Impact of Increased TOVS Signal on the NMC Global Spectral Model: A Tropical-Plume Case Study

Abstract

Information exists in tropical operational TIROS Operational Vertical Sounder (TOVS) radiance observations that is not utilized by operational sounding retrieval and analysis-initialization schemes. Temperature and moisture signals are extracted from operationally available TOVS channel radiances for a tropical plume that developed over the North Pacific in January 1989, and are inserted into the National Meteorological Center's (NMC) operational medium-range forecast model. Several 48-h forecasts are made to test the impact of the additional information. A positive impact, at least for this single case study, is shown in comparisons with satellite imagery, historical NMC forecasts, and verification analyses not only for the tropical plume but for a second tropical system evolving to the west. The patterns of corrections of 200-mb heights and winds and 850-mb winds at 48 h are qualitatively similar to 48-h historical forecast errors; amplitudes of corrections are only one-half as large as the historical errors. Perturbed upper-tropospheric relative humidity patterns are smoother and more consistent with satellite imagery than is the historical forecast. Temperature perturbations have minimal effect on the forecast unless the moisture fields are perturbed as well. Most of the evolution can be explained in terms of geostrophic adjustment of the initial unbalanced perturbations and of enhanced upward vertical motion in regions of increased relative humidity.