This study summarizes advances in radar backscattering from refractive index irregularities in the clear air and its relationship to atmospheric structure and turbulence. This recent discipline of radar meteorology originated from radar angel studies and the efforts to explain these mysterious echoes. The significance of specular atmospheric radar reflections now appears limited, and consequently this study is devoted entirely to radar backscattering from turbulent fluctuations in the clear‐air refractive index. Two regimes of atmospheric turbulence are distinguished: (1) in the convective domain, backscattering from water vapor fluctuations outlines regular, three‐dimensional clear‐air structures associated with buoyant, moist air parcels; (2) in the stable regime, horizontally stratified clear‐air echoes originate from water vapor and/or temperature perturbations due to wind‐shear‐generated turbulent mixing in zones of enhanced static stability. Theoretical relationships are given between the radar reflectivity and the refractive index microstructure for the general case, for the isotropic case, and for the inertial subrange. The connection between the microstructure and the atmospheric mean fields is illustrated for a simplified situation in the stable regime; this relationship provides insight to the radar backscattering from clear‐air turbulence (CAT). It is concluded that strong CAT will be associated with zones of increased refractive index variability and enhanced radar returns. A brief résumé of experimental work illustrates the application of radar methods in clear‐air research and summarizes characteristic features of clear‐air structures.

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Radar backscattering from the turbulent clear atmosphere is determined by the small‐scale variability in the radio refractive index and is therefore related to the mean gradient of potential refractive index and to the degree of turbulence. Consequently, the mean gradient of potential refractve index is considered in works concerned with radar detection of CAT (clear air turbulence). This note clarifies the role of potential quantities and their vertical gradients in quantitative discussions of the generation of small‐scale atmospheric inhomogeneities by turbulent mixing. If potential quantities are referred to a standard pressure of 1000 mb, as is common, the mean vertical gradient of potential refractive index cannot be used directly to evaluate the small‐scale refractive‐index variability resulting from adiabatic mixing. It is convenient to define a generalized potential refractive index, and a generalized potential temperature as potential quantities referred to the mean pressure at the level under consideration. The refractive‐index inhomogeneities produced by turbulent mixing are directly related to the mean vertical gradient of generalized potential refractive index (generalized potential temperature if specific humidity is negligible). The terminology already adopted in several works on radar detection of CAT is preserved if it is emphasized that the potential quantities employed are generalized. Errors of several decibels may result if the ordinary potential quantities referred to 1000 mb are used mistakenly to determine the smale‐scale refractive‐index variability.

Radar backscattering from the turbulent clear atmosphere is caused by irregular small‐scale fluctuations in the radio refractive index produced by turbulent mixing. This note considers the theoretical relationships between the refractive‐index microstructure and the back‐scattering at radar wavelengths from meters to centimeters. Spectral relationships are clarified and the basic difference between radar sampling and refractometer sampling of refractivity spectra is explained. Theoretical expressions are given for the relationship between the radar reflectivity and spectral representations of the refractive‐index variability for the general case, for the isotropic case, and for the inertial subrange.

Abstract. This paper reviews the remote sensing of waves and turbulence in statically stable atmospheric layers, utilizing sodar and microwave radar echoes from the small-scale inhomogeneities in gaseous refractive index caused by localized fluctuations in temperature, humidity, and velocity. Scattering theory and sounding methodology are reviewed briefly, and the relative performance of typical radar and sodar systems compared.The main section of the paper takes the form of a summary and discussion of experimental progress since 1969, showing how the echo patterns obtained may be applied to the interpretation of multiple layering, gravity waves, internal fronts and the details of dynamic instability and the genesis of turbulence in stably stratified shear layers. In addition, methods for the measurement of the intensity of the small-scale (~ ~./2) variability of wind, temperature and water vapor from the observed radar or sodar echo intensities, and the use of Doppler techniques for the measurement of mean velocity and turbulence are discussed.