Atmospheric Structure and Radar Backscattering in Clear Air

Abstract

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.

Observation of Clear-Air Echoes with 3.2-cm Radars

“…This conclusion has been supported by other experiments using ultrasensitive radars and aircrafts Konrad et al, 1968;Kropfli et al, 1968;Konrad and Robinson, 1972). It has been made clear by many authors that the fluctuations of refractive index detected as clear-air echoes were related to thin stable inversions, convective thermals, Benard convection cells, breaking gravity waves, high tropospheric layers which are sufficiently turbulent and sea breeze front (Hardy and Ottersten, 1969;Hardy and Katz, 1969;Ottersten, 1969;Atlas et al, 1970a;Konrad, 1970;Gossard et al, 1971;Konrad and Robinson, 1973;Noonkester, 1976;Yanagisawa et al, 1978). And it has been suggested that dot echoes might be usable as tracers indicating atmospheric motions (Browning and Atlas, 1966;Lhermitte, 1966;Atlas et al, 1970b;Richter et al, 1973).…”

“…Wavelike clear-air echoes have been observed in other countries with 10cm or UHF radars (Hardy and Katz, 1969;Ottersten, 1969;Atlas et al, 1970a;Gossard et al, 1971). In the Fig.…”

Comparisons between high-resolution profiles of squared refractive index gradient <i>M</i><sup>2</sup> measured by the Middle and Upper Atmosphere Radar and unmanned aerial vehicles (UAVs) during the Shigaraki UAV-Radar Experiment 2015 campaign

“…However, soon after the development of ST radars, concurrent measurements of atmospheric parameters from balloons launched near radar sites have been made for interpreting the radar echoes, with some notable successes (e.g., Röttger and Larsen, 1990;Gage, 1990). In particular, a large number of studies have reported a close relationship between the echo power P v at vertical incidence (hereafter, P v will refer to the echo power corrected from the range attenuation effects by the product, i.e., P × z 2 , where P is the echo power and z is the altitude) and the square of the vertical gradient of the generalized potential refractive index M 2 (Ottersten, 1969) in clear-air conditions, calculated from balloon data at the vertical resolution of the radar measurements (e.g., Röttger, 1979;Röttger, 1983, 1985;Tsuda et al, 1988;Hocking and Mu, 1997;Hooper and Thomas, 1998;Low et al, 1998;Hooper et al, 2004;Vaughan et al, 1995;Luce et al, 2007;Kirkwood et al, 2010). But radiosonde measurements provide "instantaneous" values only along the path of the balloon, which tends to move as much as a few tens of kilometers or more away from the radar due to wind drift.…”

Profiles of radio refractive index and humidity derived from radar wind profilers and the Global Positioning System

“…The close relationship between radio refractive index (RI) gradients with height and the distribution of Bragg backscattered power with height has been noted for decades; for example, see Figure 1, taken from Richter and Gossard [1970] [see also Ottersten, 1969;Bean et al, 1971;Chadwick and Gossard, 1983]. The RI at microwave frequencies depends mainly on temperature and humidity (and weakly on pressure) [e.g., Bean and Dutton, 1966].…”