| The extreme heavy rain caused by tropical disturbance and weak cold air in the autumn flood season to cause large-scale flooding in Hainan Island.The three major floods in the eastern half of Hainan Island in October 2000,October 2008 and October 2010 were caused by such heavy rains.In order to systematically study the mechanism of the formation,enhancement and maintenance of such rainstorms and enhance the understanding of rainstorms in tropical areas,this paper uses Hainan Province's high-altitude,ground observation data,satellite,Doppler radar and NCEP,ERA5 reanalysis data,statistical analysis The temporal and spatial distribution characteristics of the extreme rainstorm in the autumn flood season on Hainan Island caused by tropical disturbances and weak cold air.In-depth discussion of the interaction of multi-scale weather systems during the rainstorm process,the mechanism of triggering,development and maintenance of deep convection,and the dynamics of mesoscale systems,Thermodynamic characteristics,the following main conclusions are obtained:(1)According to climate statistics,the distribution pattern of precipitation over time in Hainan Island is similar to that in central and northern Vietnam,but is quite different from other regions in South China.Its bimodal structure is not obvious.As the rainstorm level increases,the unimodal phenomenon becomes more and more severe.Significantly,the annual peak precipitation and nearly 50% of the large-scale extreme precipitation events occurred in the autumn flood season,of which 58% of the extremely heavy rainy days in the autumn flood season caused by the tropical disturbance and weak cold air.In the autumn flood season,the geographical distribution of the precipitation intensity of the extreme heavy rain is very regular,and the overall trend is consistent with more east and west.The 40-year average wind field analysis found that the appearance and gradual strengthening of the low-level easterly strong wind belt in the northern part of the South China Sea is the most significant circulation feature during the autumn flood period.The increase in the land-sea thermal contrast is likely to induce strong convergence wind speeds in the lower troposphere in the central and northern South China Sea,forming a banded easterly jet.(2)From the composite field of all cases,it is found that the interaction of the South Asia High,the mid-latitude westerly trough,the subtropical high and the South China Sea tropical disturbance is the main circulation background for the formation of the extreme heavy rain in the autumn flood season.During heavy rains,the ITCZ in the northern hemisphere was extremely active in the Asian region,and the South China Sea monsoon trough and the Indian monsoon trough were retreating slowly to the south,which was anomalously northerly stronger than usual.The location of the South Asian High is significantly easterly and southerly than normal.The East Asian mid-latitude trough and subtropical high are also significantly stronger than normal.The water vapor that caused the increase in heavy rain mainly came from the northeast airflow on the southeast side of the continental cold high pressure,the easterly airflow on the south side of the subtropical high,and the southwest monsoon tributary of the Indian Ocean.(3)From the comparative analysis of cases of different intensities,it is found that the circulation configurations of extreme rainstorm cases have many similar characteristics: on upper level,South Asia high locate in the north of South China Sea(SCS)and provides strong divergence.on middle and lower level,increasing pressure gradient induced by the interactions among tropical disturbance,Western North Pacific Subtropical High(WNPSH)and East Asia middle-latitude trough is helpful to the formation of front zone,duration and reinforce of tropical disturbance,and induction of easterly low-level jet.Hainan Island is just on the left side of the exit area of this easterly low-level jet,and the wind direction and speed are convergent obviously.The intensity,length,and thickness of the jet core in the case of heavy rain during the strong autumn flood season,and the wind speed gradient above the jet stream is much larger than that in the weak case.The low-level jet core occur in the strong case is located just off the coast of eastern Hainan Island.It is stable and less moving in the horizontal direction,and pulsates sharply in the vertical direction and wind speed,which is conducive to the excitation of heavy precipitation.In the weak case,the jet core oscillates significantly from east to west in the horizontal direction,and has little change in vertical height and wind speed,which is not conducive to the maintenance of heavy rainfall in a fixed area.(4)From the simulation analysis of individual cases,it is found that wet-neutral stratification,frontogenesis caused by non-adiabatic heating and horizontal movement,and vertical wind shear at different heights are essential for the formation,development and maintenance of deep convection.The formation of the neutral stratification is the result of the vertical stratification change caused by the transition from the stable stratification zone after the weak cold front to the unstable stratification zone of the warm humid air mass caused by the southerly wind of the tropical disturbance.The non-adiabatic heating term and horizontal motion term contribute the most to the process of local frontogenesis during heavy rain.The change of echo structure,moving direction and speed under the influence of low-level and middle-level wind shear help explain the formation mechanism of ?echo training".Through the linear and nonlinear solution of the inertial gravity internal wave equations,it is found that the meso-scale vortices are generated and strengthened in the extreme rainstorms caused by tropical disturbances and weak cold air,which are related to horizontal wind shear and cumulus convective latent heat release,Vertical wind shear or low-altitude jet stream and cold air.Among them,the strong convective condensation latent heat heating plays a major role in the enhancement of the vertical motion amplitude of the tropical mesoscale vortex,which is beneficial to the development and maintenance of the vortex.(5)The results of the terrain sensitivity test show that the change of the terrain height of Hainan Island has a significant impact on the magnitude of the heavy rainfall in the east.Due to the existence of the terrain,the strong uplift of the airflow in front of the windward slope condenses to form precipitation,which leads to the release of a large amount of latent heat of condensation.The release of latent heat feeds back and strengthens the warm core structure of the convection zone,thereby strengthening its vertical movement and forming a positive feedback effect on convection.It is also an important reason for the heavy rainfal in the eastern part of Hainan Island... |
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