Research on Extreme Weather and Climate Events

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  • 1  Two extremely cold events in East Asia in January of 2012 and 2016 and their possible associations with Arctic warming
    WU Bingyi
    2019, 42(1):14-27. DOI: 10.13878/j.cnki.dqkxxb.20180724002
    [Abstract](1782) [HTML](0) [PDF 8.28 M](2312)
    Abstract:
    Warm winter prevailed in China from the late 1980s to the beginning 2000,and then cold winter seems to be more frequent.Intensity of winter extremely cold events appears to be further strengthened in the context of global warming and Arctic sea ice decline,its impacts have been concerned by the public and media.In the above context,East Asia suffered extremely cold events during January of 2012 and 2016.Thus the motivation of the present study is to investigate dominant features of the two extremely cold events and their possible linkages with Arctic warm anomalies.Evoluions of atmospheric circulation anomalies associated with the two extremely cold events are quite distinct from each other.For the extremly cold event in January 2012,SLP anomalies display a westward propogation process,during which the Aleutian Low led the Siberian High,indicating that the effect of downstream of atmospheric circulation anomalies plays important roles in resulting in the westward propogation.For another event,cold air mass exhibited a southeastward migration.The routes of the low-temperature area are also distinct for the two events.The former,cold air mass was mainly confined to the mid-and high-latitudes of Eurasia and migrated westward,and its impact on the low-latitudes of Asia was weaker relative to the latter.For the cold event in January 2016,the low-temperature area propaged southward along East Asian coast and affacted the tropic region.Atmospheric circulation anomalies exhibited a common feature prior to the outbreak of both extremely cold events:enhanced height ridges extended northward and transported more warmer air mass from the low-latitudes into the Arctic,and there was a multipole structure in the mid-troposphere of high-latitudes.This multipole sturcture is an important precoursor for outbreak of cold air mass.During wintertime,a rapid warming process in the Arctic can be attributed to enhanced warm ridge and its extension northward.Enhancement and extension northward of warm ridges over the Ural Mountain and West coast of North America and their coordinated evolution are critical for a extremely cold process to affect East Asia.The extremely cold event in East Asia in January 2016 did not exhibit a relation with a rapid Arctic warming process in December 2015.
    2  Interdecadal variation of heavy snowfall in northern China and its linkages with atmospheric circulation and Arctic sea ice
    CHEN Haishan LUO Jiangshan HAN Fanghong
    2019, 42(1):68-77. DOI: 10.13878/j.cnki.dqkxxb.20181212001
    [Abstract](1696) [HTML](0) [PDF 1.58 M](2496)
    Abstract:
    This study explored interdecadal variation of heavy snow in northern China and its relationship with atmospheric circulation and Arctic sea ice by using statistical methods such as correlation and regression.Results show that the high frequencies of heavy snow in northern China in winter are mainly located in Northeast China (NEC),and its spatial distribution shows an increasing trend from northwest to southeast,together with obvious interdecadal variations.Heavy snowfall in NEC is less during 1965-1980,but more during 2002-2011.Analysis shows that there is less water vapor transported to NEC during 1965-1980,but more water vapor transported from Northwest Pacific to NEC during 2002-2011.Meanwhile,the cold polar air guided by northerly airflow and Pacific warm moisture transport guided by southerly airflow benefit a convergence frontogenesis in NEC,providing favorable dynamic conditions for the frequent occurrence of heavy snow,and leading to the interdecadal increase of heavy snow in NEC.Furthermore,it is found that there is a close linkage between the interdecadal decrease of Arctic sea ice in early autumn and the interdecadal increase of snowstorm in NEC.Anomalous circulation pattern caused by the decreased Arctic sea ice in autumn shows that the zonal westerly decreases and NAO is in the negative phase,which can intensify atmospheric meridional activity and be conducive to the southward invasion of polar cold air,leading to the convergences of cold air and warm moist air in NEC.This is a possible important reason for the interdecadal increase of heavy snow in NEC.
