Case Study On Formation Mechanisms Of Explosive Cyclones

This paper analyzed the characteristics,evolutionary process and physical mechanism of a strong explosive cyclone which occurred over the Near Coast of East Asia from 2 to 4 May 2016 were investigated by using the Final Analyses(FNL)data with a resolution of 1 degrees x 1degrees,time interval of 6 hours provided by National Centers for Environmental Prediction(NCEP).In addition,the explosive cyclone was simulated by using the high-resolution of WRF(Weather Research Forecast Model)model,and the model results reproduced the explosive cyclone reasonably well.Further more,the temporal and spatial distribution characteristics of Explosive Cyclones over the Northern Hemisphere from October 2015 to September 2016 were carried out by summarized and analyzed.The diagnostic analysis results show that the jet of 200 hPa,warm advection before cold front,and the positive vorticity advection all strengthened the divergence of upper atmosphere during the development process of the cyclone,thus promoted the low altitude decompression and convergence,strengthen the ascending motion near the cyclone center,All the physical mechanisms provide favorable dynamic conditions for the development of the explosive cyclone.The increases of cold and warm advection intensified the temperature gradient before and after the cold front,and strengthened the baroclinic property of the frontal zone.The strong divergence near the tropopause,caused by atmospheric decompression,enhanced the pumping action between the high and low altitude atmosphere.The convergence of the lower atmosphere was intensified,the horizontal wind vorticity increased,and the cyclone developed explosively.The south jet which superposed with the water vapor flux axis and the water vapor flux convergence area,provided sufficient water vapor for the rainstorm.Warm moist air climbed upward along the cold front,atmospheric baroclinicity of the middle atmosphere enhanced,low pressure system developed,and the cyclone near ground was weakened because of the intrusion of the dry cold air.It is inconsistent of the development between the systems of the upper and lower troposphere that caused the part of the systems at the middle atomsphere.Over time,the upper system divorced from the lower system,and the center of the cyclone near the ground was occupied by cold air,that gradually gave rise to the death of the cyclone.In addition,the existence of secondary circulation near the energy front was closely related to the occurrence of heavy rainfall.The southerly current moved up along the energy front,that strengthened the upward motion near the cyclone center,and leaded to the enhancing of the closed circulation of upper atmosphere.The southward airflow produced by the secondary circulation was convergent with the downdraft at the north side of the cyclone,that caused the enhancing of the upward movement at the north side of the cyclone,and created a narrow energy front,thus causing heavy precipitation.The WRF3.3.1model reasonably reproduced the occurrence and development of the explosive cyclone.It had good simulation at the evolution of 300 hPa and 700 hPa jets and the propagation direction of jetcore before and after the outbreak of the cyclone,As well as the characteristics and evolution process of 700 hPa relative humidity field,the model was well simulated,but only slightly in strength and position.It also studied the statistical analyses of explosive cyclones occurred over the northern hemisphere from October 2015 to September 2016 in this paper.The results indicated that March was the peak month of the occurrence of Explosive Cyclones,the intensity of the winter cyclones were stronger than that of other seasons.With the increase of intensity,the occurrence of Explosive Cyclones were less and less.The western coasts of the two oceans were the major region of Explosive Cyclones,and the frequency of Explosive Cyclones in the Pacific was higher than that in the Atlantic,also in zonal distribution,the occurrence of Explosive Cyclones in the Pacific region was about 10 latitudes south of the Atlantic.There was not explosive cyclone in the Aleutian,but some weak explosive cyclones occurred in Greenland Iceland.