Current Issue

28 June 2002, Volume 21 Issue 3   
  • Numerical Study on Three-Dimensional Conditionally Symmetric Instability of "96.1" Snowstorm Event
  • WANG Wen;CHENG Lin-shing
  • 2002 Vol. 21 (3): 225-232. 
  • Abstract ( ) PDF (380KB) ( )
  • Using the output data of a MM4 mesoscale numerical model which fairly well simulated the developing structure and the evolution of the "96.1 "snowstorm event over Qinghai Xizang Plateau,a nonlinear and non zonally non parallel basic flow symmetric instability(SI) models and simulation system were developed to validate the possible effect of conditional symmetric instability(CSI) on the formation of the snowstorm process,the results showed that in a developing model of two dimensional nonlinear SI in a three dimensional basic flow with proper parameters,the use of localized perturbation of streamfunction or potential temperature resulted in circulations forming which slant to south with height in y- z cross section by symmetric instability and are vertical in the along front direction by nonlinear transversal wave instability,have samilar results as in literatures [2,10].These circulations were tilted in the horizontal plain relative to y-coordinate,and the three dimensional disposition in the fields of the streamfunction and vertical velocity is consistent with the evolution of the "96.1" snowstorm,that means the two dimensional nonlinear symmetric instability in three dimensional basic flow is a possible explanation for the organization of the "96.1" snowstorm.
  • Precipitation Characteristic and Interception of Forest in Qilian Mountain
  • CHANG Xue-xiang;ZHAO Ai-fen;WANG Jin-ye;CHANG Zong-qiang;JIN Bo-wen
  • 2002 Vol. 21 (3): 274-280. 
  • Abstract ( ) PDF (283KB) ( )
  • The long term precipitation characteristic and forest entrapment were studied at Sidalong station,Qilian Mountains.The annual average precipitation is 433.6 mm(from 1972 to 2000),and varied from 326.4 mm to 539.7 mm,annual precipitation falls in summer is 65.70%.Precipitation is altered in different altitudes,its tendency increase 4.45% of the amount with altitude arising per-100 m.The average entrapment rates are 37.5%,31.7% on canopy entrapping precipitation of Picea crassifolia forest and Sabina Przewalsskii forest,the average entrapment rate of brush forest even is as high as 66.5%.When the rainfall is intensive,its average entrapment rate gradually diminished in Picea Crassifolia forest.When the amount of precipitation is 18.67 mm,the amount of canopy entrapment of Picea Crassifolia forest is 14.72 mm.The trunk runoff of Picea crassifolia is 0.51% of the amount of precipitation and its trunk runoff began when the amount of precipitation is over 12.0 mm.The litter entrapment precipitation gradually increased when amount of rainfall increased,the entrapment rate increased when the amount of rainfall diminished.
  • Characteristics of Ground Flashes in Hefei Region
  • ZHU Bao-you;TAO Shan-chang;LIU Yi-feng
  • 2002 Vol. 21 (3): 296-302. 
  • Abstract ( ) PDF (283KB) ( )
  • Characteristics of return strokes in Hefei Area measured by a VLF/VHF radiation recording system is presented.The number of strokes per flash does not show a exponential decay distribution.However,flashes whose stroke number are below 5 to occur almost equally to each other and single stroke flashes account for about 15% of total flashes.The average number of strokes per flash is 4.2 and the maximum number ever measured is 16.Normalized(to 100 km) strength of return strokes distributes log normally.For 169 first return strokes the geometric mean of initial electric field peak normalized to 100 km is 9.3 V·m -1,the same value for 485 subsequent strokes is 4.5 V·m -1 and the most probable range is between 2 and 4 V·m -1.On a statistical base the subsequent stroke is weaker than the first stroke,while it is interesting to note that about 18% of ground flashes have at least one subsequent stroke whose initial peak is greater than the peak of the first stroke.The interstroke interval distributes log normally with the minimum value of about 1.6 ms.About 30% intervals are larger than 100 ms and intervals between 40 ms and 100 ms account for about 50%.The ratio of the field peak of two successive return strokes varies systematically with the time interval between them:Intervals less than 40 ms are dominated by the fact that the larger stroke occurs first,while about 55% of intervals more than 100 ms are featured by the preceding return stroke being weaker than the successive one.