    3  Real-time warning test of landslide and debris flow with a statistical model in large scale
    HE Shuangshuang WANG Jun WANG Huijun
    2019, 42(1):78-92. DOI: 10.13878/j.cnki.dqkxxb.20181008002
    [Abstract](825) [HTML](0) [PDF 4.94 M](2604)
    Abstract:
    A statistical model of landslide and debris flow is established by using the landslide susceptibility distribution and the rainfall threshold formula.The landslide and debris flow in large scale across the country can be warned by this model.The statistical model was driven by CMORPH satellite precipitation in this paper,and 116 landslide and debris flow events occurred from 2016 to 2017 were validated and analyzed.The results show that the model can warn most of landslide and debris flow events,and 72.1% of the events in rainy season can be warned,while only 35% of the events in non-rainy season can be forecasted.The effect of warning in rainy season is better than that in non-rainy season.Since the landslide and debris flow events mainly occurred in rainy season,the model is considered to have good performance.In addition,the model has a good warning ability for rapid landslides and debris-flows caused by heavy rainfall,however,the warning effect of the slow landslide events triggered by lower intensity and long duration rainfall needs to be improved.Using the statistical model and CMORPH satellite precipitation real-time products,a real-time forecast system for landslide and debris flow in large scale can be established,which has certain significance for landslide and debris flow reduction and disaster prevention.
    4  Interannual variation of Southern Oscillation and its association with Asia-Australian monsoon circulation and regional climate anomaly in the Maritime Continent during boreal summer
    ZHANG Mengmeng GUAN Zhaoyong ZHANG Benben
    2019, 42(1):36-45. DOI: 10.13878/j.cnki.dqkxxb.20181128002
    [Abstract](831) [HTML](0) [PDF 2.21 M](2580)
    Abstract:
    Based on the Southern Oscillation Index(SOI) from NOAA and the monthly mean data from NCEP/NCAR Reanalysis and CMAP,using correlation analysis and other methods,this paper investigated the interannual variation of SO(Southern Oscillation) and its association with Asian-Australian monsoon circulation and regional climate variation in the Maritime Continent during boreal summer.Results show that SO has significant interannual variation characteristics,which has important influences on the circulation,precipitation and temperature anomalies over Asian-Australian monsoon region and the Maritime Continent during boreal summer.When SOI is in positive phase,there is a cyclonic circulation in the upper troposphere and an anticyclonic circulation in the lower troposphere over eastern Australia and eastern sea area south of the equator.As the result the lower tropospheric divergence and upper tropospheric convergence over the region of Pacific north of the equator facilitate the maintenance of descending motion.The lower tropospheric convergence and upper tropospheric divergence near Kalimantan Island are favorable for the maintenance of ascending motion.The precipitation anomalies are significant positive in the maritime continent and weak positive in eastern China.The temperature anomalies are positive in the maritime continent and from Tibetan Plateau to East China Sea and negative over Bay of Bengal and Indian Peninsula.The results are helpful for our deeply understanding the influence mechanism of SO on regional climate variation.
    5  Analysis of extreme weather and climate events and circulation characteristics in the Northern Hemisphere in July 2018
    WANG Qian ZHAI Panmao YU Rong
    2019, 42(1):28-35. DOI: 10.13878/j.cnki.dqkxxb.20181129001
    [Abstract](1442) [HTML](0) [PDF 3.01 M](2542)
    Abstract:
    In July 2018,the weather and climate in the northern hemisphere were significantly abnormal,and extreme events occurred frequently.Much of Europe,North Africa,East Asia and many parts of North America were affected by severe heat waves.Extreme precipitation occurred frequently in southern and northern India,Southeast Asia,southwestern China,and western Japan.The tropical cyclone activity in the western Pacific was extremely active,and the movement path was northerly.The preliminary diagnosis results show that the abnormal high-pressure system maintained from the low to upper level is the direct cause of the heat wave in the middle and high latitudes of the Northern Hemisphere.The enhanced and northward subtropical high and the enhanced and eastward South Asian high are directly related to the persistent high temperatures and extreme precipitation events occurring in East Asia.The extremely active convective around the low-level Philippines and the strong southwest water vapor transport have led to extreme precipitation in South Asia and Southeast Asia.In addition,the warmer sea surface temperature conditions in many parts of the tropical Pacific and the abnormal cyclonic circulation near the Philippines are associated with unusually active typhoon activities.It is important to note that the anomalies of the atmospheric circulation in the Northern Hemisphere,especially in East Asia,are mainly affected by the thermal conditions of the ocean surface and the teleconnection of atmospheric circulation in other regions.Noticeably,the linkage between large scale extreme events in July 2018 and climate change is worthy of further study.
