Time-frequency analysis of UHF and VHF profiler observed winds over the central equatorial Pacific

Much of the precipitation over the Tropics is organize by atmospheric waves, but there are few detailed observations of the vertical structure of these waves. This study using three years of wind observations from collocated UHF and VHF wind profilers in the Central Equatorial Pacific documents the dominant waves and their vertical structures.

UHF and VHF wind profiler data at Christmas Island are combined using a new processing technique (Coplanar Spectral Averaging; CSA) that gives an observation height range of about 0.3 to 18 km above ground level. CSA increases the dectectability of the signal through averaging spectra over time from coplanar beams (Schafer et al. 2004). CSA also improves the agreement between the UHF and VHF profiler observations as it reduces differences associated with spatial variability by averaging over two spatially separated beams.

Time-frequency analysis techniques including sliding FFTs and lagged-regression are applied to the profiler data set to determine the dominant periodicities and vertical structure of the wind field. These reveal significant signals in the frequency ranges associated with Kelvin waves, mixed Rossby gravity waves, and Rossby waves. Further analysis using lagged-regression of the profiler filtered winds against directionally filtered OLR and NCEP/NCAR reanalysis fields allows determination of type of wave associated with the wind profiler observed signal (e.g. Wheeler and Kiladis, 1999)

Schafer, R., S. K. Avery, K. S. Gage, P. E. Johnston, and D. A. Carter, 2004:Improving wind profiler-measured winds using coplanar spectral averaging. J. Atmos. Oceanic Technol. , 21,1671-1678.

Wheeler, M., and G. N. Kiladis, 1999: Convectively Coupled Equatorial Waves: Analysis of Clouds and Temperature in Wavenumber-Frequency Domain. J. Atmos.Sci., 56, 374-399.