Severe convective weather strikes frequently, how do we plan ahead?
July and August are the most critical periods for flood control.
Since the flood season this year, severe weather caused by strong convection has repeatedly caused casualties and caused social concern.
"Preparing for a rainy day" is the precious experience that the Chinese nation has accumulated in the struggle between mankind and natural disasters for thousands of years.
Although the weather is unpredictable, today, with the rapid development of monitoring technologies such as Fengyun satellites and radars, in the face of strong convective weather, whether information technology and other technological means can be used to strengthen the "monitoring-research and judgment-forecasting-early warning" of risks. Weather conditions perfect emergency plan?
How will the city and the public respond to extreme rainstorms?
In this issue, we invite Zhang Tao, chief forecaster of the Central Meteorological Observatory, to explain the knowledge of weather monitoring and weather warning; at the same time, we introduce the disaster prevention and mitigation experience of the Shenzhen Meteorological Bureau and the Hunan Meteorological Bureau.
In my country, one of the main causes of meteorological disasters is extreme weather, especially severe convective weather.
What is strong convective weather?
It is mainly a collection of weather phenomena such as wind, rain, thunder, hail, etc.
Statistics released by the China Meteorological Administration recently show that since this century, my country’s average annual direct economic loss due to meteorological disasters has reached 290 billion yuan, which shows that meteorological disasters have severely affected and threatened the safety of people’s lives and property.
So, how to understand the modern weather forecast and how big the role of meteorological disaster prevention is?
Before the advent of severe convective weather, how can the government and the public take precautions?
1. Strong convective cloud system is like "elephant running wildly"
The old saying goes: "If you don't go out in the morning, the sunset will travel thousands of miles." It also goes: "The sun enters the carmine red, and there is wind without rain."
The ancients knew the weather by observing the celestial phenomena and observing by daily life experience.
Modern weather forecasting is a science. It is made comprehensively based on the scientific atmospheric detection network system, data storage and transmission system, supercomputer numerical simulation system, and scientific literacy people. Weather satellites, radars, etc. have built monitoring for it. net.
Just like refrigerators and the Internet, weather forecasting is one of the fruits of the development of modern science and technology, and the benefits are also limited by the advancement of various aspects of science and technology.
The forecast reliability of specific weather is limited and variable, and the predictability of different types of weather varies greatly.
Generally speaking, the overall reliability of weather forecasts in cold seasons can reach 5 to 10 days, and the reliability of 3 to 5 days in warm seasons will be greatly reduced.
Large-scale weather phenomena can be forecasted earlier (a few days), and local weather phenomena are sometimes impossible even a few minutes in advance.
In addition, there are two completely different concepts that may appear in a certain period of time or in a certain area, and that it will appear in a specific time and place.
For example, there is a thunderstorm in Beijing on a certain day, and a thunderstorm in a square at 2 pm on a certain day. These two expressions are fundamentally different.
In various rain cloud systems, strong convective cumulonimbus clouds can create denser raindrops.
Therefore, most torrential rains are caused by strong convection, as well as thunder and lightning, gusts, hail, and tornadoes.
The same weather type and intensity caused by different positions of the same fluid or the same piece of cumulonimbus are not the same. Typical performance is "a line of hail, a large area of wind and rain", etc. The intensity of wind and rain everywhere is different. average.
In addition, "Mountain rain is coming and wind is full of buildings" is actually a good description of the characteristic that wind usually arrives before rain.
The tornado, a system that is only generated at a certain position of a single body, generally has a diameter of only tens of meters to hundreds of meters.
Since strong convection can not only bring weather by a single individual, but also appear as a single cluster, the time and space, types and intensity of the specific impact of strong convective weather are very complicated.
The spatial scale can range from tens of meters to thousands of kilometers, and the time scale can vary from tens of seconds to several days.
In the same process, some places have mild wind and drizzle, some places are violent and rainy, some places have lightning and thunder, and some places have hail and tornadoes.
