Aspect sensitivity in the VHF radar backscatters studied using simultaneous observations of Gadanki MST radar and GPS sonde

Abstract

Abstract. Simultaneous observations made on four days using the MST radar and GPS-sonde at Gadanki (13.5 • N, 79.2 • E), a tropical station in India, are presented to address the aspect sensitivity of radar backscatters observed at different heights. The observations show that wherever stability parameter N 2 is high, vertical shear of horizontal wind is low and Richardson number (R i ) is high, the aspect sensitivity is high indicating that the aspect sensitive radar backscatters are due to thermal structures in the atmosphere. Such a case can be seen very clearly in the upper troposphere and lower stratosphere. At some heights, where N 2 is high, R i is high, but shears are relatively weak, the aspect sensitivity is found to almost disappear, indicating that some amount of shear provides favorable conditions for causing aspect sensitivity. Aspect sensitivity does not occur at all where N 2 is low or negative and R i is low in spite of wind shear being either high or low, indicating that the regions are well mixed and hence turbulent. The study also shows a power difference in the symmetric beams. A case study on this aspect suggests that this asymmetry is due to the tilting of layers by the action of atmospheric waves. There is indication that these waves are generated through Kelvin-Helmholtz-instability (KHI).

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

“…This is in line with the observations in Fig. 4, showing several instances of low values of φ s in the lower troposphere, and with results of Ghosh et al (2004) who used beam-swinging experiments to examine aspect sensitivity with the Gadanki radar and found evidence for high aspect sensitivity at times in the lower troposphere when the static stabilty was high. Even if volume scatter also does occur, it seems that the aspect sensitive echoes dominate the scattered power, at least in an average sense.…”

“…Previous results comparing radar reflectivities with sondes have been published by Ghosh et al (2004). The Gadanki MST radar is a Doppler beam-swing radar with a single receiver.…”

Measurement of Atmospheric Aspect Sensitivity Using Coherent Radar Imaging after Mitigation of Radar Beam Weighting Effect

“…To determine the locations of multiple DOAs, we employed the contour-based approach proposed by Chen et al (2008). Such an estimated aspect angle, however, could be a function of radar beam direction (u T ), owing to the following two reasons: (i) various oblique radar beams may observe some different shapes (or anisotropism) of scatterers, as discussed in the literature (Hocking et al 1990;Hooper and Thomas 1995;Ghosh et al 2004), and even the radar beams are overlapped partly; and (ii) the second term of P(u) in (1) is a Gaussian-type function for describing the aspect sensitivity of scatterers, which may not be suitable enough for observation with a beam direction at a larger tilt angle. The process of estimating an aspect angle from the characteristic P(u) is as follows: use a suitable beamwidth and give various values of u s to (1), and then compute the mean maximum center (DOA) of P(u) for each u s to yield a set of DOAs; one of the computed DOAs is expected to be the closest to the CRI-DOA, and the corresponding value of u s can be regarded as the optimal estimate of the aspect angle.…”

“…Plenty of approaches/techniques using VHF radar have been used to measure the aspect sensitivity of the scatterers, such as the comparison of echo powers between different oblique radar beams Hocking et al 1986;Hooper and Thomas 1995;Ghosh et al 2004), the narrowing effect of the aspect sensitivity on the Doppler spectrum of a vertically pointed radar beam (Chu et al 1990), spaced-antenna correlation (Briggs 1992), wind measured by oblique radar beam and spectral width (determined by the fading time of the autocorrelation function) (Hocking et al 1990), spatial interferometric (SI) method using multiple receivers (Rö ttger and Vincent 1978;Vincent and Rö ttger 1980), echo power variations/distributions of multiple radar beams tilted at different directions (Tsuda et al 1997a,b;Palmer et al 1998b;Worthington et al 1999Worthington et al , 2000, and so on. All the studies demonstrated a general drop-off dependence of echo power on the zenith angle.…”

On the optimum radar beam angle to minimize statistical estimation error of momentum flux using conjugate beam technique

“…q 0 is a measure of Gaussian e-folding half width of the antenna beam as described by the polar diagram of the vertically directed beam which is 1.8°for Gadanki MST radar [Ghosh et al, 2004]. Corrected radial velocities measured by 3°beam angle showed a few high values which were removed and interpolated back.…”