is accelerated greatly by GWs. By contrast, as the KH billows evolve and mix with mean flow, the mean wind and its peak value decrease.

Highlights

Numerical simulations and observational results indicate that Kelvin-Helmholtz (KH) billows are an important phenomenon in the atmosphere and have significant mixing effects on the mean atmospheric structures and species (e.g. Fritts et al, 2003; Hecht, 2004; Hecht et al, 2005)
The models developed by Klaassen and Peltier (1985a, b, 1991) and Hill et al (1999) and Werne and Fritts (1999) were all performed using the shear horizontal wind and temperature background fields, and the KH billows were excited by a small amplitude stochastic perturbation
Despite the fact that the upper part of the wave packet evolved into KH billows, the mean wind below 96 km is still accelerated by the lower part of unstable gravity wave (GW) and results in the increase in peak value

Summary

Introduction

Numerical simulations and observational results indicate that Kelvin-Helmholtz (KH) billows are an important phenomenon in the atmosphere and have significant mixing effects on the mean atmospheric structures and species (e.g. Fritts et al, 2003; Hecht, 2004; Hecht et al, 2005). The initial background field has a special configuration, and an initial triggering mechanism to excite KH billows In these situations, it is convenient to study the evolution of KH billows and their interactions with mean flow. Numerical simulations have shown that the nonlinear interactions between GWs and mean flow can cause very large shear wind in the MLT regions even though the background atmosphere is initially at rest We speculate that GWs are capable of generating KH billows as they propagate in an initial windless background atmosphere If this hypothesis is true, non-linear interactions between GWs and initial zero mean flow are an important physical mechanism to generate KH billows in the MLT region.

Numerical model and initial conditions

The evolution of KH billows

The evolution of GW spectrum

The evolution of mean state

Conclusions