This study uses station meteorological observation data from National Meteorological Information Center of China (http://www.nmic.cn/), the monthly climate data from the National Climate Center of China (http://cmdp.ncc-cma.net/) and the NCEP data from NOAA Physical Sciences Laboratory (https://psl.noaa.gov/).

This study mainly uses comparative analysis, differential analysis and the basic equations of atmospheric motion to analyze the data (Yun, 2021; Ibragimov et al., 2021; Haro et al., 2015). These methods will be introduced as follow.

Comparative analysis method is to compare two or more data and analyze the differences, so as to reveal the development, changes and regularity of these things. It can directly see the change or gap in a certain aspect of things, and can accurately and quantitatively show how much the change gap is.

Differential analysis means the analysis of deviation of the causes and consequences of the required goal and the stage of the system when the error completes the system function, so as to reduce the error to the minimum. Common differential analysis methods include general error analysis, special error analysis, qualitative error analysis, quantitative error analysis, forward error analysis, backward error analysis, random error analysis and interval error analysis.

The basic equations of atmospheric motion include the equation of motion, the continuity equation, state equation, thermodynamic equation and water vapor equation. Through the use of these equations, we can calculate the state change of the atmosphere when we have the initial conditions and boundary conditions.

Figure 1 shows the mid latitude of Eurasia in late October 2020, which presented a situation of “three troughs and two ridges”. The troughs and ridges deepened and the wavelength became shorter. The North China was controlled by the northwest air flow behind the troughs and the cold air activities were more frequent.

From October 20 to 23, sand blowing or sand dust weather occurred in central

and western Inner Mongolia, western Gansu and northern Shanxi. There are also small to moderate snow or sleet in the northeast of Tibet, Qinghai, the western Sichuan Plateau, the northeast of Inner Mongolia and the northwest of Heilongjiang, and heavy snow in the northeast of Inner Mongolia and the northwest of Heilongjiang; There is light to moderate rain in the middle east of the northeast China (Nie & Gao, 2021).

From the perspective of the situation, the cold air weather from October 20-23 was caused by the northwest air flow behind the troughs above the North China.

Figure 2 and Figure 3 show the 925 hPa geopotential height and temperature

fields at 8:00 on October 20. The cold center was located in the west of Lake Baikal, and there was a high-pressure center in the north of Altai Mountain, the intensity of the high pressure center value can be greater than 925 hPa. Meanwhile, there was also a low-pressure center in the west of the Greater Khingan Mountains, with the central air pressure, lower than 690 gpm and the transversal trough was located in the south of Lake Baikal, with strong cold advection behind the trough.

At 500 hPa geopotential height field (Figure 4), there is an evacuation trough in the area from the south of Lake Baikal to the south of Shanxi, which forms a forward inclined configuration with the 925 hPa transverse trough, which indicates that the system will further develop and deepen. And the cold air started to grow.

From Figure 5 and Figure 6, it can be seen that at 08:00 on October 21, the 925 hPa high pressure center moved to the lower pressure center moved to the northern China, and the transverse trough turned vertical, driving the cold air to move eastward and southward, bringing the cold air to the northern China. There was a cold center below minus 10˚C in the east of Baikal Lake.

The 500 hPa (Figure 7) trough moved eastward to Lake Hulun and the Huai River basin, and it was significantly deepened. In eastern China, westerly troughs have sunk deep into the Huai River basin, allowing cold air to seep southward. This weather situation is more conducive to guide the cold air accumulated at high latitudes southward to affect most parts of China.

From Figure 8, it can be seen that the 925 hPa trough kept moving eastward and southward, which driving the cold air to the further south part of China. On the low-level 925 hPa weather map, cold air split southward, accompanied by the prevailing cold air from the northwest, which was conducive to the accumulation of cold air and provided a cold cushion.

And Figure 9 shows that the 500 hPa trough moved to the Huang Huai Area, and became even deepened. The cold air became even strong than before. The 925 hPa temperature field at 8:00 on October 22, 2020 was shown in Figure 10, the temperature of some areas in the middle and eastern part of Inner Mongolia, North China, northeast and other places dropped seriously.

On October 23, as Figure 11 and Figure 12 shows, the mainland of China was controlled by the cold high pressure and located behind the 500 hPa trough. The temperature gradually increased and the cold air process ended.