A Spectral Approach to the Unification of Satellite and Conventional Temperature Data

Abstract

A new method to combine temperature soundings derived from VAS radiance observations with conventional data is proposed. Unlike similar previous attempts, only a portion of the signal contained in the VAS temperature soundings was combined with conventional data. Using the data collected during the AVE/VAS Ground Truth Field Experiment, temperature profiles from rawinsonde and VAS observations were represented by Fourier cosine series, then harmonic-by-harmonic comparisons were performed for collocated pairs as well as the spatial variations of harmonic coefficients. It was found that the zeroth (mean atmospheric temperature) and first harmonies in the VAS temperature profiles represent synoptic-scale variations almost the same as those in the rawinsonde temperature profiles. The second to the fifth harmonics weakly represent the variations, and also showed some discrepancy with rawinsonde soundings, in general. Harmonics higher than the fifth showed large disagreements with the same harmonics in the rawinsonde temperature profiles. The zeroth and first harmonics in the VAS temperature soundings were combined with higher harmonics determined from conventional data to form a unified dataset. The primary purpose of the unified data is to improve data quality over regions of inadequate conventional data. The quality of the unified data was examined by comparing the vertical motion fields derived from a linear balance model with satellite observations of cloud systems. Results for three case studies at 1200 UTC 22, 23 and 24 August 1983 over the western Atlantic showed, in general, that analyses of the unified data were as good as those of NMC data. Two case studies showed some improvements around a developing tropical depression over oceans. It is suggested that if VAS temperature retrievals improve the second harmonic, the potential of the unified data will be even more enhanced over ocean areas where conventional data are restricted.