Small but strong convection, how difficult is it to accurately report it?

Produced by: Science Popularization China

Author: Half Lazy (Institute of Atmospheric Physics, Chinese Academy of Sciences)

Producer: China Science Expo

At 3 pm on May 30, the weather was hot and stuffy. The workers who were still writing furiously in front of their computers might not have noticed that a storm was brewing: dark thunderclouds had quietly enveloped the area. Strong winds of more than level 10, accompanied by lightning, howled near the workstations, and then heavy rain poured down, drenching the friends who were caught off guard for outdoor activities.

On-site situation

(Photo source: Red Star News)

This was the strongest convective storm in Beijing in four years. At that time, there was not enough water vapor in the air and the rainstorm lasted for a short time. But it brought short-term strong winds of 8-10 levels in all parts of the city, and 5 weather stations recorded 12-level strong winds. Many large trees were uprooted, and the Old Summer Palace also announced that it would be closed for emergency repairs. The reason why Beijing was hit by strong winds, rainstorms and lightning was severe convective weather. So, what is severe convective weather?

Beijing Extreme Wind Map

(Photo source: Beijing Meteorological Bureau)

Strong convection: Don't underestimate me, I'm really strong

Severe convective weather is a general term for disastrous weather caused by strong vertical movement of air, including the thunderstorms, strong winds, short-term heavy rainfall, hail, tornadoes, downbursts, etc. mentioned above.

Severe convection has its own criteria. In my country's operational weather forecast, short-term heavy rainfall refers to rainfall of not less than 20 mm per hour, thunderstorm wind speed exceeding level 8 or 17 m/s, hail diameter of not less than 5 mm, or the occurrence of a tornado.

Although the physical mechanisms for the formation of various types of severe convective weather are not exactly the same, most of them require the coordination of energy, water vapor, and triggering conditions, and are formed under an unstable structure with dry and cold upper layers of the atmosphere and warm and humid lower layers.

To give a simple example, when two gentle extroverted ENFP puppies meet, it often brings a peaceful and warm atmosphere. However, if the two have very different personalities, one is hot and the other is cold, it will bring great lethality, and people within a 5-mile radius need to watch the fight with caution.

When severe convective weather occurs, the ground is often very hot and energy is easily accumulated in the atmosphere. When cold air intrudes from high altitude in some way, since low-altitude air masses are hotter and less dense, while high-altitude air masses are hotter and more dense, light things want to go up and heavy things want to fall down, which easily triggers vertical movement of air.

The violent movement of cold and warm air up and down will bring about a series of chain reactions. Take a typical thunderstorm + short-term heavy rainfall as an example: the warm air gradually cools during the rising process, and the water vapor condenses into water droplets, forming cumulonimbus clouds. The cloud droplets in the cloud collide and merge to become larger, and ice crystals are produced in the high altitude. They hug each other tightly, blocking the penetration of sunlight and refracting sunlight, resulting in the phenomenon of "dark clouds pressing down on the city and threatening to destroy it" in the middle of the day.

Ice crystals in the clouds squeeze each other and rub against each other, which generates electricity that penetrates the air, and thunderous lightning will streak across the sky. When raindrops accumulate to a certain amount, the air currents in the upper air cannot support their weight, and they fall as rain or even form hail. During the descent, they absorb heat and evaporate, causing the temperature to drop. With this hot and cold cycle, the air near the surface flows, forming strong winds. The greater the temperature difference between the upper and lower layers, the more intense this process will be.

If a downburst is triggered, it can instantly generate a strong downdraft that spreads in all directions, creating a strong wind parallel to the ground. The closer to the ground, the faster the wind speed. The maximum wind speed on the ground can reach level 15. The strong wind in Beijing this time is related to it.

Schematic diagram of thunderstorm cloud formation structure

(Photo source: China Weather Network)

Water vapor content also has a great impact on disaster risk . If the temperature in the area where strong convection occurs is high but the water vapor is low, there will not be continuous and large-scale heavy rain. For example, the average precipitation in Beijing from 13:00 to 17:00 on May 30 was 3.4 mm. However, once the water vapor conditions are sufficient, it is easy to form record-breaking extreme heavy rain.

From 8:00 to 14:00 on May 4, nine of the top ten cities in the national meteorological station in terms of precipitation were in Guangdong. Zhuhai experienced a super-torrential rain of 100.8 mm in 29 minutes, and the rainfall intensity in half an hour was comparable to the 720 rainstorm in Zhengzhou.

