At the turn of spring and summer, severe convective weather begins to be active, and heavy rainfall, thunderstorms and strong winds occur in many places. So, what is severe convective weather? What are its causes? What are its characteristics? and how can we prevent it? Let's find out↓↓↓
"风雨雷"——强对流组团揭秘
Severe convective weather can occur in all seasons, with the most frequent occurrences in spring and summer. Geographically, severe convective weather is not the "patent" of the southern region, and from the seasonal analysis, the southern part of the country tends to appear earlier than the north. Severe convective weather is menacing and hasty, and is considered to be "acute", with a short life cycle with obvious suddenness, usually one hour to more than ten hours, and the shortest is only a few minutes to one hour. Severe convective weather is sudden and destructive, and is the top priority of meteorological disaster prevention and mitigation in mainland China.
Thunderstorms Image source: World Meteorological Organization
With the arrival of summer, the influence of cold air weakens on the continent, and the warm and humid airflow increases, allowing it to move north to further north. Therefore, the Yangtze River Basin, North China and Northeast China will become areas with frequent severe convective weather.
So, what exactly is severe convective weather? In meteorology, it refers to the violent and destructive disastrous weather produced by deep wet convection, which is any one or several of these weather, such as thunder and lightning, heavy precipitation, hail, gale, tornado, etc.
Hail Image source: Qianku.com
Speaking of deep wet convection, it is a weather system caused by strong vertical movement of air, which is manifested as a convective cloud system or isolated convective cloud, which is a small- and medium-scale weather system in meteorology, and is listed as the third most lethal and catastrophic weather after tropical cyclones and floods.
Thunderstorms, which often occur in summer, are a type of weather caused by deep wet convection. In the summer afternoon, the surface is scorched, absorbing the sun's short-wave radiation, and the temperature rises. Of course, the physical processes of severe convective weather vary from one type to another. Severe convective weather is the background of large-scale weather systems, which affect or determine the generation, development and movement of medium- and medium-scale weather systems. In addition, the influence of strong convective weather on the dynamic and thermal effects of the underlying surface is also very important.
"Canyon Wind" - Cracking the "Effect" of Strong Convection
Nowadays, with the change of seasons and the frequent occurrence of severe convective weather, words such as narrow tube effect and train effect have gradually entered the public eye. So, what exactly are these effects in severe convective weather, and what are their powers?
The so-called narrow tube effect is also called the canyon effect. In the literal sense of the word "canyon", the wind in the canyon is often more violent than the wind on the plain, and the air flow from the open area into the narrow terrain becomes smaller, the passage becomes smaller, the air circulation speed increases, and the wind force increases suddenly. The effect of this topography on airflow is known as the narrow tube effect, also known as the "canyon wind".
The Narrow Tube Effect Image source: Jiupai News
Nowadays, with the acceleration of urbanization, high-rise buildings have become the "standard" in major cities. However, there is also a narrow tube effect between tall buildings in cities.
Between tall buildings, local strong winds occur due to the narrow tube effect. The more tall buildings, the larger the volume, and the closer the spacing, the more likely it is that the narrow tube effect will occur. In the narrow area between the high-rise buildings in the city, the wind is generally stronger, and the instantaneous wind force between some narrow buildings often exceeds level 7, so that the driving car will shake. "Canyon wind" in cities is a new problem faced by major cities, and the narrow pipe effect caused by high-rise buildings will aggravate the impact of wind disasters, and relevant international organizations have listed it as one of the 20 new urban disasters faced by large cities.
Another term, the train effect, is first associated with trains. Yes, it does have similarities with the effects of trains.
The train has many carriages, and as it passes, multiple carriages pass one by one, and those standing by the railroad track will constantly feel the tremendous momentum of the train.
This is like the precipitation of convective clouds arranged in a row, each convective cloud (called a convective cell) will produce short-term heavy precipitation, and when multiple convective clouds pass through a place in turn, the amount of precipitation produced by them will accumulate, resulting in heavy rainstorms or even extremely heavy rains, a phenomenon known in meteorology as the train effect.
Different from the precipitation brought by stable clouds, the short-term heavy precipitation under the train effect is more likely to accumulate and produce heavy rainfall or heavy rain, which also makes the train effect an important bridge between short-term heavy precipitation and heavy rainfall.
"Flash" - the cause of severe convective weather
On the mainland, severe convective weather is generally more frequent in spring and summer, mainly due to the specific climatic conditions of spring and summer. At this time, the air is humid and the water vapor content in the atmosphere is high, which provides a material basis for the formation of severe convective weather.
At the same time, the uplift of the airflow in spring and summer also provides dynamic support for the occurrence of severe convective weather. The reasons for this are: first, the convergence of cold and warm air, and the dynamic uplift of the atmosphere allows the condensation of water vapor in the atmosphere to release latent heat; second, the solar radiation heats the ground, causing the lower atmosphere to form an unstable state of hot and cold, and the thermal uplift occurs. All of this provides a "flash" impetus for the occurrence of severe convective weather.
It is important to note that although severe convective weather is more common in spring and summer, it does not mean that other seasons will not occur. In fact, severe convective weather can occur in any season, as long as the corresponding meteorological conditions are met. For example, in winter, although cold air predominates, strong convective weather, such as "thunder and snow", can also occur under certain special circumstances, such as a sudden increase in warm and moist airflow or under the action of factors such as terrain uplift.
