Fresnel scatter revisited – comparison of 50 MHz radar and radiosondes in the Arctic, the Tropics and Antarctica

Abstract

Abstract. High-resolution radiosondes and calibrated radars operating close to 50 MHz, are used to examine the relationship between the strength of radar scatter and refractive index gradient. Three radars are used, in Kiruna in Arctic Sweden, at Gadanki in southern India and at the Swedish/Finnish base Wasa/Aboa in Queen Maud Land, Antarctica. Calibration is accomplished using the daily variation of galactic noise measured at each site. Proportionality between radar scatter strength and the square of the mean gradient of potential refractive index, M 2 , is found in the upper troposphere and lower stratosphere at all three sites, confirming previously reported results from many VHF radars. If the radar scatter is interpreted as Fresnel scatter, the constant of proportionality between radar scatter and M 2 is found to be the same, within the calibration uncertainties, for all three radars. The radiosondes show evidence of distinct layering with sharp gradients, extending over 10s of kilometers horizontally, but the scatter is found to be two orders of magnitude weaker than would be expected from true Fresnel scatter from such layers. Using radar reflectivities resolved to a few 100 ms, we show that this is due to strong temporal variability in the scattering conditions, possibly due to undulations of the scattering layers. The constancy of the radar scatter -M 2 relationship between the different sites suggests an unexpected uniformity in these perturbations between very different regions of the globe.

Polar mesosphere summer echo strength in relation to solar variability and geomagnetic activity during 1997–2009

“…Each block consists of 12 kW power and feeds a separate 1/6th of the antenna array, so such dropouts also affect A e (for transmission). In order to correctly estimate η, allowing for major dropouts in P t and possible changes in L, we use the result found by Kirkwood et al (2010a) concerning radar-radiosonde comparisons in the upper troposphere/lower stratosphere (UTLS) region. For the UTLS region, Kirkwood et al (2010a) showed that Fresnel scatter, as quantified by the normalised power reflection coefficient ρ 2 / r, is proportional to M 2 , where M is the dry term of mean vertical gradient of generalized potential refractive index.…”

Aspect sensitivity of polar mesosphere summer echoes based on ESRAD MST radar measurements in Kiruna, Sweden in 1997–2010

“…(4) for various values of aspect sensitivity for the actual ESRAD transmitted beam width. 6 rectangular antenna subarrays were used for reception (each 6 × 8 Yagi antennas, 4 × 6 before 2004), with mean beam width θ R of 6 • (9 • before 2004) as described in more detail by Kirkwood et al (2010). In Fig.…”

“…Moreover, isotropic (turbulent) and Fresnel scattering mechanisms lead to very different cross-sections (e.g. Kirkwood et al, 2010). Therefore in the cases of isotropic and aspectsensitive PMSE, estimation of their cross-sections using observations requires quite different calculations to account for radar characteristics (for a given echo power at the radar receiver, volume reflectivity of isotropic scattering depends on the antenna effective area, whereas Fresnel reflectivity depends on the square of that area).…”

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.…”