At 03:00 on May 19, the rainfall at Longmen Port in Qinnan, Qinzhou, Guangxi Zhuang Autonomous Region reached 189.6 mm, breaking the rainfall record for the entire Guangxi region. The sliding 60-minute rainfall (the cumulative rainfall over a continuous hour) exceeded the hourly rainfall and sliding 60-minute rainfall of the 720 heavy rain in Zhengzhou. The cumulative precipitation from 08:00 on May 18 to 08:00 on May 19 reached 610.5 mm.

Guangxi automatic station rainfall in the 24 hours before 16:00 on May 19, 2024

(Photo source: Guangxi Meteorological Bureau, China Meteorological Enthusiasts)

You may have noticed that severe convection is not a big guy. It belongs to the small and medium-scale weather system, and the range of influence is often from a few kilometers to 200 kilometers. It is also impatient, and its life cycle is generally a few hours to more than ten hours, and it may be as short as a few minutes. It can be said that it comes and goes in a hurry. It has an unpredictable temperament and will come out in spring, summer, autumn and winter to join in the fun. It also loves to stack various BUFFs. "Calling the wind, rain and thunder" is its own BGM when it appears.

Due to its high lethality, its impact is second only to typhoons, earthquakes and floods, and it can be called a small-scale lethal weapon. The heavy property losses and casualties caused by the strong convection in just a few hours may take a long time for many cities to recover. In the early morning of March 31, 2024, strong winds, thunderstorms and heavy rainfall occurred in Nanchang City, Jiangxi Province. Under the influence of downburst and narrow channel effect, 4 people were killed and more than 10 people were injured in Nanchang City.

On-site photos of strong winds, thunderstorms and heavy rainfall in Nanchang, Jiangxi Province

(Photo source: Jiangxi TV Metropolitan Scene)

What are the characteristics of severe convective weather in my country?

Severe convection in my country mainly occurs from April to September, especially in the afternoon in the eastern region, and is usually related to cold vortices, plum rain fronts and landfalling typhoons. Among them, **short-term heavy precipitation occurs most frequently, thunderstorms and gale winds mainly occur in the north and Guangdong Province, hail occurs more frequently in mountainous areas and plateaus, and **tornadoes are extremely rare, with an annual frequency of less than one-tenth of that in the United States.

Figure: Distribution characteristics of various types of severe convection in China. (a) Frequency of short-term heavy precipitation events with hourly rainfall >= 20 mm from April to September (%); (b) Spatial distribution of normalized thunderstorm winds from 2010 to 2014 (unit: number of events per station per unit time); (c) Average annual hail frequency distribution from 1980 to 2015; (d) Frequency of tornadoes

(Image source: Reference 1)

As for long-term changes, from 1961 to 2013, the number of thunderstorm days and lightning days from April to October showed a stable and significant decreasing trend, with a reduction rate of -2.6d/decade and -6.5d/decade respectively. The downward trend was most significant in South China. The relative humidity in the lower troposphere and the 0-6km vertical wind shear also decreased significantly in most parts of mainland China. The convective effective potential energy, relative scarcity of water vapor, weakened vertical wind shear and enhanced atmospheric stability may be the reasons for the decrease in the number of thunderstorm days and lightning days.

Spatial distribution of the average number of thunderstorm days in the warm season from 1961 to 2013 (a), anomaly trend of the grid average number of thunderstorm days (b), and anomaly series of the national average number of thunderstorm days (c)

(Image source: Reference 3)

However, the frequency and intensity of short-term heavy precipitation, especially hourly extreme precipitation, showed a significant increasing trend. Compared with 1970-1999, the wetter trend of extreme precipitation in 2000-2018 was more frequent and more intense. This was most significant in South China and East China (the results were slightly different depending on the extreme precipitation index selected). In general, severe convective events showed an increasing trend.

Figure: Probability density function of the extreme precipitation index R95pw1hr (hourly precipitation > 95th wet hour percentile of total precipitation) at (a) seven river basin stations in China; (b) spatial distribution of long-term (1970-2018) trend; (c) probability density function of the next-day extreme precipitation index at all stations during 1970-1999 (cinnabar) and 2000-2018 (medium cyan).

(Image source: Reference 2)

Severe convection: How difficult is it to report accurately?