On September 5, 2023, due to the impact of extreme precipitation, the water level of many rivers in Fujian exceeded the warning level. The Lianjiang Shanzi Reservoir in Fuzhou City comprehensively considered the heavy precipitation in the upstream and the heavy rainstorm in Lianjiang County, and opened the flood gates. Photo/Li Na
Due to the vast size of the continent, the frequency and duration of severe convective weather are significantly different between the south and the north, and the overall performance is that the south is earlier than the north.
Around April, the convergence of cold and warm air is frequent, especially in the southern part of the Yangtze River and southern China, where the cold air and the warm and humid airflow from the South China Sea form a confrontation, resulting in the occurrence of severe convective weather. In summer, the cold air weakens, the warm and humid airflow becomes stronger, and the frequent areas of strong convective weather will move northward to the Yangtze River Basin, North China and Northeast China.
So, is the emergence of the severe convective weather family related to the geographical environment?
The topography and geomorphological characteristics of the continent significantly affect the distribution and characteristics of severe convective weather. In general, there are more places in the lower latitudes than in the middle latitudes. This is due to the high temperature and rainy temperature in the low latitudes, while the temperature and humidity of the near-surface atmosphere rise in the summer and half of the summer in the middle latitudes, which increases the instability of the atmosphere and triggers convective activity. In areas of the same latitude, there are more severe convective weather in the mountains than in the plains. This is due to the fact that topographic conditions such as mountains and hills tend to cause atmospheric instability, and the air is prone to uplift to form convection.
Tornado Image source: World Meteorological Organization
In recent years, tornadoes have gradually entered the public eye, and they tend to occur at low altitudes where thunderstorms are frequent. The formation of tornadoes is closely related to the strong updraft and downdraft in severe thunderstorm clouds, and strong vortices are formed when strong shear occurs between these air currents. In plains where the terrain is flat and less easily disturbed, such as the Yangtze River basin on the mainland and the Midwest of the United States, such as the Yangtze River basin on the mainland, this vortex is more likely to sustain and evolve into tornadoes.
"Defense" - scientific response to severe convective weather
Imagine such a scene: when you just go out, the sky is clear, a thunderstorm and a strong wind are in unison, and a moment of lightning and thunder, and the rain is like a pour, standing on the street that instantly becomes a river, and the rain next to the sewer at your feet is like a strong whirlpool at this time...... This is a scenario of severe convective weather, and it is also a reality that may occur at any time, if severe convective weather occurs, how to respond quickly to protect our own safety?
Severe convective weather such as thunderstorms, gales, tornadoes, and hailstorms have the characteristics of sudden occurrence, rapid movement, small range, and strong destructive power. It is indeed difficult to prevent the rapid and "violent" severe convective weather, but the risk of disasters can still be reduced by doing the following two things well.
First of all, it is necessary to grasp the weather information and understand the occurrence time of severe convective weather, so as to "prevent it". Severe convective weather mostly occurs in spring and summer, and the continent is vast, and from February, severe convective weather usually occurs from south to north, and after April, strong convective weather also begins to occur in the north. Know the time and place of high and frequent occurrence of severe convection, and pay attention to weather forecasts, early warnings and early warning signals in advance during key seasons. At present, the Central Meteorological Observatory will issue early warnings in the early stage of severe convective weather, and the national severe convective weather warning standards are divided into blue, yellow, and orange from low to high. The warning level is determined according to the level of thunderstorms, strong winds and hail, and the intensity of short-term heavy precipitation, and the time and space scale of the warning is as follows: strong convective weather is expected in some areas of three or more adjacent provinces (autonomous regions and municipalities) in the next 24 hours.
On March 31, a landslide occurred in Changtanhe Village, Jinci Street, Cili County, Zhangjiajie City, Hunan Province, and a mudslide roared down, causing serious damage to houses. Guided by meteorological warning, 130 people were evacuated in time and successfully avoided danger. Photo/Zhuo Renqing
In addition to predicting the occurrence time and intensity of severe convective weather through early warnings and early warning signals, you can also learn to read the radar echo map to grasp the occurrence trend of severe convection. The official website of the Central Meteorological Observatory publishes live weather every day, including radar echo maps. The colors in the radar echo map are marked from blue to purple according to the intensity of the echoes, and the intensity, range, and trend of rainfall can be determined by the echoes of different colors and their direction of movement. Therefore, when there are red and purple echoes in a certain area on the radar map, strong convective weather may occur in this area, and if the red and purple echoes move in the direction where they are located, you need to be vigilant.
How do we deal with heavy precipitation in outdoor streets? First, we need to stay away from whirlpools, manhole covers, sewers, etc., and second, we need to avoid the "narrow pipe effect" and stay away from the narrow passages between high-rise buildings. If hail is also present at this time, you should look for shelter, quickly enter the room or take shelter under a strong shelter, if there is no suitable shelter, squat down with the wind on your back, hold your head in your hands, protect your head, chest and abdomen from attack, and pay attention to stay away from glass objects such as windows.
What should you do if you encounter strong convective weather while driving? In case of short-term heavy precipitation weather, you should not enter the tunnel, and you should detour when you encounter culverts, tunnels, low-lying areas and under overpasses and other unknown waterlogged road sections. In the event of a tornado, if you stay in your car, drive in the opposite direction or leave the car immediately and take shelter in a low-lying area. If you are experiencing strong convective weather in your home, be sure to stay away from doors, windows, and the outer walls of your house, and take refuge in a wall or small room that moves in the opposite direction of the wind, where the basement or semi-basement is the safest place to shelter from the wind. In case of thunderstorms, it is strictly forbidden to use the water intake device connected to the solar water heater, and do not go near trees, metal doors and windows, etc.
Source: China Meteorological Administration