VHF radar observation of atmospheric winds, associated shears and Cn at a tropical location: interdependence and seasonal pattern

The turbulence refractivity structure constant (Cn) is an important parameter of the atmosphere. VHF radars have been used extensively for the measurements of Cn. Presently, most of such observations are from mid and high latitudes and only very limited observations are available for equatorial and tropical latitudes. Indian MST radar is an excellent tool for making high-resolution measurements of atmospheric winds, associated shears and turbulence refractivity structure constant (Cn). This radar is located at Gadanki (13.45 N, 79.18 E), a tropical station in India. The objective of this paper is to bring out the height structure of Cn for different seasons using the long series of data (September 1995 – August 1999) from Indian MST radar. An attempt is also made to understand such changes in the height structure of Cn in relation to background atmospheric parameters such as horizontal winds and associated shears. The height structure of Cn, during the summer monsoon and post-monsoon season, shows specific height features that are found to be related to Tropical Easterly Jet (TEJ) winds. It is important to examine the nature of the radar back scatterers and also to understand the causative mechanism of such scatterers. Aspect sensitivity of the received radar echo is examined for this purpose. It is observed that radar back-scatterers at the upper tropospheric and lower stratospheric heights are more anisotropic, with horizontal correlation length of 10–20 m, as compared to those observed at lower and middle tropospheric heights.