Abstract
A numerical prediction experiment using an operational Northern Hemispheric prediction Model (8L-NHM) at Japan Meteorological Agency was performed on an upper level cyclone observed in the upper troposphere around 20°N of western North Pacific late July in 1979. Low and high level cloud motion vectors derived from GMS (Geostationary Meteorological Satellite) pictures were the major data sources for the initial analyses.
Four forecast experiments have been performed using 00Z analyses of July 20 to 23 1979. In most cases, the westward motion of the upper level cyclone was fairly well predicted, but its intensity was underestimated. One of the reasons of the poor prediction of the intensities was considered to be due to the defect of the formulation of the diabatic heating process in the model. Despite the deficiency, the prediction seems to be useful for the operational forecast of the upper level cyclone in the Western Pacific.
In order to investigate the mechanism of the westward movement of the cyclone, a method to diagnose the phase velocity of a vortex from the vorticity budget equation was developed. The application of the method revealed that the westward movement could be mostly explained by the advection of vorticity by mean flow. The divergence effect may become more important when the cyclone is in the stage of development or decay.