If a single strong convective weather system is likened to an elephant, then when the strong convective weather system appears in large-scale clusters, the strong convective weather cloud system is like an "elephant running wild".
For example, the severe convective weather in the south from May 10 to 11 this year was large and very severe, like a rushing "elephant group" that ran all the way from Chongqing and Guizhou to Zhejiang and Jiangsu from west to east, affecting the middle and lower reaches of the Yangtze River in my country. The vast area.
2. Strong convective weather forecast: potential forecast and near-warning/warning
The weather forecast issued by the meteorological department generally only talks about the results: it mainly contains information such as time, location, type, intensity, etc. The core is the weather zone, sometimes it is a "circle" on the map.
Sometimes the people who eat melons have something to say: A small half of China has been circled in, and the time span has spanned a full 12 hours, that's it?
But weather forecast products are like this, which is actually a potential forecast: that is, strong convective weather has not happened yet, but it is very likely to happen.
From a scientific point of view, the emergence of a strong convective weather system requires many physical conditions. These data will be affected by errors in observation and simulation, as well as by some unknown factors.
These are enough to affect the accuracy of the forecast results.
This is the main difficulty of potential forecasting.
That is, no matter whether the forecast is big or small, it can only be a possibility. The term is called probability.
The potential forecast of severe convective weather can generally be made a few hours to a few days in advance. It can be said that there is a high degree of credibility in the approximate time, approximate area, approximate weather type and intensity, but it is not true. Know exactly when and where it appears.
Potential forecast is to predict the possibility of its appearance when it does not appear. Forecasts that have already appeared or partially appear are called nowcasts/warnings.
The production of the approaching pre-alarm report relies on monitoring, and the general form is to issue four-level early warning signals of blue, yellow, orange and red.
The color upgrade means the greater the intensity and range of influence of the weather.
The early warning of the Central Meteorological Observatory is actually a potential forecast regardless of the color. The various warning signals of the three-level local stations of the province, city and county are equivalent to the immediate warning (alarm) report, and some are similar to the potential forecast, depending on the weather. The situation is different for different publishing units. The provincial level is closer to the potential forecast, and the city and county level is closer to the nowcast.
Is the approaching warning accurate?
How long can it be in advance?
If compared with the potential forecast, of course it is too accurate, because I have already seen it.
But compared with precision in the absolute sense, it still can't be done. Although there are rules, there are no definite numbers.
The approaching warning (alarm) is generally relatively accurate, but this time can only be a few minutes to a few hours in advance.
How to monitor and approach early warning?
Part of this technology is connected with potential forecasting. We currently mainly use weather radar, weather satellites, and encrypted ground weather observations to capture strong convective weather, and then predict its movement and evolution based on its traces and surrounding physical conditions.
There are actually many difficulties, such as catching small fish with big nets.
What's the matter with big nets and small fishes?
In principle, we can basically monitor accurately by applying the advantages of various observation methods.
However, each observation method has its limitations and shortcomings.
For example, the diameter of a tornado is generally only a few tens of meters to a few hundred meters. Let's take a look at how to grasp it with the most important and powerful tools.
Ground weather station: The distance between the current encrypted observation stations is several kilometers apart recently (it was hundreds of kilometers apart a few decades ago).
Meteorological radar: Just talk about myopia (resolution), that is, the actual space size represented by the size of a pixel on the radar image. It can be several hundred meters away from the radar, and several kilometers or even ten kilometers away from the radar. , The standard value of the pixel is actually the average value of this large area.
Meteorological satellite: A satellite that can stare at a place 24 hours a day is called a geostationary meteorological satellite, but it is 36,000 kilometers above the equator, and one pixel of the visible light image represents at least 1 kilometer in radius.
A polar orbiting meteorological satellite that is close to each other can reach several meters or tens of meters per pixel, but generally it can only look at the same place twice a day, 12 hours apart.
