What's the difference between a thunderstorm caused by a trough and a thunderstorm caused by a cold vortex?

The method of forecasting hail by weather chart is usually based on historical cases, and the previous hail weather conditions are divided into several types, and the forecast indexes are given respectively. In practical work, hail weather is predicted by similar method.

The basic characteristics of each type are briefly described as follows.

1. High altitude cold trough type

There is a cold trough in the troposphere, with a clear front and obvious movement. Because of the different moving speeds of the upper and lower front areas, there may be a forward trough and a backward trough, and the hail can be in front or in front. The main hail suppression mechanism is that the front movement promotes the warm and humid air in front to rise. This kind of hail can be divided into two sub-categories according to the mutual position between the hail area and the high-altitude trough, that is, the hail behind the trough and the hail before the trough.

2. High altitude cold vortex type

There is a closed low pressure center at a height of 500 millibar, which is equipped with a cold box or cold center. The cold vortex in the upper air is a deep convergence system with a strong positive vorticity center, which is beneficial to the rise of warm and humid air in the lower layer, and the moving speed is slow, and hail can appear for several days. According to statistics, hail areas mostly appear in the southeast quadrant of the cold vortex and within 7 latitudes from the center of the cold vortex. The cold vortex centers that affect hail in China are generally generated in Lake Baikal and the People's Republic of Mongolia, and then move to the southeast of China. This cold vortex can be divided into northeast cold vortex, Mongolia cold vortex and northwest cold vortex. The main influence area is east of east longitude 100 degrees and north of 35 degrees north latitude.

3. Northwest airflow pattern at high altitude

There is a stable long wave trough on the isobaric surface of 500 mbar, the temperature trough lags behind the height trough, and the long wave trough inclines backwards with the height. There is a small stream of cold air sliding down in the northwest airflow behind the trough, and the vertical shear of wind speed is strong, which can promote the unstable energy release of warm and humid air in the lower level and form hail. Because the long wave trough is relatively stable, it can hail for several days in the same area. This type can appear from April to September. The hail area is mainly east of east longitude 100 degrees and north of 32 degrees north latitude.

4. South branch trough type

In spring, over the vast area south of the Yangtze River in China, northwest polar front jet and southwest subtropical jet appear alternately, that is, cold and warm air meet frequently. The Eurasian circulation is characterized by the polar front retreating to 40 degrees north latitude, a wide low-pressure area from western Siberia to the People's Republic of Mongolia, a low-pressure active area around Lake Baikal, cold air moving from west to east, and the southern ridge moving on isobaric surfaces of 500 mbar and 300 mbar. There is an obvious south branch trough in the Bay of Bengal. The southwest airflow in front of the trough merges with the southwest airflow on the west side of the western Pacific subtropical high, forming a strong wind belt-subtropical jet at high altitude. When convergence center, convergence zone, shear line and other systems appear in the warm region of the middle and lower troposphere, the warm and humid airflow rises, which is easy to cause strong convective weather.