Observed and Navy Global Model Climatologies of Synoptic Disturbances over the Tropical Western Pacific during Northern Winter 1991/92: A Spectral Analysis

Abstract

This study compares the spectral characteristics of tropical synoptic (5-day) disturbances over the western Pacific contained in radiosonde observations and the initial analysis and 48-h forecast of the navy's operational global model during the northern winter of 1991/92. The power spectra of the analysis agree closely with observations, but the forecast shows larger differences. The westward-propagating horizontal structure is well represented in both the analysis and the forecast, although they tend to produce a higher spatial coherency than that determined from the observations. The observed vertical structure, with a very small low-level tilt in the eastern region (east of 140°E) and an eastward tilt throughout the troposphere in the western region (west of 140°E), contrasts sharply with the typical summer structure observed previously. This contrast, when compared with the seasonal difference in vertical mean wind shear due to the Fast Asian monsoons, is consistent with the theoretical effect of the vertical shear on tropical wave structure. The east-west variation of the vertical structure is reproduced by the analysis, but the forecast structure has a weaker vertical organization over areas where a well-organized structure is observed, and a stronger organization to the west where it is not observed. It appears that the model's dynamics produces a downstream propagation that results in a wave structure that is partially artificial.