Space-Time Spectral Analysis of 2-D Signal on the Globe Using Spherical Harmonics and Wavelet Transform Methods

Abstract

In this research, a method to analyze wave-number and frequency spectra from time series of global 2-D geophysical data has been developed. Spherical Harmonics (SH) transform method is used to analyze wave structures in both longitude and latitude, while the temporal evolution of frequency content in the data is analyzed using complex wavelet transform. The frequency spectra of longitudinally propagating waves are separated into their westward- and eastward components by wavelet transforms of the corresponding to complex SH coefficients and their complex conjugate. The method was applied to analyze daily OLR (Outgoing Long-wave Radiation) data of 27 years record (1987-2013) as a proxy for large-scale convective pattern. It proved that the method enables us to identify the existence of various wave structures representing the convectively coupled planetary atmospheric waves. As SH and wavelet transform methods are generic tools for signal analysis, our space-time spectral analysis method should also be applicable to other geophysical data whose properties are characterized with embedded planetary scale waves.