Fist-sized hailstorm hits Guangzhou, smashing through car windows and roofs! How could the hail be so big?

From the afternoon to the evening of April 27, many places in Guangxi and Guangdong were hit by heavy rain, thunderstorms, strong winds, hail, and tornadoes, and the roofs of some factories were blown off.

According to the Guangzhou Meteorological Department, preliminary assessment is that the tornado that occurred in Guangzhou's Baiyun District was at the level of a strong tornado (equivalent to wind force levels 15 to 17, which means wind speeds are as high as 56.1 to 61.2 meters per second, an extremely strong storm).

In addition, hail fell in some places in Guangxi and Nanning. Some hailstones were larger than 5 cm in diameter, larger than eggs. Many vehicle windows and roofs of simple houses were damaged.

Image source: CCTV News

Why did strong tornadoes and hail as big as fists appear? Will there be similar disastrous weather in the near future? We invited Xin Xin, a senior engineer from the China Meteorological Administration, to answer the questions.

Why did such large hail occur?

To sum up in one sentence: there is sufficient water vapor and the updrafts are strong enough to support the hailstones to continue growing until they are big enough.

Judging from the cloud maps and radar, multiple "supercell storms" appeared in Guangxi and Guangdong this time.

The obvious bulge seen on the cloud map on April 27 (above) is a large cumulonimbus cloud that looks like bubbles in boiling water. The nearly elliptical red-purple echo seen on the radar map (below) is a supercell storm. Image source: provided by the author

The so-called "supercell storm" is essentially a strong rotating updraft, usually accompanied by strong winds, local heavy rains, hail, and even tornadoes. It has extremely destructive power and is the most dangerous weather phenomenon in the world.

If an ordinary cumulonimbus cloud is compared to a child, this supercell storm is like the "Hulk". Compared with ordinary thunderstorm cells, the "supercell storm" has a stronger upward movement inside, with an upward speed of 10 meters per second or even more than 15 meters per second. Such an upward airflow can hold hail embryos of 15 to 20 mm.

Moreover, if there is large vertical wind shear and spirality at low altitude in a supercell, it may also cause a tornado - the inconsistent wind direction at low altitude will form a horizontal vortex tube. Under the influence of the rising and falling airflows of the supercell, the vortex tube changes from horizontal to inclined, stands upright and then touches the ground, which is a tornado.

We know that hail is formed by water droplets in cumulonimbus clouds. The rising air currents in cumulonimbus clouds carry water droplets from the lower part of the cloud to the middle and upper layers of the cloud. As the altitude increases, the temperature gradually decreases. When the temperature drops to a certain level, the water droplets begin to freeze and turn into small ice crystals. These small ice crystals are the embryos of hail.

As the air flow moves up and down, the hail embryos will continue to encounter supercooled water droplets and freeze on the outside of the hail embryos, causing the size of the hail to continue to increase.

The process of forming large hailstones is actually similar to "rolling Yuanxiao". Dip the Yuanxiao fillings in water and roll them continuously in glutinous rice flour. Corresponding to hailstones, the embryos of hailstones (small ice crystals) keep going up and down in the cumulonimbus clouds, constantly colliding with the supercooled water droplets in the clouds, and then growing larger.

The stronger the updraft in a cumulonimbus cloud, the bigger the hail that can be lifted, making the hail grow bigger until the hail is big enough that the updraft can no longer hold it up and it falls to the ground.

During the falling process, in the air temperature above 0℃, the surface of the hail melts a little. Due to the turbulence at low altitude, the hail may stick together and merge, forming hail as big as eggs.

Figure 1. The cross-sectional structure of a supercell hail cloud. Source: Principles and Methods of Meteorology, 4th edition

If you look closely, some large hailstones may even look like coronaviruses. The bulges on the outside are actually the initial shapes that were partially retained when smaller hailstones merged together but did not completely fuse.

Finally, when the hail grows to a certain size, its weight will exceed the supporting force of the updraft, and the hail will fall from the clouds, forming what we call hail weather.

