Implications of global atmospheric spatial spectra for processing and displaying data

The information available on different scales in the atmosphere for a number of different variables is explored using the global ECMWF analysis by examining the spatial spectra at T106 resolution. The widespread practice of using a coarse grid without the appropriate truncation or smoothing first can result in unresolved scales being aliased and excessively noisy fields; an example is the 2.5° gridded fields made available by ECMWF. Appropriate procedures are described for truncating the T106 ECMWF spectral archive for scalar and vector fields. T42 resoltuion is an adequate representation for diagnostic calculations depicting most quantities within a few percent accuracy, although some spatial structure, which may partly be noise, is lost for the ω, divergence, and moisture fields. In contrast, errors greater than 10% can occur at T21 or R15 resolution, although these truncations can be useful for displaying results. -from Authors