Since severe convection is so terrible, can we avoid risks by forecasting in advance? **Unfortunately, severe convection forecasting has always been a difficult point in global forecasting business. **The forecasting ability of severe convective emergencies caused by local small-scale convective systems is still relatively low, among which the forecasting ability of thunderstorms, gales and hail is far inferior to that of short-term heavy precipitation. At the World Meteorological Organization Summit on May 27, 2023, the nowcasting of precipitation within three hours was listed as one of the important unresolved scientific problems.

Annual forecast performance scores for the National Weather Service's 12-hour and 24-hour forecasts for thunderstorms, short-duration heavy precipitation, gusty winds (i.e., thunderstorm gales), and hail from 2010-2018.

(Image source: Reference 1)

This is because, on the one hand, the 0-2h forecast interval is the key period for forecasting small-scale severe convective weather. At present, mainstream weather forecasts are still completed through numerical forecast models. The time and space scales of small, strong and fast severe convective weather are too small, making it difficult for models to capture their subtle changes from a larger grid. It is precisely because of the high requirements for time and space resolution that models find it difficult to complete this task.

On the other hand, people always look for objective laws from events they know about, but severe convective weather events occur much less frequently than general weather processes, there are too few cases available for scientific research, and the mechanisms of occurrence and development are not yet clear, which undoubtedly raises the threshold for uncovering its sudden laws.

In fact, my country has had severe convective weather forecasting services since 2009. With the deployment of China's new generation of Doppler weather radars, the launch of the Fengyun series of meteorological satellites since 1998, the construction of China's lightning detection network, and the construction of a large number of automatic weather stations since 2000, severe convective weather research has made great progress.

Currently, the severe convective weather warning signal is issued 43 minutes in advance, but there is still a long way to go before accurate forecasting. Therefore, when facing severe convection, please automatically trigger the dodge skill.

Main weather observation methods

(Image source: Reference 1)

Will there be more severe convective weather in the future?

my country's special geographical location determines that meteorological disasters will leave a painful mark every year. According to the National Climate Center's meteorological disaster database, in the past 30 years (1991-2020), meteorological disasters have caused an average of 3,039 deaths (including missing persons) and direct economic losses of 258.4 billion yuan in China each year.

One fact that cannot be ignored is that "low-probability, high-impact" events such as severe convective weather will become more likely to occur in the future - extreme weather and climate events will become more frequent and more severe.

Specifically, as the climate warms, the atmospheric water holding capacity increases, and the global water cycle will continue to intensify. On a global scale, this is manifested in an increase in total precipitation and an increase in precipitation extremes (IPCC).

In the future, the increase in extreme precipitation in China may be greater than the average precipitation, and the variability will increase, which means that global warming is or will make the climate system more variable and uneven. On average, extreme precipitation in China, especially in the east, will intensify at a rate of 6.52% (5.22%-8.57%) for every 1 degree of global warming. At the same time, hourly extreme precipitation in eastern China will become more frequent and intense in the future, and extreme strong winds in central and southeastern China may increase slightly. We just ended the "hottest year in history" in 2023, and at the beginning of 2024, new high temperature records for the earth will be set one after another.

Currently, five climate tipping points are in the "danger zone" and have been or will soon be crossed, and the remaining 11 tipping points are classified as "likely" to be activated. If the critical threshold from one stable state to another is crossed, the impact will be immeasurable.

"Record breaking" is becoming the "new normal", and it's time to actively adapt - at least close all the windows tightly before going to work when the next severe convection is forecast to occur.

Global distribution map of climate tipping points, with icons representing tipping points that are likely to be activated under this warming scenario

(Image source: McKay et al., 2022, Science)

References

1. Zhang, XL, JH Sun, YG Zheng, et al., 2020: Progress in severe convective weather forecasting in China since the 1950s. J. Meteor. Res., 34(4), 699–719.

2. Li, Xin & Zhang, Ke & Bao, Hongjun & Zhang, Hengde. (2022). Climatology and changes in hourly precipitation extremes over China during 1970–2018. Science of The Total Environment. 839. 156297.

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4. Zhang, W., Zhou, T., 2020. Increasing impacts from extreme precipitation on population over China with global warming. SCIENCE BULLETIN 65, 243–252.

5.Tang, J., Lu, Y., Wang, S., Guo, Z., Lu, Y., & Fang, J. (2023). Projection of hourly extreme precipitation using the WRF model over eastern China. Journal of Geophysical Research: Atmospheres, 128, e2022JD036448.

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7. World Meteorological Organization: Climate in Asia 2023

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