Experimental Evaluation of Theoretical Formulations for the Correction of Spectral Widths of MST Radar Spectra

Abstract

The turbulent kinetic energy (TKE) per unit mass estimated from the spectral width of very high frequency/ultra high frequency (VHF/UHF) radars often produce unrealistic negative values, particularly in the presence of strong winds, due to excess correction of nonturbulent factors. An experiment has been conducted with the newly acquired capabilities of the Advanced Indian MST Radar (AIR), in which several antenna configurations, providing different beamwidths, have been employed to evaluate various existing theoretical formulations for the estimation of nonturbulent factors. These formulations include traditional single-beamwidth, symmetric dual-beamwidth, and asymmetric beamwidth methods. The large variation is seen in the observed spectral widths with different antenna apertures with broader beams showing larger biases due to beam and shear broadening even in moderate winds. Wind-driven biases in spectral width are found to be larger for beams pointed perpendicular to the wind direction than those pointed parallel, particularly at heights of strong wind. After employing the above-mentioned correction methods, all profiles of mean spectral widths obtained with different antenna configurations converge and produce nearly equal values, indicating that the corrected spectral width ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sigma ^{2}_{\mathrm{ turb}}$ </tex-math></inline-formula> ) values may represent realistic estimates of turbulence intensity. Comparison of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sigma ^{2}_{\mathrm{ turb}}$ </tex-math></inline-formula> estimated using east and south beams indicates that the turbulence (on average) is isotropic. Among all the correction methods, the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sigma ^{2}_{\mathrm{ turb}}$ </tex-math></inline-formula> obtained by employing the asymmetric dual-beamwidth method has relatively larger bias than other methods. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sigma ^{2}_{\mathrm{ turb}}$ </tex-math></inline-formula> values are used to estimate the vertical eddy diffusion coefficients and eddy dissipation rates to have direct comparisons with those available in the literature.