Hail is a layered structure. Source: Wikipedia

Therefore, the reason why the hailstones were "bigger than eggs" this time was mainly because they met the following conditions:

First, the "supercell storm" makes the lifting capacity of the updraft particularly strong. According to data from the National Oceanic and Atmospheric Administration (NOAA), an updraft of 103 kilometers per hour can produce hail the size of a golf ball, and if the air speed reaches 130 kilometers per hour, it will produce hail the size of a baseball.

Second, there is abundant water vapor in the middle and low altitudes, and the hail particles rise and fall many times in the cumulonimbus clouds, collide and stick together, making the hail bigger.

On the afternoon of April 27, the atmospheric conditions in Guangzhou were such that the 0℃ layer was just about 4,000 meters (600 hPa). There was a strong upward movement near and above the 0℃ layer, which, combined with good water vapor conditions, was conducive to the formation of hail. Note: The thick red line is the 0℃ layer, the ordinate is the pressure height, the abscissa is time, and the dotted line is the upward movement. Green is for high relative humidity. Source: Comprehensive profile map of Guangzhou from European numerical simulation

In the future,

Will there be more severe weather conditions such as storms and hail?

This spring, severe convective weather in my country was extremely active and of extraordinary intensity.

On the one hand, the warm and humid air flow is abnormally strong, and the corresponding subtropical high pressure is stronger to the north. The southerly wind on the north side is stronger than usual, making the humidity and heat level of the low-altitude atmosphere comparable to the level after the summer monsoon breaks out in May and June. The water vapor and energy of the low-altitude atmosphere in the south are very sufficient.

On the other hand, since late March, the upper-altitude troughs have also been very active, with multiple upper-altitude troughs moving eastward from the Qinghai-Tibet Plateau, making the convective middle atmosphere dry and cold. A vertical structure of warm and humid below and dry and cold above has been formed many times, which is a typical structure of strong convection.

A high-altitude trough refers to a low-pressure area in the high altitude, which is usually represented by the low-value area of ​​the height field. On the 500 hPa contour map, the area that generally bends downward is the high-altitude trough. The dry and cold air brought by the high-altitude trough is the key factor in intensifying convection.

When the low-altitude warm and humid air rises, the temperature drops, and the water vapor reaches the saturation point, it will condense. The condensation process of water will release heat, making it warmer and lighter, and continue to rise. If dry and cold air is involved at this time, the contrast between cold and warm will increase, which will further increase the rising speed, forming a strong rise, rushing straight to an altitude of 10,000 meters and reaching the top of the troposphere.

At 14:00 on April 27, the 500 hPa height field was superimposed on the 850 hPa wind field. Source: provided by the author

After reaching the top of the troposphere, the air flow turns to descend. At this time, if dry air is drawn into the precipitation particles in the cloud during the falling process, they will evaporate or sublimate, and then strongly absorb heat and cool down. They will become colder and heavier and continue to accelerate their descent.

When air flows to the ground and disperses, it will form thunderstorms and strong winds (especially strong ones are called downbursts). Therefore, from the end of March to April this year, strong convection in southern my country frequently saw record-breaking thunderstorms and strong winds of 11 to 12 levels. This level of strong winds was rare on land, but this year it has become common.

According to the current forecast, from the night of April 28 to April 29, there will be a high-altitude trough + low-altitude warm and humid air flow in eastern Sichuan, Chongqing, Guizhou, and Hunan. Be alert to the possibility of thunderstorms and gales with winds of 8 to 10 or even 11, as well as short-term heavy rainfall of 30 to 50 mm.

Looking ahead to the May Day holiday, there will be significant convective rainfall in the south from May 3 to 5. For specific details, please pay attention to the upcoming warning forecasts from the meteorological department.

Author: Xin Xin, Senior Engineer, China Meteorological Administration

Review | Zhou Bing, Chief of Climate Services, China Meteorological Administration