Nanchang’s severe convective weather has become a hot topic. Is such severe weather normal in spring?

Recently, severe convective weather has occurred frequently in Jiangnan and South China. In the early morning of March 31, severe convective weather broke out in Nanchang, Jiangxi Province. The strongest period was around 3:00. Nanchang City and Nanchang County observed 9-level gale, and Jinxian County and other places even saw 11-level gusts. Then the news #Nanchang suddenly had severe convective weather and 3 people fell to their deaths# became a hot search.

We saw a scene like this: the entire French window was blown down by the strong wind in an instant, as if it had experienced an explosion. This also shows that at that moment, there may have been a wind stronger than level 11.

So what is severe convective weather? Is it normal to have such intense weather in spring?

Q1: What is severe convective weather?

Severe convective weather refers to disastrous weather that occurs suddenly, moves quickly, is severe, and is highly destructive. Common ones include thunderstorms, gales, hail, and short-term heavy rainfall; there are also some less common ones, such as tornadoes and downbursts.

Q2: How does severe convective weather occur?

Severe convective weather is caused by strong vertical movement of air, and often occurs suddenly in spring and summer. The physical mechanisms of various types of severe convective weather are not exactly the same, but most of them are formed under the unstable structure of dry and cold upper layers of the atmosphere and warm and humid lower layers.

Q3: What kind of severe convective weather occurred in Nanchang at that time?

Specifically for the process in Nanchang, there was an obvious dry and cold layer in the high altitude, and a warm and humid layer in the low altitude, showing a typical unstable structure, and there was a shear line triggered in the low altitude. This situation is very conducive to the formation of severe convective weather.

Figure: Weather conditions near Nanchang at 02:00 on March 31

From the radar echoes observed at that time, we can see that an obvious bow-shaped echo passed through Nanchang, which is a typical feature of a squall line. A squall line is a mesoscale severe convective weather system, which is a discontinuous line of air pressure and wind with a small range and a short life history. Squall lines are usually accompanied by severe convective weather phenomena such as thunderstorms, strong winds, hail and even tornadoes.

Figure: Radar echo on March 31

The possibility of downbursts in this squall line cannot be ruled out. A downburst refers to a localized strong downdraft in a thunderstorm cloud. When it reaches the ground, it will produce a straight-line strong wind. The closer to the ground, the greater the wind speed. It is a sudden, localized, low-probability, severe convective weather. Downbursts often occur in mature strong thunderstorm clouds. When drier air mixes in around the thunderstorm clouds, it will cause the water droplets in the thunderstorm clouds to evaporate strongly. During the evaporation process, heat is continuously absorbed, the air cools rapidly, and the density increases, accelerating the sinking, forming a strong downdraft. This airflow hits the ground and radiates in all directions, forming a short-term strong straight-line strong wind. (The capsizing of the Eastern Star cruise ship in Jianli, Hubei in 2015 was a combination of a squall line and a downburst.)

Figure: Schematic diagram of the formation principle of downburst

In addition, in cities, there are many tall buildings, and there may be a narrow channel effect, which will add a buff to the strong winds and cause greater damage.

Q4: What is the constriction effect?

The narrow channel effect refers to the phenomenon that when air flows from an open area into a narrow area, such as a canyon or a narrow area between high-rise buildings in a city, the air mass cannot accumulate in large quantities, so the flow rate will increase, and the wind speed will increase. In simple terms, it is like when you spray water with a water pipe and block part of the water outlet, the water will spray out faster. It is a similar principle.

Figure: Schematic diagram of the formation principle of the narrow tube effect

Q5: Is it normal to have such terrible severe convective weather in spring?

The peak period of severe convective weather varies from place to place, and it is earlier in the south than in the north. Usually in March and April, severe convective weather enters its peak period in the Jiangnan area.

For example, in Nanchang, thunderstorms began to increase significantly from March. In April, the average number of thunderstorm days was 6.2 days, second only to July.

Compared with summer, the air in the south is still dry and cold in spring, so thunderstorms, gales and hail are more likely to occur. In summer, the air becomes warmer, so hail and thunderstorms are generally less common than in spring.

The north generally enters the peak period of severe convective weather in summer.

Q6: How to prevent this kind of severe convective weather?

Severe convective weather is a small- and medium-scale weather system in meteorology. The general horizontal range is about 10 kilometers to 200 to 300 kilometers, and some horizontal ranges are only tens of meters to more than 10 kilometers. Its life history is short and has obvious suddenness, about one hour to more than ten hours, and the shorter is only a few minutes, which has always been a difficult point in forecasting. If the weather forecast indicates the possibility of severe convective weather, or even issues a warning message, please be vigilant, do not take it lightly, and take protective measures in advance, for example, clean up the debris on the balcony in advance; hide in a safe room, close the doors and windows; when severe convective weather occurs, do not stay on the balcony for a long time.

Photo and text editor: Pang Lan and Zhang Dongting

Data visualization: Xu Xiaoyuan Yu

Weather Consultant: Xin Xin

Reviewer: Zhai Yuxinxin