Low-frequency Characteristics Of Persistent Heavy Rain In The Pre-flood Season In South China For 10-30 Days And Its Extended Forecast

In this paper,the daily re-analysis grid data provided by ECMWF and the daily precipitation data provided by 110 regional automatic stations in South China are used to analyze and analyze South China using statistical methods such as Morlet wavelet analysis method,first-order Butterworth band-pass filter analysis method and classification synthesis analysis.The climatic characteristics of the continuous rainstorm process in the first flood season,the water vapor channel changes in extreme precipitation events,and the low-frequency circulation characteristics of10-30 days.Based on the low-frequency circulation characteristics,the relevant physical conceptual model for the prediction of the extended rainstorm in the first flood season in South China is proposed.First of all,define the continuous heavy rain process,and then screen the continuous heavy rain in the first flood season of South China in the study year,and statistically analyze the frequency,period and precipitation intensity to provide research for the low frequency characteristics of precipitation and the low frequency of circulation background.A total of 176 continuous rainstorm processes occurred during the first flood season in South China from 1980 to 2016.The continuous rainstorm processes in key areas lasted for 4-18 days,which corresponded to the occurrence of heavy rainstorm processes.The rainstorm process with a duration of 5days is the most frequent;the single-station daily rainfall with a duration of 6 days is the highest;the continuous rainstorm process with the highest frequency occurs in May;there is a significant main cycle of quasi-3 years and quasi-8-9 year sub-cycle.Secondly,classify 24 extreme precipitation events,study the water vapor channels corresponding to various weather situations,and analyze the impact of different high and low altitude jets on energy and water vapor supply.The precipitation in southern China during the first flood season in Guangxi and Guangdong coastal areas The coastal areas of the junction,central Guangdong,and southern Guangdong have reached high values,and the humid and cold air that affects the precipitation mostly enters the south China region from the southwest,gradually providing a driving force for the occurrence and maintenance of precipitation.Based on the presence of high-and low-level jets,the 24 extreme precipitation events are divided into three categories: "high-altitude jets","low-altitude jets" and "high-low jets".When the "high-altitude jet pattern" occurred,the water vapor convergence area in the southeast of the South China area was obvious,and the high-altitude jet flow strongly dispersed the cold air at high altitude.When the "low trough-low-altitude jet flow" occurs,on the right side of the study area,there is a stable development of water vapor sink area,which provides the necessary water vapor supply for the occurrence and development of precipitation.When the "cyclone-low-altitude jet" occurred,a large-span low-altitude jet was located on the right side of the study area,which led to the occurrence and development of low-level convective instability.There was a water vapor sink area corresponding to the entire layer of water vapor,which was maintained steadily.When the "high-low jet" type occurs,the circulation situation of the upper and lower layers is coordinated,water vapor is transported through the low-level jet,and the high-level jet establishes symmetry and instability,providing important dynamic conditions for extreme precipitation events.Finally,the first-order Butterworth filter was used to extract and synthesize the low frequency components of 10-30 d for the 176 continuous heavy rains in the first flood season in South China from 1980 to 2016.The characteristics of low-frequency components and vertical circulation are summarized,and then a physical conceptual model for the prediction of the continuous rainstorm extension period in the early flood season of South China is obtained: in the upper troposphere,the intensity divergence over South China corresponds to the original field.The weather system is the right side of the high-level jet divergence area,South Asia High pressure and northeast cold vortex.In the middle troposphere,continuous rainstorm processes are classified according to five circulation configurations,which are two ridges and one trough,two troughs and one ridge,one trough and one ridge,one ridge and one trough,and multi-wave patterns.In the lower troposphere,the low-frequency flow field and the low-frequency vorticity field are classified and analyzed according to the South China Sea low-frequency anticyclone and the Yangtze-Huaihe River basin low-frequency cyclone.The Bay of Bengal provides warm and humid air and the southward cold air to meet to provide power and water vapor support for continuous heavy rain.The water vapor channel classifies the continuous rainstorm process according to the channel type of water vapor flux.The Bay of Bengal-Indochina Peninsula,the Indian Ocean-Northern South China Sea,and the Bay of Bengal-Northern South China Sea are maintained in response to the Yangtze-Huaihe River basin shear line and low-level jet convergence area in the weather system.Vertical circulation,the warp-direction average analysis of the vertical velocity of110 ° E-117 ° E,in which there is the coupling of high and low jets,and the secondary warp-direction circulation which is conducive to the continuous development of rainstorms.Provide favorable conditions.