Aspect sensitivity of the received radar backscatter at VHF: preliminary observations using the Indian MST radar

Abstract

The main mechanisms that give rise to atmospheric radar echoes at VHF are isotropic, anisotropic backscattering and Fresnel reflection/scattering. Anisotropic turbulence and Fresnel reflection/scattering make the radar echo aspect sensitive, which in turn influences the beam pointing angle and beam width and thereby the determination of horizontal wind components and turbulence parameters. Therefore a series of experiments are conducted on 4 days, using the newly established Indian MST radar, located at Gadanki and operating at 53 MHz, to study the aspect sensitivity in the troposphere-stratosphere region. Results of these experiments have shown that aspect sensitivity is more prominent between the altitude range of 15 and 21 km, and the same is noticed to be reduced above 21 km. Results also show the presence of scatterers with different horizontal correlation lengths. Aspect sensitivity measurements for different beam combinations show significant differences for two orthognal planes, i.e., E-W and N-S, indicating the azimuthal anisotropy of the scatterers. The effect of aspect sensitivity on beam pointing angle and wind measurements is estimated. For beams with small zenith angle (i.e., ~4°), the underestimation of winds can be as much as 30%, whereas for larger zenith angles (≥8°), the same would be ≤5%. Results presented thus give some insight into the scattering processes and also yield valuable information on the structure of the scatterers.