    6  Real-time prediction for 2018 JJA extreme precipitation and landslides
    MA Jiehua SUN Jianqi WANG Jun YU Entao HE Shuangshuang
    2019, 42(1):93-99. DOI: 10.13878/j.cnki.dqkxxb.20181214001
    [Abstract](1334) [HTML](0) [PDF 4.22 M](2355)
    Abstract:
    In this study,based on a dynamical downscaling prediction system,four-month leading real-time prediction experiments for JJA extreme precipitation and landslides were performed in February 2018 by the Nansen-Zhu International Research Centre,Institute of Atmospheric Physics,Chinese Academy of Sciences.The study results show that the spatial pattern of predicted summer extreme precipitation days is quite similar to the observations,yet with a significant underestimation over most regions of mainland China.The distribution of the predicted landslides is basically consistent with the reported landslide events caused by precipitation.The results show that the prediction system has a certain degree of predictability for summer extreme precipitations and landslide disasters,and possesses prospects for real-time prediction.
    7  Persistent extreme high temperature event in southern China in late spring of 2018 and the effect of Madden-Julian Oscillation
    ZHANG Fanghua TAO Yiwei GAO Hui CHEN Tao
    2019, 42(1):100-108. DOI: 10.13878/j.cnki.dqkxxb.20180928002
    [Abstract](1250) [HTML](0) [PDF 2.99 M](2220)
    Abstract:
    A persistent extreme high temperature(HT) event occurred in southern China in late spring of 2018.It began on May 15 and developed rapidly in the Jianghuai region,then spread southward to southern China beginning May 17,and remained in the region until early June.The HT days in the southern reaches of the Yangtze River and east part of southern China are approximately 6-10 days greater than the climatology.The accumulated HT frequencies with maximal air temperatures over 35℃ reach nearly 2 000,which is 2.6 times greater than the second largest value in the research period.In May 2018,more than 1/3 of the stations in southern China broke the historical records of HT days of May.The statistical analyses show that the HT days in southern China in late spring (May 16-31) are significantly related to the 500 hPa geopotential heights and convection over the area,as well as the 850 hPa meridional wind speed over its west side.However,the effect of the convection is more advanced,with a leading influence of about one week.Aside from the local circulations,the convection over the tropical Indian Ocean has a significant negative correlation with the HT frequency.A case study of circulations in 2018 also supported this statistical result.The spatial differences of convection may be linked to the activities of MJO.In May 2018,MJO remained in phases 1 and 2 for 20 days with amplitudes exceeding 1.The days in these two phases are the largest in May since 1981.Both the statistical analysis and case study indicate that MJO has an important impact on the HT events in southern China in late spring of 2018.
    8  Nonlinear response of Arctic sea ice to EP ENSO in autumn
    QI Li MA Weiwei HE Jinhai GENG Xin
    2019, 42(1):117-128. DOI: 10.13878/j.cnki.dqkxxb.20180719001
    [Abstract](1216) [HTML](0) [PDF 1.70 M](2291)
    Abstract:
    Based on the monthly Hadley Center sea surface temperature data,Hadley Center Arctic sea ice concentration data,and NCEP/NCAR reanalysis data during 1961-2015,this paper analyzed the abnormal response of Arctic sea ice to EP (eastern Pacific) ENSO events in autumn,and further discussed the possible causes of such an abnormal response.Results show that the response of Arctic sea ice to EP ENSO in autumn is nonlinear.Particularly,the sea ice concentration anomalies in the Kara Sea area (70°-80°N,60°-90°E) are significantly negative both in the EP El Niño and EP La Niña phases.Further analyses reveal that the mechanisms of sea ice anomalies in different EP ENSO phases are significantly different.For EP El Niño years,the suppression of convective activity in the waters near the Philippines stimulates the meridional wave train in the extratropical area.Meanwhile,the southward wind component of the anomalous anticyclonic circulation in response to the wave train in the high latitudes forms warm convection transport to the Kara Sea area and causes sea ice retreat as a result.However,the Kara Sea area is mainly influenced by the westerly anomalies from the open Atlantic Ocean during EP La Niña years.Moreover,both the composite results and the analysis results of individual EP La Niña event show that there is a remarkable westerly jet center over the North Atlantic in autumn,which is conducive to downstream transport of warmer seawater from the open North Atlantic,affecting sea ice in the Kara Sea as a result.The above results show that the nonlinear response of circulation field in the extratropical areas to EP ENSO leads to a significant nonlinear response of Arctic sea ice to EP ENSO in turn.
