Observed and Navy Operational Global Model Climatologies of Synoptic Disturbances Over the Tropical Western Pacific During Winter 1991-92

Abstract

This study compares the structure and characteristics of tropical synoptic timescale wave disturbances over the western Pacific, particularly around the Caroline Islands where six upper air stations are present, as contained in radiosonde observations and the initial analysis (tau00) and 48-h forecast (tau48) of the Navy Operational Global Atmospheric Prediction System (NOGAPS) during northern winter of 1991-1992. The observed, analyzed and forecast disturbance climatologies are all determined from power spectral and cross-spectral analyses focusing on the synoptic frequency band encompassing three adjacent period points: 4.6-, 5.3- and 6.4-day. The meridional wind at 850 hPa is used as the principal parameter for defining the disturbances. The power spectra of the NOGAPS rau00 analysis agree reasonably closely with the radiosonde observations, but the tau48 forecast shows larger differences. Nevertheless, the observed horizontal structure of the disturbances are all well represented in both the analysis and the forecast at all levels. These include the westward propagation, a 850 hPa wavelength near 33 deg along 10 deg N and near 38 deg along the equator, a 200 hPa wavelength near 50-60 deg, and a northeast-southwest meridional tilt. In general, the analysis and the forecast produce a higher spatial coherency than that shown from the radiosonde observations.