Amplification of Gravity Waves by the Exothermic Chemistry of the Nighttime Upper Mesosphere and Lower Thermosphere

Abstract

While it is well known that exothermic chemistry and dissipating gravity waves provide the dominant energy inputs continuously driving the nighttime upper mesosphere and lower thermosphere (UMLT), possible effects of exothermic chemical heating on evolving UMLT gravity‐wave dynamics remain largely unquantified. By deriving and combining analytical solutions of diabatic UMLT gravity‐wave dynamics and of wave‐perturbed heating rates for the seven exothermic chemical reactions most relevant to the heat budget of the nighttime UMLT, we show that an analytical thermal rate term captures the first‐order diabatic feedback of wave‐perturbed chemical heating on gravity‐wave dynamics. Although complex in general, the rate coefficient is real to a very good approximation throughout the UMLT and thus acts primarily to either amplify or damp wave amplitudes. Evaluating this coefficient over all seven exothermic reactions reveals that wave‐perturbed exothermic chemical heating amplifies gravity‐wave amplitudes from 80 to 98 km at rates peaking near 1  near 90 km. These amplification rates vary substantially with local time due to strong tidal modifications of the background chemical heating rates. Largest amplification rates arise from monatomic oxygen recombination and from ozone destruction by atomic hydrogen, the latter producing excited hydroxyl emissions used to image UMLT gravity waves from the Atmospheric Waves Experiment (AWE). Accumulated chemical amplification is greatest for those waves with slower vertical group velocities and hence longer residence times in the 80–98 km height interval. Simplified approximations to the exact analytical rate expressions are provided for efficient numerical implementation in models and parameterizations.