    9  Definition and characteristics of extreme events of ozone valley over the Tibetan Plateau
    XU Wenwen GUO Dong SUN Xiaojuan SHI Chunhua CHEN Dan DUAN Mingkeng RAO Jian DU Yahan TANG Zhou
    2019, 42(1):129-137. DOI: 10.13878/j.cnki.dqkxxb.20180829001
    [Abstract](1350) [HTML](0) [PDF 21.83 M](2280)
    Abstract:
    Studies on the extreme events of ozone depletion in the polar region are many but studies on the ozone extreme events of ozone valley over the Tibetan Plateau(OVTP) are few and mainly focus on case analysis.Therefore,this paper defined the extreme and ordinary events of OVTP and analyzed their characteristics based on the ERA-interim daily reanalysis data from 1979 to 2016.Firstly,this paper selected the characteristic isolines of extreme(ordinary) strong and weak OVTP according to total column ozone(TCO) and zonal deviation of TCO (TCO*) after plus or minus 1.96(1.29) standard deviation of TCO and TCO*,respectively.Secondly,this paper defined the extreme strong OVTP with TCO ≤ 250 DU and TCO* ≤ -55 DU,the ordinary strong OVTP with 250 DU* ≤ -40 DU,the ordinary weak OVTP with 285 DU* ≤ -6 DU,and the extreme weak OVTP with-6 DU* ≤ 0 DU.Then this paper analyzed the characteristics of the extreme and ordinary OVTP.The results as follows:1) Extreme strong OVTP frequently occurs in summer and autumn,and it appears most in October when the occurrence is up to 2.0%.Ordinary strong OVTP frequently occurs in spring and summer,with the most occurrences in July when the occurrence is up to 1.7%.Extreme weak OVTP frequently occurs in autumn and winter,with the most occurrences in December when the frequency is up to 3.8%.Ordinary weak OVTP frequently occurs in winter,with the most occurrences in January when the frequency is up to 2.0%.2) Trends of extreme OVTP are significant,while trends of ordinary OVTP are not significant.Extreme strong OVTP increased significantly with 0.004%·a-1.Extreme weak OVTP decreased significantly with 0.015%·a-1.3) Area and strength of extreme strong OVTP are the largest in autumn and reach the maxima in October,with 4.3×105 km2 and 1.5×105 t.Area and strength of ordinary strong OVTP are the highest in summer and peak in July,with 1.7×105 km2 and 4.1×103 t.Area and strength of extreme weak OVTP are small in spring and summer and reach the minima in April,with 3.2×104 km2 and 1.1×102 t.Area and strength of ordinary weak OVTP are small in spring,summer and autumn,and reach the minima in April (October),with 2.5×104 km2and 68 t (2.2×104 km2 and 97 t).4) The significant trends of area and strength of extreme (ordinary) strong OVTP are 2.5×102 (4.5×102) km2·a-1and 2.5×102(4.5) t·a-1,respectively.On the other hand,the significant trends of area and strength of extreme (ordinary) weak OVTP are -1.7×104 (-2.3×103) km2·a-1 and -7.0×103 (-2.7×102) t·a-1.
    10  Extreme temperature projection over northwestern China based on multiple regional climate models
    YU Entao SUN Jianqi
    2019, 42(1):46-57. DOI: 10.13878/j.cnki.dqkxxb.20181109001
    [Abstract](1400) [HTML](0) [PDF 11.50 M](2342)
    Abstract:
    In this study,the spatial and temporal changes of future extreme temperature over northwestern China was evaluated using 11 regional climate models in the context of the Coordinated Regional Climate Downscaling Experiment for East Asia (CORDEX-EA) project.The main findings are as follows:1) The regional climate models could reproduce the spatial distribution of extreme temperature over northwestern China.2) The multi-model ensembles indicate that the FD and ID will decrease,while the TR and SU will increase in the 21st century over northwestern China.3) The WSDI and GSL of northwestern China show increasing trend,while the CSDI and DTR show decreasing trend in the 21st century.4) Due to the increase of temperature,TX90p and TN90p will increase,while TX10p and TN10p will decrease in the future.5) TXx,TNx,TXn and TNn show an increasing trend in the 21st century.Therefore,in the future,the probability of cold extreme events will decrease,and the probability of warm extreme events will increase,along with distinct spatial heterogeneity.
