Signature of an overturning gravity wave in the mesospheric sodium layer: Comparison of a nonlinear photochemical‐dynamical model and lidar observations

Abstract

In this paper we study the evolution of the sodium layer in the presence of an overturning (or convectively unstable) gravity wave using model simulations and lidar observations. The simulations employ a time‐dependent, nonlinear, photochemical‐dynamical model. The observations are a 9‐day (210‐hour) set of sodium density and temperature lidar measurements from Fort Collins, Colorado (41°N, 105°W). We model the evolution of large‐scale (vertical wavelength of 30 km) and small‐scale (vertical wavelength of 10 km) waves and the associated evolution of the sodium layer. We use filtering methods to identify waves of similar scales in the lidar measurements. The semidiurnal tide is the dominant large‐scale wave in the lidar observations. We present the observed evolution of sodium density, mixing ratio, temperature, potential temperature, and buoyancy period over 24‐hour periods. We find that the model and observations show similar behavior in the evolution of the sodium densities, mixing ratios, and potential temperature in response to large‐ and small‐scale waves. The model and observations indicate that the sodium density perturbation has a more pronounced overturning behavior in the bottomside of the layer than the topside of the layer. The sodium density also has a more pronounced overturning behavior than the mixing ratio and potential temperature. The overturning signatures in the sodium density due to small‐scale waves occur periodically at the wave period even before the wave itself becomes completely unstable. The study suggests that observations of single overturning events in sodium densities should be interpreted with caution and may not indicate complete overturning of a wave.