Since July 16, many parts of Henan Province have been hit by historic persistent heavy rainfall, with Zhengzhou as the center of the rainstorm.

Photo source: Xinhuanet.com

Why is there such extreme weather as heavy rain in Henan? Can artificial rain be used to intervene? What changes in climate conditions are we facing?

Taking Zhengzhou as an example, some relevant observation data show that when the rainfall is the strongest, 201 mm of rain falls in Zhengzhou in one hour, and the rainfall in 24 hours exceeds 600 mm, which is equivalent to the total precipitation of Zhengzhou in a year.

If you don't feel the above figures, you can look at our country's rainstorm level, the daily precipitation is more than 250 mm, even if it is a heavy rainstorm, it is the highest level of rainstorm, Zhengzhou far exceeded this time.

Photo source: Rainfall Process Grade released by China Meteorological Administration

The cause of this precipitation is very complex, not only the vortex and subtropical high.

First of all, there was a very strong low vortex system in the local area, and the low vortex system generally corresponds to the convergence rising area, while the precipitation precisely needs a very strong rising movement.

Not just the vortex, but a very strong subtropical high, known in synoptic terms as a high pressure dam, which acts as a dam to block the precipitation system moving from west to east and keep the rain in one place.

Schematic diagram of subtropical high

In the eastern part of our country, if high pressure DAMS are formed in a large area from North China all the way to North Korea and the Bohai Sea, then generally speaking, there will be heavy rains in North China, because a very strong easterly wind will form on the south side of the high pressure DAMS, continuously transporting water vapor from the sea to the inland.

Schematic diagram of Typhoon impact

However, the reason why this rainfall is stronger is that there are two typhoons to the south of the typhoon. The pressure gradient between the low pressure center of the typhoon and the high pressure center of the auxiliary high pressure formed a very strong pressure gradient force, which would transport more steam than the normal high pressure dam, so the extreme rainstorm can be formed.

Image source: http://bzdt.ch.mnr.gov.cn/

In addition, because the northern and western areas of Henan are located at the junction of the third and second steps of the country's topographic ladder, the topographic uplift is also very strong.

Several conditions combined to produce a very strong rainstorm.

This rainfall is indeed very extreme, rainfall of more than 250 millimeters will generally bring a variety of disasters, floods, mudslides and other disasters are difficult to avoid. The accuracy of an extreme weather forecast like this depends on how big the system is. The bigger the system, the easier it is to see it and see it many days in advance. The smaller the system, such as a small area of instantaneous heavy rain, is much harder to predict.

This time the weather system is really relatively large, so its predictability is good, the whole meteorological system for this weather process forecast is more accurate. Heavy rain is a disaster-causing factor, and whether it can eventually form a disaster is closely related to the degree of response of the whole society. If we can respond to it in time, we can try our best to avoid related casualties and reduce economic losses. Therefore, forecasting is only the first step in disaster prevention and mitigation.

This idea is called "weather modification" from a professional point of view, and weather modification is indeed a relatively fast developing field in our country in recent years. But there are certain conditions for weather modification, and only when those conditions are met can we exert a slight influence.

Photo credit: Qinghai Station of China Weather Network

For example, we really can't make a cloud in a clear sky and then have a rainstorm, which is obviously not in accordance with the law of conservation of matter. There was nothing we could do about this rainstorm, because it lasted for several days on a time scale and hundreds of kilometers in space. Such large-scale precipitation could not be eliminated by weather modification, and it was impossible to move it from one place to another, which was unrealistic.

Weather modification can only be done in accordance with the trend and under certain conditions. For example, the current hail removal and fog removal, artificial rain enhancement, can only be done if they meet very strict conditions. The idea that humans can do whatever they want to do with the weather should be a wonderful expression of the "rainmaking" ability, but it is not feasible under current conditions and may not be realistic for a long time to come.

Global warming, which speeds up the water cycle, will indeed increase global rainfall. According to the current study, for every 1 ° C increase in global temperature, precipitation around the world will increase by about 2 to 3 percent, so precipitation in all regions will increase overall.

But global warming will also have a "drier, wetter" effect, with more rain in the rainy season and more severe droughts in the dry season. Although global warming contributes to the increase of global rainfall, it may be manifested in the extreme rainfall in various regions. This situation may be more prominent in our country, because our country is a typical monsoon area, the precipitation mainly appears in the summer, no matter what has happened, the next is basically heavy rain, so we can see more heavy rain.

We have noticed that there have been more extreme weather events around the world this year.

It's all in Fahrenheit, with Oregon's Eugene (111 degrees), Portland (112 degrees) and Vancouver (112 degrees) breaking local records.

Some time ago, the temperature in the western part of North America was very high, and the latitude of many places was higher than that of the northeast of our country. Theoretically, this season should be relatively warm and comfortable, but the highest temperature in some places like Canada exceeded 49.5℃, resulting in a significant increase in the mortality rate. Over the past few days, heavy rains have hit Western Europe, killing more than 100 people in Germany. The storms, tornadoes, sandstorms and cold waves in our country are also very serious this year.

So we feel more extreme weather, and it does make sense in theory, because global warming will contribute to the occurrence of extreme weather, such as tornadoes, strong typhoons, heavy rains, including heat waves, which is the manifestation of global warming in all regions.

It is true that there have been many periods in Earth's history that have been much warmer than today, and even at the warmest, when there was no ice at the poles, you can imagine how much warmer it would have been.

But all of the previous climate changes in Earth's history have been caused by various feedbacks in the Earth's climate system, by natural causes and natural processes, which are completely different from the causes of global warming. The current warming is caused by human activity emitting greenhouse gases much faster than natural processes.

When we compare the concentration of carbon dioxide in Antarctic ice cores, the current rate of increase in greenhouse gases is hundreds to thousands to tens of thousands of times higher than the fastest period in history, completely breaking the rules of the Earth's climate system, and its future trends are dominated by human activities.

Historically, the climate has changed at completely different rates, for completely different reasons, and most importantly, for completely different effects. Climate change in the history of the Earth, whatever it was, had nothing to do with human beings, had nothing to do with human civilization, and now climate change is highly affecting human beings, so it's a completely different process.

At present, the most important measure of the international community to deal with global warming is through carbon emission reduction, including our country is carrying out carbon peak and carbon neutrality and other actions, which is an aspect of tackling climate change.

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