Computation of clear‐air radar backscatter from numerical simulations of turbulence: 3. Off‐zenith measurements and biases throughout the lifecycle of a Kelvin‐Helmholtz instability

Abstract

[1] Previous papers by Franke et al. (2011) and Fritts et al. (2011) described the computation of radar backscatter power and vertical velocities from numerical simulations of turbulence arising due to Kelvin-Helmholtz (KH) shear instability. Comparisons of backscatter power and inferred velocities with the distributions of turbulence and the true velocities revealed biases in the identification of active or intense turbulence and in the inferred Doppler spectrum and vertical velocities throughout the flow evolution. This paper extends these analyses to off-zenith viewing angles typical of multiple-beam MF, HF, and VHF radars. These reveal similar biases in the identification of turbulence occurrence, Doppler spectra, and inferred radial velocities, with additional sensitivity to the off-zenith angle relative to the mean shear across the turbulence layer. Radial velocities are typically underestimated during turbulence generation and breakdown of the KH billows, except where turbulence refractive index gradients are strong. Doppler spectra are biased toward regions retaining strong refractive index gradients, implying strong aspect sensitivity at later stages in the evolution. Persistent tilted structures at late stages of the evolution contribute to radial velocity measurement biases that also are functions of off-zenith angle and time.

Derivation of turbulent energy dissipation rate with the Middle Atmosphere Alomar Radar System (MAARSY) and radiosondes at Andøya, Norway

“…Kelvin-Helmholtz instabilities occurring under conditions of wind shear are well known to cause turbulence (e.g. Fritts et al, , 2012. The statistical results reveal that the turbulent energy dissipation rate does indeed follow the dependence of the corresponding wind shears.…”

“…Hence the wind shears play an important role (through the Kelvin-Helmholtz instability; e.g. Fritts et al, , 2012 in the generation and/or maintenance of turbulence.…”

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.…”

Investigation of a mesospheric gravity wave ducting event using coordinated sodium lidar and Mesospheric Temperature Mapper measurements at ALOMAR, Norway (69°N)

“…A number of studies have also addressed the specific dynamics needs noted above. These include estimates of MLT GW momentum fluxes employing various techniques [e.g., Vincent and Reid, 1983;Fritts and Vincent, 1987;Reid and Vincent, 1987;Reid et al, 1988;Meyer et al, 1989;Tsuda et al, 1990;Wang and Fritts, 1990;Hitchman et al, 1992;Murphy and Vincent, 1993;Nakamura et al, 1993;Swenson et al, 1999;Gavrilov et al, 2000;Espy et al, 2004Espy et al, , 2006Suzuki et al, 2007;Antonita et al, 2008;Fritts et al, 2010Fritts et al, , 2012. Other studies yielded direct evidence for local instabilities suggesting GW "breaking," dissipation, and energy and momentum deposition [e.g., Swenson and Mende, 1994;Hecht et al, 1997;Yamada et al, 2001;Xu et al, 2006;Li et al, 2005Li et al, , 2007.…”