Analysis on characteristics and causes of a heavy snowstorm event in southern Henan in the beginning of January 2018

Based on conventional observation data and NCEP/NCAR 6-hour reanalysis data with 1°×1° resolution, we analyzed the characteristics and causes of a heavy snowstorm event in southern Henan in the beginning of January 2018. The results show that this event, which is characterized by long duration, more total snowfall, large precipitation intensity and deep snow cover, occurred under a circulation environment of the stable maintenance of blocking high in eastern part of Ural Mountains in 500 hPa with multiple short wave troughs moving westward and superposing in same phase in the mid- and high- latitudes. During the severe snowfall period of this event, there are two sources supplying water vapor to southern Henan in the lower troposphere, i.e., the water vapor transported by 700 hPa southwest jet from the Bay of Bengal and by 850 hPa southeast airflow from the south of the East China Sea, respectively. The intensive transportion and convergence of water vapor in the low-level, the wet layer through whole troposphere and the continuance of high specific humidity and whole layer precipitable water are of good indicative for the origination and continuance of snowstorm. The cold cushion in the low-level is helpful for southwest warm and wet airflow climbing above the inversion layer, and the variation of vertical wind shear in the low-level is a better indication for the change of snowfall intensity. The establishment of the high-level jet stream precedes that of southwesterly low-level jet. The southward movement of high-level jet axis makes the low-level jet develop northward, and the coupling of the high-level jet and the low-level jet stimulates the ascending branch of the secondary circulation, which provides the intensive ascending movement for the occurrence and development of snowstorm. The frontogenesis in 700 hPa and 850 hPa in the lower troposphere is conducive to the strengthening and continuation of snowstorm.