Vertical variation of low-frequency oscillations in UHF radar derived winds at a tropical Indian station

Abstract

[1] Signatures of the low-frequency intraseasonal oscillations and their vertical variation in the lower troposphere are studied using multilevel wind observations made at a tropical Indian station, Pune (18°32′N, 73°51′E), from wind profiler/UHF radar during the period June 2003 to May 2006. Spectral analysis using fast Fourier transform showed the presence of oscillations (30–40 days and 50–70 days) in daily mean zonal winds and revealed their evolution in height-period structures. The 30–40 days oscillation mode has the largest amplitude compared to the other modes in zonal and meridional winds. All oscillation modes have much less amplitude in the vertical wind component. The high temporal (3 hourly) zonal and meridional wind observations obtained during the southwest monsoon period (June to September, JJAS) during the years 2003, 2004, and 2005 showed a prominent regime of oscillation, namely, 30–40 days and a moderate signature of 10–20 day mode. In all three years it is seen that these oscillations are not occurring as a single-frequency sinusoidal oscillation but are seen as broadband spectrum. The amplitude of these wave modes showed significant vertical variation. The vertical structures of radar-observed intraseasonal oscillations (30–40 days mode) and Tropical Rainfall Measuring Mission derived mean precipitation intensity during the monsoon period (JJAS) during the years 2003–2005 showed a reasonable comparison. The results presented in this study show significant intraseasonal and interannual variability in lower tropospheric winds, which are closely related to the performance of the southwest monsoon.