Recently, the southern regions of Jiangnan and Huanan have been experiencing frequent severe convective weather events.
In the early hours of March 31, a sudden severe convective weather event hit Nanchang City, Jiangxi Province, peaking around 3:00 AM. Nanchang City and Nanchang County recorded level 9 strong winds, while places like Jinxian County experienced gusts reaching level 11.
Following this, news that the #SevereConvectiveWeatherInNanchang caused 4 deaths and injured more than 10 people# quickly rose to the top of Weibo's trending topics.
We witnessed this scene—huge floor-to-ceiling windows were knocked down by a gust of wind in an instant, as if they had been blown up by an explosion. It's estimated that, in that moment, the wind speeds might have exceeded those of a level 11 storm.
What is severe weather? Is it normal to have such intense weather in spring?
Severe weather refers to sudden, rapidly moving, extreme, and destructive weather events. Common examples include thunderstorms with strong winds, hail, and short-duration heavy rainfall. There are also less common events such as tornadoes and downbursts.
Yes, it's relatively normal to experience severe weather in spring, as the changing temperatures and atmospheric conditions can lead to the development of these intense weather phenomena.
Several Common Types of Severe Weather | China Meteorological Administration
How does severe weather occur?
Severe weather is caused by strong vertical movements of air, often occurring suddenly in the spring and summer seasons.
While the specific physical mechanisms leading to various types of severe weather may differ, most of them form within an unstable structure where the upper atmosphere is dry and cold while the lower atmosphere is warm and moist.
Severe Convective Weather Formation Diagram | China Meteorological Administration
What type of severe convective weather occurred in Nanchang at the time?
Specifically, regarding this event in Nanchang, there was a distinct cold and dry layer aloft, paired with a warm and moist layer at lower altitudes, presenting a typical unstable structure. Additionally, the presence of a low-level shear line acted as a trigger. This situation is highly conducive to the development of severe convective weather.
On March 31st at 02:00 near Nanchang, a weather phenomenon was observed. Based on the radar echoes at that time, there was a distinct bow-shaped echo passing through Nanchang, which is a typical characteristic of a squall line.
A squall line is a mesoscale convective system characterized by a small-scale, short-lived line of discontinuity in air pressure and wind. Squall lines are typically associated with severe convective weather phenomena such as thunderstorms, strong winds, hail, and even tornadoes.
March 31st Radar Echo | Chinese Weather
In this squall line, there might also be occurrences of downburst.
Downburst refers to a localized strong downdraft within a thunderstorm cloud, which upon reaching the ground generates a linear gust of wind, with wind speeds increasing closer to the surface. It's characterized as sudden, localized, low probability, and severe convective weather.
Downbursts often happen within mature strong thunderstorm clouds. When drier air mixes into the vicinity of a thunderstorm cloud, it causes intense evaporation of water droplets within the cloud. During this evaporation process, heat is continuously absorbed, rapidly cooling the air, increasing its density, and accelerating its downward movement, forming a strong downdraft. This downdraft impacts the ground and diverges outward, creating a short-lived intense linear wind. The 2015 capsizing of the Eastern Star cruise ship in Hubei's Jianli was attributed to a combination of squall line and downburst.
Moreover, in urban areas with skyscrapers, there might be a presence of the "chimney effect," which amplifies the wind, causing even greater destruction.
The chimney effect refers to the phenomenon where airflow speeds up when transitioning from open areas to narrow ones, such as canyons or narrow areas between tall buildings in cities, due to the inability of air masses to accumulate in large quantities. Simply put, it's similar to when you use a hose to spray water and partially block the outlet; the water flow accelerates, based on the same principle.
Is it normal to have such terrifying severe convective weather in spring?
The peak period for severe convective weather varies across different regions, with the south experiencing it earlier than the north.
Typically, in March and April, areas like Jiangnan enter the peak period for severe convective weather.
Image source: China Weather
For instance, in Nanchang, the frequency of thunderstorms noticeably increases starting in March. By April, the average number of thunderstorm days hits 6.2, second only to July.
Image Source: China Weather
Compared to summer, the upper atmosphere in the southern regions remains dry and cold during spring, making it more prone to occurrences of thunderstorms, strong winds, and hail. As summer approaches, the upper atmosphere gradually warms up, leading to fewer occurrences of hail, thunderstorms, and strong winds compared to spring.
Image source: China Weather
In the north, the high-incidence period for severe convective weather typically begins in summer.
How can we prepare for severe convective weather?
Meteorologically, severe convective weather is considered a mesoscale weather system. It generally spans a horizontal range of about several kilometers to two or three hundred kilometers, though some systems may only extend from a few dozen meters to several kilometers.
Its lifespan is short and marked by sudden onset, ranging from an hour to several hours, with the shortest instances lasting only a few minutes. This has always made it a challenging aspect of weather forecasting.
If the weather forecast indicates a possibility of severe convective weather or if a warning has been issued, it's crucial for everyone to stay alert and not take it lightly.
National Severe Convective Weather Warning Standards | China Meteorological Administration
When the meteorological department issues a severe convective weather warning, it's crucial to take preventive measures in advance. For instance, clean up any clutter on your balcony beforehand; seek shelter in a safe indoor area and secure all doors and windows; avoid spending extended periods on the balcony during severe convective weather events.
Severe Convective Weather Preparedness Guide | China Meteorological Administration
