張利雄

An Objective Analysis in the Vorticity and Divergence Fields in the Vicinity of Taiwan

Lihsiung A. Chang

Abstract

　　By using the Bellamy (1949) method, objective calculations on the horizontal velocity divergence and vorticity have been done for four different triangle areas and for seven isobaric surfaces in the troposphere, for the season of 1971 summer. Vertical velocity are also kinematically computed for different levels for each triangle area.

　　The estimated divergence, vorticity and vertical motion have order of magnitude comparable to that of the synoptic scale system, except for that of the Taoyuan-Makung-Tungkong triangle which has only few hundred kilometers in extent.

　　The fluctuation spectra show two major periodicities in which 5-7 day fluctuation has been selected for intensive studies. Divergence has maximum amplitude of fluctuation around 200mb and a secondary peak around 850mb・ Consistent out-of-phase relationship between the upper level and the lower level fluctuations are observed for all triangle areas. Vertical motions has consistent phase relation among different levels and a maximum amplitude of fluctuation around 300mb.

　　The inter-triangle cross-spectral analysis shows very good coherence between different triangle areas in their divergence and vertical motion series for the 5-7 day disturbances. The phase relationship shows these waves have horizontal wavelength of about 3000km and propagating westward as well as northward in this region.

　　The inter-parameter cross-spectral analysis also shows that divergence and vertical motion at 300mb are good indicaters for weather fluctuation in the tropical-subtropical area. They are in good coherence with temperature and relative humidity. Different vertical profiles in temperature fluctuations are noted between the larger triangle Koror-Guam-Clark in the lower latitude and the smaller triangle Ty-Mk-Tk in the higher latitude. The former shows temperature fluctuation at lOOmb is in phase with divergence fluctuation at 300mb (hence in phase with vertical motion and relative humidity and out of phase with divergence in low levels), while the latter shows a cold core structure at lOOmb.