Research On The Spatiotemporal Distribution Characteristics And Meteorological Conditions Of Dry And Wet Low Temperature Disasters In Winter In Chin

Cold hazard is one of the main meteorological disasters in China,including extreme low temperature,abrupt temperature drop,freezing(referred to as cold hazard without precipitation),and rain&snow,snowstorm(referred to as cold hazard with precipitation).Cold hazard exerts a huge influence on economic construction and people’s lives,resulting in huge social and economic losses.Therefore,it is very important to clarify the temporal and spatial distribution,meteorological conditions,and mechanisms.Based on the nationwide records of cold weather hazards combined with the high-precision reanalysis data NASA MERRA2 during wintertime(November-March)in the period of 1980-2019,a daily dataset of low-temperature disasters(low temperature,abrupt temperature drop,freezing,rain&snow,and snowstorm)in China has been constructed.Using linear trend analysis,ensemble empirical mode decomposition(EEMD)method,and wavelet analysis,the temporal and spatial distribution characteristics of various kinds of cold hazards in China are studied.Comparisons of meteorological factors thresholds,large-scale circulation conditions,and main mechanisms for hazardous and non-hazardous extreme cold events over the Yangtze River in Winter are revealed.Results show that cold hazards without precipitation(low temperature,abrupt temperature drop,and freezing)are often co-caused by two or more kinds of cold hazards,while cold hazards with precipitation(rain&snow and snowstorm)are mostly caused by a single kind.The occurrence frequency of different kinds of cold hazards has significant regional and seasonal differences: the low temperature hazard had the widest affected area in January,and the occurrence frequency presents a "high-low-sub-high" distribution pattern from south to north;The frequency of abrupt temperature drop hazard occurred the most in December,whose occurrence frequency is "high in the East and low in the west";Rain& snow hazard occurred most in southern China from January to February;The occurrence of snowstorm hazard is significantly more frequent than the other four kinds of cold hazards,mostly distributed in susceptible regions such as pastoral areas.As far as the whole country is concerned,the frequency and affected area of most cold hazards(except snowstorm)show a long-term increasing trend.The snowstorm hazard,however,has an increasing trend in the affected area,mainly contributed from January,and a decreasing trend in the frequency,mainly contributed from March;The interannual variability of the frequency of cold hazards(except snowstorm)increased significantly after 2005.For single-kind disasters,only the affected area of abrupt temperature drop hazard showed a significant increasing trend,mainly due to January.For the compound-kind cold hazards,the combination of abrupt temperature drop and cold hazards with precipitation increased significantly.(2)Whether extreme cold events cause hazards mainly depend on the range and duration of moderate cold,extreme cold,and precipitation over the Yangtze River.However,the difference between hazardous and non-hazardous extreme cold events is minor for the sharp temperature drop area.Strengthened tropospheric Blockings,stronger Siberian High,and more westward deepening East Asia trough are the tropospheric features for hazardous extreme cold events.Besides,the split-type sudden stratospheric warming(SSW)and persistently strong stratospheric polar vortex that shifts toward North America are also favorable for the hazardous extreme cold events over the Yangtze River.The theory of moist isentropic mass circulation(MIMC)further indicates that wet and cold air dominated in the early stage(-7～+1d)for hazardous extreme cold events,mainly due to the intensification of the Ural Blocking and the enhanced Siberian High transporting extremely warm and humid air through the middle and low latitudes,and the convergence of the equatorial cold and humid air transporting from the high latitudes over the Yangtze River.While dry and cold air dominated in the later stage(+1d),which is mainly caused by the convergence of dry and cold air near 60°N transporting to the equator.