Tornadoes are these small fish that slip through the observation net.
This is why if there is no witness, we can only confirm whether there is a tornado by investigating the characteristics of the disaster.
In short, the forecast of severe convective weather is roughly divided into two steps and two types.
Potential forecast: Severe convective weather system has not happened yet, where might it happen?
Approaching early warning/alarm: Severe convective weather system has occurred or partially occurred, how does it occur and disappear?
Where will it go?
What weather is it?
What intensity?
The two technical route methods, accuracy and time advance are actually quite different.
3. Two major reasons for this year's flood disasters: rare extreme heavy rains and unfavorable topography in the disaster area
This year's extreme rainstorms in Henan and Hubei are extremely rare in recorded meteorological history.
I can't find a more appropriate and accurate statement than "extremely heavy rain that is rare in history."
For example, Zhengzhou, Henan, broke the record by a large margin: (here based on the daily rainfall of the national station between 20:20 and 20:00) The original record was 189.4 mm (July 2, 1978), and the original record was 552.5 mm on July 20. Increased to nearly 3 times.
Heavy rain in some areas, heavy local heavy rain: 400-600 mm in central and northern Henan, 600-800 mm in some areas, nearly 1000 mm in local areas (extra heavy rain reaches the standard 250 mm), which has greatly exceeded the local average annual rainfall .
Breaking the national record: The 1-hour rainfall at Zhengzhou Station from 16:00 to 17:00 on July 20 was 201.9 mm, breaking the original historical record of 198.5 mm of hourly rainfall in mainland China.
The general heavy rain event is like a "shower", and the heavy rain in Henan is like a "bath".
Not to mention that the "sponge city" has not been built. Even if the "sponge city" is built, it may only be able to cope with the "shower", and there is a high probability that it will not be able to do anything when it encounters a "bath" rainstorm.
The super-strong rainfall in a large area in a short period of time is the main cause of the disaster situation.
The topography of the inland alluvial plain is the second important cause of severe flooding.
The area where Henan encountered extreme rainfall this time was the beginning of the lower Yellow River. Floods in the Yellow River for thousands of years carried a large amount of sediment from the Loess Plateau. After washing down the second step of the Chinese terrain, the deposits were spread in the lower reaches of the gentle terrain. , This is the origin of the Yellow River alluvial plain.
In the historical stories that we all know, Yu's father Gun failed to stop the water control, but Dayu finally succeeded in opening the mountains to dredge.
But in fact, under the combined action of monsoon rains, a large amount of sediment and flat terrain, it is impossible for dredging to be successful alone. The combination of dredging and blocking is the true history of water control.
For thousands of years, the Yellow River has built dikes on both sides of the river to prevent floods, and the flood channel has been siltated and raised. As early as thousands of years ago, the Yellow River has become a "overground river" above the two banks. road.
The severity of migration and changes in the lower reaches of the Yellow River is unique in the world.
This torrential rain in Henan caused a disaster on a unique terrain, and it fell in a short period of time, even in the history of monsoon climate, extremely rare extreme rainfall.
The Yangtze River Basin is also a place where floods are relatively prone to occur.
From 21:00 on August 11 to 9:00 on August 12, the cumulative rainfall in Liulin Town, Suixian County, Hubei Province was 503 mm; the rainfall reached 373.7 mm from 4 am to 7 am on the 12th, and the rainfall exceeded for two consecutive hours at 5 am and 6 am 100 mm is the extreme value since the weather record.
The township of Liulin Township is surrounded by mountains on three sides and became a flood gathering place. Later, the average depth of water accumulation reached 3.5 meters, and the deepest point reached 5 meters.
Generally speaking, the terrain of many places in my country is prone to flood disasters. With climate change, extreme rainstorms become more and more common. All localities need to pay more attention to disaster prevention and mitigation construction, and take precautions for flood prevention.
(Author: Zhang Tao, chief forecaster of the Central Meteorological Observatory)
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