Computation of clear-air radar backscatter from numerical simulations of turbulence: 2. Backscatter moments throughout the lifecycle of a Kelvin-Helmholtz instability

Abstract

[1] Franke et al. (2011) describe a numerical simulation of the instability and turbulent breakdown of Kelvin-Helmholtz (KH) billows at a high Reynolds number, numerical assessment of radar backscatter, and accuracies of inferred Doppler spectral moments for one test volume. Those results suggest a potential for significant measurement biases for radars that obtain backscatter from refractive index fluctuations. We present in this paper the morphology of computed radar moments throughout the KH instability lifecycle for two radar configurations in order to reveal the evolving character of radar backscatter and compare the radar velocity estimates with true velocities throughout the evolution, and to provide guidance, and cautions, for the interpretation of these dynamics in observational data. Results reveal strong variations in backscatter moments and character, and dependence on radar measurement parameters, that should be beneficial in the interpretation of such measurements in the atmosphere. Backscatter power predictions agree reasonably with observations of such events and their temporal evolutions. Our results also reveal a potential for significant measurement or sensitivity biases, some of which were predicted previously. Examples include a lack of significant backscatter power in well-mixed billow cores, suggesting possibly weak turbulence where in fact it may be strongest, maximum backscatter power in the billow exteriors, where refractive index fluctuations are large but turbulence is weak, underestimated vertical velocities within the KH billows at early times, and inferred significant vertical velocities where true vertical velocities are near zero at late stages of restratification, especially in the edge regions of the turbulence layer.

Aspect sensitivity of polar mesosphere summer echoes based on ESRAD MST radar measurements in Kiruna, Sweden in 1997–2010

“…Rapp and Lübken, 2004). Intensive modelling studies of turbulence generated at mesopause altitudes showed anisotropy of the turbulence fields and radar backscatter (Gibson-Wilde et al, 2000;Fritts et al, 2003Fritts et al, , 2009Fritts et al, , 2011. However, to our knowledge there is no quantitative estimation of aspect sensitivity for such turbulence so far (perhaps with the exception of Hocking and Hamza (1997) who analytically considered anisotropic turbulence due to wind-shear only).…”

Quantifying Kelvin‐Helmholtz instability dynamics observed in noctilucent clouds: 2. Modeling and interpretation of observations

“…Our initial high‐resolution numerical studies of KHI [ Werne and Fritts , , ; Fritts et al ., ; Werne et al ., ] employed a pseudo‐spectral DNS code that solves the nonlinear Navier‐Stokes equations subject to the Boussinesq approximation in a Cartesian domain to explore the dependence of these dynamics and effects for various environmental parameters. A LES extension of the DNS modeling capability allowing application to a higher Reynolds number was also employed to explore the nature and implications of radar backscatter from KHI structures in the atmosphere [ Franke et al ., ; Fritts et al ., , ]. Here we summarize these methods and illustrate some of the results that are pertinent to our application to KHI dynamics observed in NLC images.…”

Comparisons between high-resolution profiles of squared refractive index gradient <i>M</i><sup>2</sup> measured by the Middle and Upper Atmosphere Radar and unmanned aerial vehicles (UAVs) during the Shigaraki UAV-Radar Experiment 2015 campaign

“…We note that M 2 UAV and M 2 radar agree well in the turbulent layer during UAV7 (descent). Radar echoes are now at a minimum, and this feature may indicate the late decay stage of turbulence (e.g., Fritts et al, 2011). 3. M 2 UAV ≤ M 2 radar in convective clouds.…”