Extended-range Forecast Of Persistent High Temperature Events In Jiangnan Region Of China

Under the background of global warming,persistent high temperature events(PHTEs)occur more than before,and pose widespread risks as they not only affect social ecosystems directly but also damage public production and life.Therefore,it is of great significance to strengthen the extended-range prediction of PHTEs for meteorological disaster prevention and reduction and to ensure the development of social economy.Based on the daily maximum air temperature data from 1981 to 2018 provided by China Meteorological Data Network,the intra-seasonal oscillation(ISO)characteristics of surface maximum temperature in the extended summer in Jiangnan area are explored and the PHTEs related to ISO in 38 years are counted.The low frequency PHTEs are classified according to the position variation of the Western Pacific subtropical high during the occurrence of PHTEs,and the intra-seasonal circulation characteristics and evolution are discussed,respectively.Finally,the extended-range prediction model based on spatial-temporal projection model(STPM)is constructed by using the predictability sources affecting the low-frequency surface maximum temperature in the extended summer of Jiangnan area during 1981-2010,and the independent forecast for the low-frequency surface maximum temperature during 2011-2018 is further carried out.The main conclusions are as follows:(1)The surface maximum temperature in the Jiangnan area exhibits a vigorous ISO of 10-30 days in the extended summer,and is largely regulated by intra-seasonal signal.Total 37PHTEs related to ISO are identified from 1981 to 2018.The influence of diabatic heating on the position variation of subtropical high in a case of PHTE in the middle reaches of the Yangtze River is analyzed by using the complete form of vertical tendency equation.The results show that the apparent heat source(Q₁)in the northern of Bay of Bengal in the early stage of PHTE has certain indicative for the westward extension of the subtropical high.(2)The first type of high temperature events is mainly characterized by the obvious westward extension of subtropical high during the high temperature process.The Q₁ in the Indo-China peninsula is beneficial for the westward extension of subtropical high in the early stage of high temperature process.According to evolution of circulation anomalies,it clearly can be seen that the circulation structure in the upper troposphere is characterized by the southwestward propagation of mid-latitude low-frequency wave-train.In the mid-lower troposphere,the most remarkable characteristic is a westward movement of anticyclonic anomaly in the east of Taiwan,which leads to the western extension of subtropical high.The diagnosis of 925h Pa thermodynamic equation indicates that the ISO features of the surface maximum air temperature in the core region is determined by the intra-seasonal variation of the diabatic variation in the early stage of heating process,and the low-frequency adiabatic variation plays the major role in the later stage.(3)The second type of high temperature event is mainly characterized by weak subtropical high activities during the heating process which is always located in the southeast coastal area of China,and its horizontal circulation field is characterized by low-frequency wave-train propagating southwestward in the middle latitude of the troposphere.Diagnostic analysis shows that the low-frequency meridional temperature advection plays a positive role in the early stage of heating process,and the low-frequency adiabatic variation and diabatic variation contribute mainly in the later stage.The low-frequency vertical circulation analysis of the two types of high temperature events shows that the increase of temperature in the core region is mainly caused by the combined effect of air column heating induced by descending airflow and southward movement of the warm anomaly in the north of core region.(4)Based on the relationship between the low-frequency surface maximum temperature in Jiangnan area and the large-scale low-frequency circulation field in the early stage,the STPM prediction model is constructed.Through the cross-validation approach of the training period,five best predictors are selected for optimal ensemble and multiple linear regression forecast.For the independent forecast period,the STPM model can predict the 10-30d surface maximum temperature at the lead time of 10-15 days,30-60d and 10-60d low-frequency component at20-25 days.According to TCC and RMSE,the forecast by multiple linear regression is better than that by optimal ensemble.In addition,the prediction results of different low-frequency components by the STPM model have obvious interannual variation characteristics.