Time‐height evolution of intraseasonal oscillations in the tropical lower atmosphere: Multilevel wind observations using UHF radar

Abstract

Signatures of the low frequency intraseasonal oscillations (ISO) and their time‐height evolution in the lower atmosphere are studied using multilevel wind observations made at a tropical Indian site, Gadanki using UHF radar during the period from March 1999 to September 2000. Spectral analysis using Fast Fourier transform (FFT) and wavelet transform showed the presence of oscillations in zonal wind and revealed their evolution in height and time during the observational period. 10–25 day and 30–60 day oscillations are predominantly observed during pre‐monsoon and southwest (SW) monsoon respectively. The amplitude of these oscillations are found to be weak in the lower altitudes and strengthened across the lower Atmospheric Boundary Layer (ABL) and this is quite prominent in the case of 30–60 day oscillations. All these oscillations strengthened at higher altitudes especially above mean ABL height >1 km. It is observed that intense 10–25 day oscillations present during pre monsoon become 30–60 day oscillations as monsoon intensifies. Spectral analysis carried out using NCEP wind for the same period also showed the existence of increasing time scale along SW monsoon season even though 10–25 day and 80–90 day oscillations are found to be relatively weak. Further use of NCEP zonal wind in 2001 and 2002 showed a remarkably different behaviour during the dry year of 2002 with an early development of low‐frequency oscillations prior to the monsoon season.