Author:
De-Leon Yair,Paldor Nathan
Abstract
Abstract. This work examines the accuracy and validity of two
variants of Radon transform and two variants of the two-dimensional fast
Fourier transform (2-D FFT) that have been previously used for estimating the
propagation speed of oceanic signals such as sea surface height anomalies
(SSHAs) derived from satellite-borne altimeters based on time–longitude
(Hovmöller) diagrams. The examination employs numerically simulated
signals made up of 20 or 50 modes where one, randomly selected, mode has a
larger amplitude than the uniform amplitude of the other modes. Since the
dominant input mode is known ab initio, we can clearly define “success” in
detecting its phase/propagation speed. We show that all previously employed
variants fail to detect the phase speed of the dominant input mode when its
amplitude is smaller than 5 times the amplitude of the other modes and that
they successfully detect the phase speed of the dominant input mode only
when its amplitude is at least 10 times the amplitude of the other modes.
This requirement is an unrealistic limitation on oceanic observations such
as SSHA. In addition, three of the variant methods detect a dominant
mode even when all modes have the exact same amplitude. The accuracy with
which the four methods identify a dominant input mode decreases
with the increase in the number of modes in the signal. Our findings are
relevant to the reliability of phase speed estimates of SSHA observations
and the reported “too fast” a phase speed of baroclinic Rossby waves in the
ocean.
Subject
Cell Biology,Developmental Biology,Embryology,Anatomy