Extreme Cold and Snowstorm Event in North America in February 2021 Based on Satellite Data

Using meteorological satellite data and the reanalysis dataset from European Centre for Medium Range Weather Forecasts（ERA5） and others,based on the accuracy evaluation of FY-3 D/VASS temperature in North America,the climatic background,development and evolution of North America winter storm Uri in2021,the triggering effects of polar vortex activities on Uri and the characteristics of atmospheric environment causing extreme low temperature and snowfall are studied.The results show that the average absolute error of FY-3 D/VASS temperature at 100,400 hPa and 850 hPa are 1.14℃,1.44℃ and 2.63℃ respectively.It shows that FY-3 D/VASS temperature can meet the demand of global extreme cold event monitoring,and it is an important data source in regions where conventional meteorological observation is insufficient.FY-3 D/VASS temperature analysis shows that in February 2021,when winter storm Uri is active,the temperature in the western part of the key area of polar cold air activity in North America（50°-150°W,50°-80°N） is 4-8℃ lower than climate mean.The cold air intensity is the strongest in the first ten days of February,and the maximum percentage of temperature anomaly is about 70%.Under the guidance of the warm high ridge in the northeast Pacific,the polar vortex intensifies and extends southward,and the cold air breaks out southward during the vertical rotation of the horizontal trough on the west side of the polar vortex center.The abnormal southward extension of the upper troposphere potential vorticity in high latitude provides upper-level dynamic forcing for the formation of Uri.The cold air in the low layer extends southward and intersects with the warm and humid airflow transported northward over the Gulf of Mexico along the west side of the subtropical high in southern United States.They trigger the low-level vortex of storm Uri and the rapid development of cloud system.The cloud causing strong snowfall of storm Uri shows convective characteristics,with the cloud top brightness temperature reaching lower than-40℃ and even lower than-52℃ in some areas.Lightning is also active in the storm cloud system which causes heavy snow.