COMPARATIVE ANALYSIS OF THE INTENSIFYING AND WEAKENING LANDFALL TROPICAL CYCLONES DURING EXTRATROPICAL TRANSITION OVER CHINA

Based on the Tropical Cyclone(TC) Yearbooks data and JRA-25 reanalysis data from the Japan Meteorological Agency(JMA) during 1979-2008, dynamic composite analysis and computation of kinetic energy budget are used to study the intensifying and weakening TCs during Extratropical Transition over China. The TCI shows strong upper-level divergence, strengthened low-level convergence and significantly enhanced upward motion under the influence of strong upper-level troughs and high-level jets. The TCI is correspondingly intensified after Extratropical Transition(ET); TCW exhibits strong upper-level divergence, subdued low-level convergence and slightly enhanced upward motion under the influence of weak upper-level troughs and high-level jets. It then weakens after ET. The increase(decrease) of the generation of kinetic energy by divergence wind in TCI(TCW) at low level is one of the major reasons for TCI's intensification(TCW's weakening) after transformation. The generation of kinetic energy by divergence wind is closely related to the development of a low-level baroclinic frontal zone. The growth of the generation of kinetic energy by rotational wind in TCI at upper level is favorable for TCI's maintenance, which is affected by strong upper-level troughs. The dissipation of the generation of kinetic energy by rotational wind in TCW at upper level is unfavorable for TCW's maintenance, which is affected by weak upper-level troughs.

Impact of the Monsoonal Surge on Extreme Rainfall of Landfalling Tropical Cyclones

A comparative analysis and quantitative diagnosis has been conducted of extreme rainfall associated with landfalling tropical cyclones(ERLTC)and non-extreme rainfall(NERLTC)using the dynamic composite analysis method.Reanalysis data and the tropical cyclone precipitation dataset derived from the objective synoptic analysis technique were used.Results show that the vertically integrated water vapor transport(Q_(vt))during the ERLTC is significantly higher than that during the NERLTC.The Q_(vt)reaches a peak 1−2 days before the occurrence of the ERLTC and then decreases rapidly.There is a stronger convergence for both the Q_(vt)and the horizontal wind field during the ERLTC.The Q_(vt)convergence and the wind field convergence are mainly confined to the lower troposphere.The water vapor budget on the four boundaries of the tropical cyclone indicates that water vapor is input through all four boundaries before the occurrence of the ERLTC,whereas water vapor is output continuously from the northern boundary before the occurrence of the NERLTC.The water vapor inflow on both the western and southern boundaries of the ERLTC exceeds that during the NERLTC,mainly as a result of the different intensities of the southwest monsoonal surge in the surrounding environmental field.Within the background of the East Asian summer monsoon,the low-level jet accompanying the southwest monsoonal surge can increase the inflow of water vapor at both the western and southern boundaries during the ERLTC and therefore could enhance the convergence of the horizontal wind field and the water vapor flux,thereby resulting in the ERLTC.On the other hand,the southwest monsoonal surge decreases the zonal mean steering flow,which leads to a slower translation speed for the tropical cyclone associated with the ERLTC.Furthermore,a dynamic monsoon surge index(DMSI)defined here can be simply linked with the ERLTC and could be used as a new predictor for future operational forecasting of ERLTC.

Dynamic Recrystallization Behavior of Bimodal Size SiCpReinforced Mg Matrix Composite during Hot Deformation

The（submicron＋micron） bimodal size Si Cp-reinforced Mg matrix composite was compressed at the temperature of 270–420 °C and strain rate of 0.001–1 s^-1. Then, dynamic recrystallization（DRX） behavior of the composite was investigated by thermodynamic method and verified by microstructure analysis. Results illustrated that the composite possess the lower critical strain and higher DRX ratio as compared to monolithic Mg alloys during hot deformation process. The predicted DRX ratio increased with the proceeding of compression, which was well consistent with the experimental value. Results from thermodynamic calculation suggested that the occurrence of DRX could be promoted by Si Cp, which would be further proved by microstructure analysis. Formation of particle deformation zone around micron Si Cp played a significant role in promoting DRX nucleation. Nevertheless, the distribution of submicron Si Cp was increasingly uniform with the proceeding of compression, which could fully restrain grain growth. Therefore, the corporate effects of micron and submicron Si Cp on DRX contributed to the improvement of DRXed ratio and the refinement of grain size for the composite during compression process.

Large-scale characteristics of landfalling tropical cyclones with abrupt intensity change

Data from the China Meteorological Administration and ERA-Interim are used to examine the environmental characteristics of landfalling tropical cyclones(TCs)with abrupt intensity change.The results show that,of all 657 landfalling TCs during 1979-2017,71%,70%and 65%of all landfalling TDs,TSs and TYs,respectively,intensify.Of all the 16595 samples,4.0%and 0.2%of typhoons and tropical storms,respectively,experience over-water rapid intensification(RI)process during their life cycle.Meanwhile,4.5%and 0.6%of typhoons and tropial storms,respectively,undergo overwater rapid decay(RD).These two kinds of cases,i.e.,RI and RD,are used to analyze their associated large-scale conditions.Comparisons show that the RI cases are generally on the south side of the strong western Pacific subtropical high(WPSH);warm sea surface temperatures(SSTs)and sufficient water vapor fluxes existing in RI samples is a dominant feature that is conducive to the development of TCs.Also,the moderate low-level relative vorticity is favorable for TC intensification.On the contrary,the RD TCs are located on the west side of the WPSH;significant decreasing SSTs and low-level water vapor transport may synergistically contribute to RD.Simultaneously,low-level relative vorticity seems to be unfavorable for the development of TCs.