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

摘要

Simultaneous observations made on four days using the MST radar andGPS-sonde at Gadanki (13.5° N, 79.2° E), a tropical stationin India, are presented to address the aspect sensitivity of radarbackscatters observed at different heights. The observations show thatwherever stability parameter N is high, vertical shear of horizontalwind is low and Richardson number (R) is high, the aspect sensitivity ishigh indicating that the aspect sensitive radar backscatters are due tothermal structures in the atmosphere. Such a case can be seen very clearly inthe upper troposphere and lower stratosphere. At some heights, where Nis high, R is high, but shears are relatively weak, the aspectsensitivity is found to almost disappear, indicating that some amount of shearprovides favorable conditions for causing aspect sensitivity. Aspectsensitivity does not occur at all where N is low or negative and Ris low in spite of wind shear being either high or low, indicating that theregions are well mixed and hence turbulent. The study also shows a powerdifference in the symmetric beams. A case study on this aspect suggests thatthis asymmetry is due to the tilting of layers by the action of atmosphericwaves. There is indication that these waves are generated throughKelvin-Helmholtz-instability (KHI).