    11  The mechanism of transition of linear mesoscale convection system mode in a continuous rainstorm process in the Jiangnan region
    WANG Xue LIN Yonghui LIU Shanfeng
    2019, 42(1):138-150. DOI: 10.13878/j.cnki.dqkxxb.20170428001
    [Abstract](900) [HTML](0) [PDF 22.41 M](2304)
    Abstract:
    In this study,based on intensive surface observational data,sounding data,NCEP reanalysis data,Doppler radar data,and resolution data produced by the WRF model,the evolution process,organizational structure and modal transformation mechanism of the mesoscale convective system during the process in a continuous rainstorm in the Jiangnan region from 20 to 21 June 2014 are studied.The rainstorm process which formed in the region was ahead of the eastward trough at 500 hPa,between the shear line and the wet and warm low-level jet at 850 hPa.An organizational modal transformation occurred during the MCS evolution.In the eastward development,the convection formed TS MCS,then gradually changed to PS MCS.
    In order to analyze the mechanisms of the transition,the WRF model was used to simulate the process.The results of the simulation were nearly in accordance with the observation facts in the isochrones of the squall line,the 24 h simulated cumulative precipitation,and the transition process from TS mode to PS mode.The main results could be summarized as follows:
    1)Blocked by Fengshan,Hunan Province,the airflow of the climbing slope forms scattered convection in the leeward slope of the mountain,while the convection moved eastward and continuously developed.Under the combination of terrain blocking and wind field convergence,the convection developed into TS MCS,then rapidly into a bow-shaped echo.Under the push of the low vortex,the arcuate echo quickly moved toward the northeast.Blocked by the front,the bow-shaped echo moved at a slower speed,and the direction of movement changed,while a new convection at the front side of the north-south convective zone was constantly forming.Combined with the initial convection,the new convection gradually transformed into PS MCS.
    2)In the environmental wind field,the relative inflow of TS MCS was a vertical convection line,and the cyclone was generated on the north side of the arcuate echo.The circulation distribution of PS MCS is such that the lower level is the relative inflow of the vertical convection line and the middle and upper layers exhibit the relative inflow of the parallel convection line.A smaller scale cyclonic circulation formed at the tail of the convection zone.
    3)In TS MCS,the strong convection zone was located in the positive disturbance zone,forming vertical force acting on the convection line,and thus resulting in a relative inflow perpendicular to the convection line.However,in the parallel convection line direction,the disturbance pressure change was small,thus the parallel relative airflow was weak,and the overall presentation of the relative airflow was a vertical convection line.In the PS MCS,the overall performance of the pressure distribution characteristics included the southwest positive pressure disturbance and northeast negative pressure disturbance.The pressure gradient force in the northeast and northwest formed a relative inflow of the parallel convection line,which led to a transformation of the MCS microstructure.
    12  The influence mechanism of the Indian Ocean Dipole on precipitation anomaly during the post-flood season over the Guangdong-Guangxi region in 1994
    CHEN Junzhi JIN Dachao WANG Guodong QING Tao
    2019, 42(1):151-160. DOI: 10.13878/j.cnki.dqkxxb.20180912011
    [Abstract](746) [HTML](0) [PDF 2.30 M](2163)
    Abstract:
    In this paper,based on the daily precipitation data of 174 stations in Guangdong Province and Guangxi Zhuang Autonomous Region(hereinafter collectively referred to as the Guangdong-Guangxi region) at a total of 2 400 China surface meteorological stations from 1979 to 2017,monthly sea surface temperature data of the Hadley Center in the UK and monthly NCEP/NCAR reanalysis data,the influence mechanism of the Indian Ocean Dipole(IOD) on the extreme positive anomaly of precipitation in southern China during the late flood season (July-September) of 1994 is analyzed.The study results show that the positive IOD event in 1994 can affect the above-normal precipitation over Guangdong-Guangxi region during post-flood season.In addition,when the sea surface temperature in the tropical southeastern Indian Ocean(eastern pole of IOD) is negative,water vapor is transported to the Guangdong-Guangxi region along the abnormal water vapor transport channels in the northern part of India,northern part of the Bay of Bengal,central part of the Bay of Bengal,central part of the Central South Peninsula,and two parts of the Bay of Bengal,resulting in an abnormal convergence of water vapor in the Guangdong-Guangxi region.At the same time,when the SST anomaly in the eastern pole of IOD is low,the anomalous convergence of the lower troposphere and anomalous divergence of the upper troposphere in the Guangdong-Guangxi region will then be controlled by the anomalous upward motion,which will result in meridional vertical circulation.The above physical mechanisms work together,thereby leading to extreme positive precipitation anomalies in the Guangdong-Guangxi region in the flood season after 1994.
