Collaborative Research: Numerical Evaluation of Radar Backscatter from KH Instability and Gravity Wave Breaking for Realistic Radar Parameters and Configurations

University Of Illinois At Urbana-Champaign, Urbana IL

Investigators

Abstract

The goal of this project is to identify the sources and effects of some of the systematic biases that occur in radar measurements of neutral and plasma dynamics in the atmosphere and provide a means to correct for their effects. This will be done by using simulated radar backscatter in high-resolution numerical simulations of turbulence processes for realistic atmospheric sources. In previously funded research, a method was developed to assess radar backscatter from a modeled turbulence field that was computed by direct numerical simulations (DNS) of Kelvin-Helmholtz (KH) instability at high Reynolds numbers. The method makes no assumptions about the character of radar backscatter (turbulent, specular, or other). The method was tested with, and applied to, very high resolution DNS of KH billow evolution, turbulent breakdown, and restratification in order to aid in the interpretation of, and the potential measurement biases induced by, radar measurements of such dynamics throughout the lower and middle atmosphere. These potential biases in radar estimates may have important implications for the radar measurements of both the small-scale dynamics and the mean quantities, among them mean horizontal and vertical velocities, wind shears, and turbulence intensities and dissipation rates. This project will extend the radar backscatter computations to a number of other cases, including both zenith and off-zenith measurements of KH instability for a range of Richardson numbers, given their very different dynamics, turbulence evolutions, and likely implications for mixing and measurements, and both zenith and off-zenith measurements accompanying turbulence due to gravity wave (GW) breaking for a range of GW parameters, superpositions, and mean wind environments. The KH instabilities are expected to exhibit specular reflections that bias horizontal and vertical velocity estimates and depend on radar frequency and beam width, and spectral widths (often used to estimate energy dissipation rates) that are difficult or impossible to quantify due to turbulence impacts on refractive index fluctuations and distributions. GW applications will examine the impacts of turbulence and refractive index fluctuations that are known to vary strongly within the GW field and with time on radar measurements of GW parameters, including amplitudes, momentum fluxes, dissipation rates, mixing, and heat fluxes, all of which have substantial, but poorly quantified, impacts on atmospheric structure and variability, as well as implications for modeling and parameterization. Undergraduate and graduate students will participate in the project as well as postdoctoral scholars. The broader impacts of the research are that it will provide an improved understanding of radar backscatter and will enable correction of measurement biases. This may lead to improvements in quantifying the dynamics underlying turbulence generation, spectral evolution, and mixing and transport processes in geophysical fluids and better constraints on parameterizations of small-scale GW and turbulence effects in atmospheric (and oceanic) circulation, weather, and climate models.