F. Lott | Searcharxiv

arXiv2019

Study of gravity waves distribution and propagation in the thermosphere of Mars based on MGS, ODY, MRO and MAVEN density measurements

By measuring the regular oscillations of the density of CO$_{2}$ in the upper atmosphere (between 120 and 190~km), the mass spectrometer MAVEN/NGIMS (Atmosphere and Volatile EvolutioN/Neutral Gas Ion Mass Spectrometer) reveals the local impact of gravity waves (GWs). This yields precious information on the activity of GWs and the atmospheric conditions in which they propagate and break. The intensity of GWs measured by MAVEN in the upper atmosphere has been shown to be dictated by saturation processes in isothermal conditions. As a result, GWs activity is correlated to the evolution of the inverse of the background temperature. Previous data gathered at lower altitudes ($\sim$95 to $\sim$150~km) during aerobraking by the accelerometers on board MGS (Mars Global Surveyor), ODY (Mars Odyssey) and MRO (Mars Reconnaissance Orbiter) are analyzed in the light of those recent findings with MAVEN. The anti-correlation between GW-induced density perturbations and background temperature is plausibly found in the ODY data acquired in the polar regions, but not in he MGS and MRO data. MRO data in polar regions exhibit a correlation between the density perturbations and the Brunt-V\"{a}is\"{a}l\"{a} frequency, obtained from Global Climate Modeling. At lower altitude levels (between 100 and 120~km), although wave saturation might still be dominant, isothermal conditions are no longer verified. In this case, theory predicts that the intensity of GWs is no more correlated to background temperature, but to static stability. At other latitudes in the three aerobraking datasets, the GW-induced relative density perturbations are correlated with neither inverse temperature nor static stability; in this particular case, this means that the observed activity of GWs is not only controlled by saturation, but also by the effects of GWs sources and wind filtering through critical levels.

astro-ph.EP↗

arXiv2007

The instability of counter-propagating kernel gravity waves in a constant shear flow

The mechanism describing the recently developed notion of kernel gravity waves (KGWs) is reviewed and such structures are employed to interpret the unstable dynamics of an example stratified plane parallel shear flow. This flow has constant vertical shear, is infinite in the vertical extent, and characterized by two density jumps of equal magnitude each decreasing successively with height, in which the jumps are located symmetrically away from the midplane of the system. We find that for a suitably defined bulk-Richardson number there exists a band of horizontal wavenumbers which exhibits normal-mode instability. The instability mechanism closely parallels the mechanism responsible for the instability seen in the problem of counter-propagating Rossby waves. In this problem the instability arises out of the interaction of counter-propagating gravity waves. We argue that the instability meets the Hayashi-Young criterion for wave instability. We also argue that the instability is the simplest one that can arise in a stratified atmosphere with constant shear flow. The counter propagating gravity waves mechanism detailed here explains why the Rayleigh criteria for shear flow instability in the unstratified case does not need to be satisfied in the stratified case. This illustrates how the Miles-Howard theorem may support destabilization through stratification. A normal mode analysis of a foamy layer consisting of two density jumps of unequal magnitude is also analyzed. The results are considered in terms of observations made of sea-hurricane interfaces.

physics.geo-ph↗