    13  Outline of the real-time seasonal haze pollution prediction in China in recent years
    YIN Zhicong WANG Huijun DUAN Mingkeng
    2019, 42(1):2-13. DOI: 10.13878/j.cnki.dqkxxb.20181224001
    [Abstract](1538) [HTML](0) [PDF 2.36 M](2907)
    Abstract:
    In recent years,severe haze pollution has been damaging human health,traffic security,the ecosystem and social economy in eastern China.In addition to the haze forecast within 1 week,seasonal haze prediction provides scientific support for longer periods to the decisions of emission reduction.In this study,taking the annual increment as the predictand,monthly prediction models were trained for the Beijing-Tianjin-Hebei and Yangtze Delta regions.The performances of the built models were similar,with 2 days of root-mean-square error and a>80% simulation rate of the anomalies' mathematical sign.In the real-time seasonal prediction for Beijing-Tianjin-Hebei haze days in the winter of 2016,the results with respect to the climate mean(the previous year) were completely (mostly) accurate.During the winter of 2017,the predicted biases for the December and January haze days in the Yangtze River Delta were very small,and the bias of February was nearly 2 days.
    14  The influence of temporally-random precipitation on monthly total precipitation
    SU Jingzhi
    2019, 42(1):109-116. DOI: 10.13878/j.cnki.dqkxxb.20180929001
    [Abstract](1245) [HTML](0) [PDF 5.36 M](2213)
    Abstract:
    Due to the random nature of precipitation events,the accumulated precipitation within the first (final) 1-5 days of a given month is likely to exceed 50% or more of the total precipitation in that month.Using the daily rainfall data from more than 2 400 stations in China from the period of 1961 to 2017,the statistical analyses in this paper show that such events occur with significant features in both the seasonal and spatial distribution:1)The cumulative precipitation in the first 1-5 days can influence the monthly precipitation during the fall and winter,and the cumulative precipitation in the final several days can influence the monthly precipitation during the period of January to April.2) The number of stations significantly affected by accumulated precipitation of the first several days has a maximum value in some years and months.The number of stations significantly affected by such event reaches about 30-50% of the total number,which corresponds to a large-space extreme heavy precipitation event.3) The stations significantly affected by such events show features of spatial concentration in September.For example,the stations affected by the first 5 days of September are mainly concentrated around Shandong and Hebei Provinces,and such events in those regions have occurred about 20 times during the past 57 years.
    15  Diagnosis features of extreme temperature variations in China based on the NCEP-GEFS reforecasts
    GAO Li REN Hongli ZHENG Jiawen CHEN Quanliang
    2019, 42(1):58-67. DOI: 10.13878/j.cnki.dqkxxb.20180911001
    [Abstract](1189) [HTML](0) [PDF 12.81 M](2608)
    Abstract:
    In this study,based on the reforecast data of NCEP Global Ensemble Forecast System(GEFS) and China homogeneous grid-point observational data,features of extreme temperature variations in the past 30 years are analyzed,and the performance of the NCEP-GEFS in representing these kinds of features is thoroughly investigated.By estimating the historical climatic percentile of 2 m temperature in the observational and model data,the characteristics of extreme temperature in winter and summer,along with the spatial distribution and multi-year trend of extreme temperature days,are analyzed.The results show that some strong regional features exist in the spatial distributions of winter extreme low temperature(ELT) and summer extreme high temperature(EHT) in China,i.e.there are relatively lower temperatures corresponding to the percentile thresholds of the winter ELT in northeastern China,northern China and the Qinghai-Tibet Plateau,with higher temperatures corresponding to the percentile thresholds of the summer EHT in southern China,northwestern China and the Yangtze River Basin.Both the summer mean temperature and EHT days throughout China show increasing trends in the past 30 years,and the winter mean temperatures are also increasing throughout most of China,yet decreasing in northwestern and northeastern China.Correspondingly,the numbers of days of the winter ELT are decreasing in most areas,and only slightly increasing in small parts of northwestern,northeastern and southern China.The NCEP-GEFS reforecasts are able to accurately reproduce the climatic trends and interannual variations of the seasonal mean temperature and extreme temperature days in the winter and summer of China,yet varying degrees of cold biases exist in the different regions.The biases in winter are significantly larger than those in summer,and as the forecast length increases,these cold biases are gradually strengthened in winter,while gradually weakened in summer.Therefore,it is suggested to adopt the relative definition of extreme temperature based on the percentile threshold,which can automatically correct these systematic biases in the model analysis and prediction products.

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