Volume imaging of aspect sensitivity in VHF radar backscatters: first results inferred from the Advanced Indian MST radar (AIR)

Structures and backscattering characteristics of CUSAT 205 MHz Stratosphere-Troposphere Radar at Cochin (10.04 o N, 76.3 o E) -First results

“…Quantitatively a rapid decrease of signal strength at an average of about 1.2 dB per degree till 10˚ tilt and at 0.6 dB per degree beyond that (Tsuda et al,1997a;Anandan et al,2008;Das et al, 2022) are attributed to Fresnel reflection/scattering and anisotropic turbulence up to 10˚ (Gage and Balsley, 1980) and those beyond 10˚ are attributed to Bragg scale isotropic turbulent scattering (Rao et al, 1997) as observed for 49-53 MHz VHF radars. While previous studies focused mostly only on the orthogonal north-south, east-west variation of the echo strength (Damle et al, 1994;Jain et al, 1997;Qing et al, 2018;Ghosh et al, 2004;Das et al, 2008Das et al, , 2016 due to the limitation of experiments, others have found an azimuthal dependence too (Tsuda et al, 1997b;Worthington et al, 1999;Das et al, 2022). The angular variation of the echo strength has been attributed to either diffuse reflection from stable temperature sheet structure or the presence of corrugated sheets or anisotropic turbulence (Das et al, 2014).…”

“…Earlier studies have shown that aspect sensitivity of backscatter echoes can be linked to the presence of a thin stable layer which is effectively a single sharp gradient in radio refractive index or to the presence of shear-generated steep layer structures (Hocking et al, 1986(Hocking et al, , 1990Tsuda et al, 1997a, Das et al,2008. A strong thermal gradient in the vicinity of tropical tropopause (16-18 km) acts like a perfect reflector causing high aspect sensitive VHF radar echoes (Jain et al, 1997;Das et al, 2008Das et al, , 2016Das et al, , 2022. Quantitatively a rapid decrease of signal strength at an average of about 1.2 dB per degree till 10˚ tilt and at 0.6 dB per degree beyond that (Tsuda et al,1997a;Anandan et al,2008;Das et al, 2022) are attributed to Fresnel reflection/scattering and anisotropic turbulence up to 10˚ (Gage and Balsley, 1980) and those beyond 10˚ are attributed to Bragg scale isotropic turbulent scattering (Rao et al, 1997) as observed for 49-53 MHz VHF radars.…”

“…Unlike isotropic scattering, anisotropic scattering mechanisms lead to a deterioration of the signal with increasing off-zenith angle, thus rendering the echoes 'aspect sensitive' (Röttger and Liu, 1978;Jain et al, 1997). Such aspect sensitivity effectively alters the radar parameters such as beam pointing angle and hence the quantities derived from them i.e., underestimation of the horizontal wind (Hocking et al,1990;Damle et al, 1994;Das et al, 2022). Thus, the study of the aspect of sensitivity of such VHF radars is of utmost importance given their use as powerful tools in deciphering winds and turbulence parameters from clear air and during disturbed conditions.…”

“…A strong thermal gradient in the vicinity of tropical tropopause (16-18 km) acts like a perfect reflector causing high aspect sensitive VHF radar echoes (Jain et al, 1997;Das et al, 2008Das et al, , 2016Das et al, , 2022. Quantitatively a rapid decrease of signal strength at an average of about 1.2 dB per degree till 10˚ tilt and at 0.6 dB per degree beyond that (Tsuda et al,1997a;Anandan et al,2008;Das et al, 2022) are attributed to Fresnel reflection/scattering and anisotropic turbulence up to 10˚ (Gage and Balsley, 1980) and those beyond 10˚ are attributed to Bragg scale isotropic turbulent scattering (Rao et al, 1997) as observed for 49-53 MHz VHF radars. While previous studies focused mostly only on the orthogonal north-south, east-west variation of the echo strength (Damle et al, 1994;Jain et al, 1997;Qing et al, 2018;Ghosh et al, 2004;Das et al, 2008Das et al, , 2016 due to the limitation of experiments, others have found an azimuthal dependence too (Tsuda et al, 1997b;Worthington et al, 1999;Das et al, 2022).…”

“…Thus, an understanding of the causative mechanisms for aspect sensitivity has been found in the above-mentioned atmospheric processes. But two salient points from the aforementioned studies are firstly that very few of these have been done to understand the total spatial dependence of aspect sensitivity i.e., both in the off-zenith and azimuthal directions mostly due to the limitation of radar scanning patterns and most of the studies have been performed in the higher and mid-latitudes with only a handful of them being in the tropical low latitude belt, of which most are limited to the 53 MHz radar at Gadanki (Das et al, 2022).…”