Electromagnetic wave propagation through simulated atmospheric refractivity fields

Abstract

Abstract. Large-eddy simulation (LES) provides three-dimensional, time-dependent fields of turbulent refractivity in the atmospheric boundary layer on spatial scales down to a few tens of meters. These fields are directly applicable to the computation of electromagnetic (EM) wave propagation in the megahertz range but not in the gigahertz range. We present an approximate technique for extending LES refractivity fields to the smaller scales needed for calculating EM propagation at gigahertz frequencies. We demonstrate the technique by computing refractivity fields through 128 3 LES, extending them to smaller scales in two dimensions, and using them in a parabolic equation EM propagation model. At 0.263 GHz the very small scale structure in the extended fields has a negligible effect on the predicted EM levels. At 2 GHz, however, it increases the predicted levels by 15-25 dB. We relate these results to the refractivity structure sampled by EM waves at 0.263 and 2 GHz. We also show that at long range an EM field calculated through an LES-based refractivity field is generally less coherent and significantly weaker than one computed from a "plywood" (i.e., stratified, range-independent) model of the small-scale refractivity field. We give a physical explanation for the differences in the EM fields computed in these two ways. Finally, although the plywood model gives results that fit the EM levels observed in the recent Variability of Coastal Atmospheric Refractivity (VOCAR) experiment, it is not physically realistic. The instantaneous top of the atmospheric boundary layer is known to be sharp and horizontally varying, and we show that using such a top also yields a fit to the VOCAR data. IntroductionThe research discussed here is an interdisciplinary effort between boundary-layer meteorologists and physicists working in electromagnetic (EM) propagation. The purpose of the collaboration was to use large-eddy simulation (LES) methods and a parabolic equation ( Large-Eddy SimulationTurbulence is characterized by three-dimensional, stochastic motions (eddies) of a wide range of spatial scales. These motions generate corresponding stochastic structure in advected scalars such as refractive index. The largest eddies in the turbulent atmospheric boundary layer scale with the integral scale, which is of the order of the boundary-layer depth zi (-1 km). The smallest eddies scale with the Kolmogorov microscale, which is typically of the order of 1 mm. A direct numerical solution of the governing equations for such a turbulent flow is far beyond the present or expected capabilities of computers. 1413

Effect of superrefraction on inversions of radio occultation signals in the lower troposphere

“…The structure of refractivity in the moist troposphere, induced mainly by the structure of water vapor, is complicated and not well known. In some studies of the tropospheric propagation [see Gilbert et al, 1999] they use two ''extreme'' models of irregu- larities: statistically isotropic (based on large-eddy simulations), and horizontally homogeneous (''plywood'') and conclude about their validity on the base of comparison of the modeled and observed signals. In this study we utilize the refractivity spectra obtained from the high resolution tropical radiosondes used by Sokolovskiy [2001a].…”

Clear-Air Effects on Airborne Sensors

“…These effects remain relatively small with the simulated refractivity and turbulence profiles used here, unless we assume very strong range correlation. The most striking effects are obtained for frozen-in-range turbulence, a phenomenon that was already been noticed in [6].…”

“…This assumption may not always be realistic [16], but there is currently no real alternative in the absence of sufficiently resolved computational fluid dynamics models. Hybrid models extending large eddy simulations to subgrid scales have recently been introduced [6,21], but computational requirements are still prohibitive for the very large domains required for our application.…”

“…In particular it is not known whether atmospheric turbulence enhances or mitigates radar holes caused by atmospheric layering. Although some work has been carried out on this topic for ground-based sensors [8,6], very little information is available so far on the airborne case, mainly because of the high computational burden associated with it. Here we use parabolic equation (PE) techniques [11] to model propagation in a stochastic medium, using phase-screen techniques along the lines of [22].…”

A new phase-screen method for electromagnetic wave propagation in turbulent flows using large-eddy simulation

“…The only attempt in this regard was made in [23], in which the authors used the LES data to study two-dimensional (2D) propagation of EM waves in gigahertz range. They used the Fourier based sub-grid regeneration technique of [24], to extend the resolved field of refractive index to higher grid resolution (hence P1 is resolved), and used it to study the propagation L 0H Average value of L 0 in the propagation domain k…”