What is the weather system that caused the cold on March 9?

During the winter solstice, the daytime is the shortest, the angle of direct sunlight on the ground is the smallest, and the sunshine intensity per unit area on the ground is also the smallest, so the ground gets the least solar heat every day, and the daily heat budget deficit on the ground has not reached the maximum. On the solstice after winter, although the day is growing day by day, the growth rate is very slow. From the winter solstice to the "March 9" period, the average daily growth is only about one minute. At the same time, the angle between the direct angle of the sun and the ground is slowly increasing. The daily heat budget of the ground is still in deficit, so it continues to lose heat and the ground temperature continues to drop. The deficit of total heat balance on the ground only reaches the maximum in the season of March 9. Because the temperature is mainly affected by the ground temperature, this leads to the lowest temperature in the "March 9" season. In addition, around March 9, the cold air mass in Siberia also developed to a very strong stage, and the frequent invasion of strong cold air is also one of the reasons for the low temperature. The earth absorbs heat from solar radiation during the day and releases heat at night. In summer, you absorb more heat during the day and emit less heat at night, so you can accumulate some heat every day. In winter, less heat is absorbed during the day and more heat is released at night. Not only can you not accumulate it every day, but you also need to use some of the heat accumulated in summer. During the winter solstice, less heat is absorbed during the day, but there is still some stored heat on the earth. After the winter solstice, less and less heat is stored. By March 9, the earth's heat storage is the least in a year, so it is the coldest.