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

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.