A DIAGNOSTIC ANALYSIS OF THE WEAKENING OF WEST PACIFIC SUBTROPICAL ANTICYCLONE DURING THE PERIOD OF SOUTH CHINA SEA SUMMER MONSOON ONSET IN 1998

The onset of South China Sea summer monsoon in 1998 occurred on May 21st. Using the U.S. National Centers for Environmental Prediction reanalysis data, this paper examines the physical process of the weakening of a subtropical anticyclone in West Pacific during the onset period using the Zwack-Okossi vorticity equation. Results show that during the pre-onset period, the positive vorticity advection in front of an upper tropospheric trough was the most dominant physical mechanism for the increase of the cyclonic vorticity on the 850-hPa layer over the South China Sea and its nearby region. The secondary contribution to the increase of the cyclonic vorticity was the warm-air advection. After the onset, the magnitude of the latent-heat warming term rapidly increased and its effect on the increase of the cyclonic vorticity was about the same as the positive-vorticity advection. The adiabatic term and divergence term contributed negatively to the increase of the cyclonic vorticity most of the time. Thus, the positive vorticity advection is the most important physical mechanism for the weakening of the West Pacific subtropical anticyclone over the South China Sea during the onset period.

Causes of Continuous Cold Weather in Middle and Lower Reaches of Yellow River

Based on conventional observation data and the temperature data of meteorological stations in China,the circulation patterns,weather systems and causes were analyzed by using principles of synoptic meteorology,weather dynamics and thermodynamics diagnostic analysis method for the strong cooling and continuous low-temperature weather process in early and middle January of 2010 in middle and lower reaches of Yellow River.The results showed that the cold air from polar region was transported towards south by northerly steering airflow behind trough at 500 hPa level.The stable cold vortex center in Northeast Asia played a role in blocking the cold air eastward,promoting and strengthening the accumulation of cold air.The deep low trough in Siberia oppressed high pressure ridge to collapse,thus the strong cold air broke out and moved southward.The main reason for the rapid temperature decrease and continuous cold weather was the intense cold advection of temperature at 850 hPa level.The evolution of the cold advection center was consistent with the moving path of strong cold air.There was negative vorticity advection from the upper to the lower layer in middle and lower reaches of Yellow River.The strong negative vorticity advection was conducive to the movement of the high-pressure system.The smaller the value of negative vorticity advection,the stronger cold air.The negative vorticity advection center corresponded with the strong cooling center.The cold advection center appeared 12-24 h in advance compared with strong cooling center and the 'negative vorticity advection core' appeared earlier than strong cooling weather.All these features provided favorable consultation materials for the forecast of strong cooling weather.

NUMERICAL STUDY ON FUNCTIONS AFFECTING TROPICAL CYCLONE STRUCTURE AND MOTION

By using a beta-plane quasigeostrophic barotropic model,four numerical experiments with integration time more than five days are performed.Results show that the vorticity advection term and beta term are two basic factors significantly affecting tropical cyclone structure and motion.