Characteristics of the Eccentric Structure of Severe Tropical Storm Doksuri and Analysis of the Causation
Using observations,data from the weather satellite FY2 /MTSAT and NCEP reanalysis,this work performed a preliminary study on the eccentric structure of Severe Tropical Storm Doksuri( coded1206) and its causation by synoptic analysis and diagnoses based on physical quantities. The results are shown as follows. Doksuri was significantly asymmetric in both the dynamic and thermodynamic structure so that deep-convection cloud systems were almost all on the southwest side of the storm,low-cloud lines distributed around the eye that were both shallow and far between,and its mid-and higher-level wind field was only with easterly waves that were slightly cyclonic and much deviated to the southwest side of the eye. The strengthening and westward extension of the western Pacific subtropical high( WPSH) are mainly responsible for the eccentric structure of Doksuri at the mid- and higher-levels. On the one hand,the WPSH advanced rapidly to the west while the geopotential gradient strengthened between it and the storm,constituting to a powerful easterly airflow that checked the development of convection. On the other hand,the airflow was dry and descending inside the WPSH,inhibiting the development of cyclonic circulation north of the storm. At lower levels,the WPSH was less stripe-distributed and west-extended as at the mid-and higher-levels,contributing to inconsistent location of the storm circulation between the high and low levels. Besides,lower levels were prevalent with a strong southwest monsoon and cross-equatorial airflow so that the eccentricity of the low-level cyclonic circulation was not as obvious. As the southwest airflow entrained into the mid-level storm was both weak and broken-off,cyclonic circulation was not helped develop and maintain at that level. Furthermore,the presence of strong wind shear was also one of the factors accounting for the vigorous development of convection south of the storm.