Current Issue

28 April 2005, Volume 24 Issue 2   
  • Impact of Pressure System Anomaly over Mid-High Latitude on the Interdecadal Change of East Asia Summer Monsoon
  • LIU Yan-xiang;GUO Yu-fu
  • 2005 Vol. 24 (2): 129-136. 
  • Abstract ( ) PDF (321KB) ( )
  • The interdecadal changes of both the East Asia summer monsoon(EASM) and the pressure system anomalies over China-Mongolia areas(30°~55°N, 70°~120°E) as well as their relationship have been analysed, utilizing NCEP/NCAR reanalysed gridded data from 1958 to 2000, the result show that, in the lower and middle layers of troposphere, from the surface to 500 hPa or more, the negative correlation between EASM and the pressure system anomalies is quite clear. The coefficients are -0.67 (for sea level pressure) and -0.69 (for geopotential height on 500 hPa) respectively, which is significant at 99.9% confidence level. A clear change appears in the late of 1970's. In the period of 1960-1976, both the surface and 500 hPa, the pressure system anomalies were negative centre over the China-Mongolia areas. It indicates that the areas dominate low pressure trough or cyclone activities. The strong southerly wind enhances the EASM and result in wet summer in North China. However, in the period of 1977-2000, in the China-Mongolia areas dominate the high pressure ridge or anticyclone activities, the stronger northerly wind over North China preveates the EASM flow form moving northward, and causes the weak EASM and the dry North China. Interestingly, on 100 hPa, mean geopotential height anomaly appears a dipole pattern: During the period of 1960-1976 it was higher in the east, lower in the west; vise versa during the period of 1977-2000. It would be needed further to study.
  • Spatial and Temporal Variation Characteristics of Arctic Oscillation on 500 hPa in Recent Fifty-One Years
  • YAN Hua-sheng;DONG Hui-lin;CHENG Jian-gang;CAO Jie;WANG Qing-chuan
  • 2005 Vol. 24 (2): 187-195. 
  • Abstract ( ) PDF (420KB) ( )
  • The monthly mean normalized data of 500 hPa geopotential height field over Northern Hemisphere in 1951-2001 is used to do the EOF analysis according to the each of 12 month, respectively, and then the power density spectrum is used to study the time coefficient changes of the first eigenvector of EOF. The result show that the Arctic Oscillation is the first character of 500 hPa geopontential field circulation anomaly over Northern Hemisphere in recent fifty-one years, and the variance contribution rate is between 10% and 20%. The center at polar area has a remarkable eccentric construction. The areas of the center extend to high latitude in winter and spring, are larger and that of the center in summer and autumn are smaller to polar contracting. The central area at middle-high latitude is also larger in winter than in summer. The regions of from polar to Greenland, from Southwest Europe to Etesian, from Baikal lake to the Japanese Sea and the Southeastern of North America are the major occurring areas of 500 hPa circulation anomaly over Northern Hemisphere. For positive periods of AO, winter is in 1990's; spring, before the middle of 1970's; summer, in 1960's and 1970's; autumn, in the mid-later period of 1980's to the early 1990's. For negative periods of AO winter are in 1950's; spring, after middle period of 1970's, 1950's and 1990's; autumn, the later half of 1990's. And 1998 is the most notable year of AO.
  • Mesoscale Analyses on A Moist Convective Storm Rain Behind Trough
  • LIU Yong;ZHANG Ke-xiang
  • 2005 Vol. 24 (2): 247-254. 
  • Abstract ( ) PDF (1318KB) ( )
  • Mesoscale analyses were carried out for a moist convective storm rain behind trough in Shaanxi Province on August 5, 2002. The primary results are as follows:This heavy storm rain was of long-time duration, wide influence and high density, which reflected the synoptic feature of moist convection storm rain. The southward movement of cold advection behind 500 hPa trough was direct factor of this process; θse on 850 hPa, frontal zone in the northern part, and energy zone in the mid- and south- parts supplied the energy and caused strong unstable zone of potential for this process. Typhoon named "kammuri"located in the South China Sea, transformed water moistureto the continent, and provided the ample moisture for this process. Moist in the whole atmospheric layer, moist and cold in the upper, caused the moist convection wind storm; second-circulation formed by coupling of high- middle and low-level jets resulted in successive violent updraft. Evere rainfall weather in the northern part of shannxi was caused by one of the two mesoscale cyclones, and short-term storm rainfall and heavy storm rain in the middle and southern parts were caused by amalgamation of two mesoscale cyclones. Effect of sub-circulation might be main reason of surface mesoscale cyclone. The density of radar echo appeared in pieces for this process varied from 45 to 55 dBz, and its cloud height reached to 12~14 km, the maximum was 16 km.
  • Analysis on A Mesoscale Convective System in North China Plain
  • Lü Sheng-hui;GAO Yan-hong;LIU Wei
  • 2005 Vol. 24 (2): 268-274. 
  • Abstract ( ) PDF (1252KB) ( )
  • Meteorological routine observation data are used to analyze a Mesoscale Convective System (MβCSs) on the Meso-β scale which takes place in the North China Plain on June 1, 2002. The results show that the convective system occurs in the wind shear zone, and it locates on the right-hand side of upper-level jet stream, and which exists in the northeast part of North China Plain, due to the influence of the Northeast Cold Vortex. The collective effect of upper-level positive vorticity advection and low-level negative vorticity advection results in both upper-level convergence and low-level divergence, which tend to maintain the ascending motion of middle troposphere in this region. Low-level moisture flux convergence provide plenty of water vapor. Then decreasing(increasing) the total energy of the upper(low)-level airso that a large amount of unstable energy develops over the North China Plain. This circulation pattern favors convective systems and finally triggers the mesoscale convective system. This paper sheds light on the causes of MβCSs and provides some experiences for forecasting MβCSs in the future.