[Creative Cultivation Program] Why does the promised rainstorm never come?

Author: Wang Zifeng

Reviewer: Wang Jinhu, Associate Professor, Department of Disaster Prevention and Mitigation Science and Engineering, Nanjing University of Information Science and Technology

Have you ever experienced such a moment?

As the dark clouds gather outside the window and the gloomy weather seems to be brewing a rainstorm, you hear the "heavy rain warning" on TV, smile slightly, and put down the rain gear that you had prepared long ago.

Heavy rain warnings are like reverse predictions to you. The weather forecast always sets the mood, and you wait eagerly for a day, but the imagined scene of "rain is coming" disappears quietly. Where is the shadow of the heavy rain? Why does the heavy rain never come?

Let us first understand how heavy rain is formed?

Under the sun, water on the surface of land and ocean evaporates into water vapor. When water vapor rises to a certain height, it turns into small water droplets when it encounters cold, and the small water droplets form clouds. These small water droplets are not quiet, but keep colliding in the clouds. When the clouds can no longer bear their weight, they will fall from the sky and become rain.

But the process of turning ordinary rain into a "rainstorm" is not simple. It also needs to meet the following factors:

First, there is abundant water vapor transport. Heavy rain is made up of water vapor. These water vapors are equivalent to "small soldiers". They gather in large numbers in a certain area at a lightning speed, and all gather together to form a "large army" of heavy rain.

The second is rapid upward movement. According to calculations, in a mesoscale system with a horizontal scale of 100-300 km, the air rises at a speed of 101 cm/s, and water vapor rises rapidly to a designated location in the sky, just like taking an elevator.

In addition, the two types of convection in the atmosphere collide with each other, and cumulonimbus clouds and stratiform clouds coexist, resulting in an unstable rain structure, disturbance collision, and turning into heavy rain.

The topography also plays a close role. Many rainstorms often occur in mountainous areas. This is because my country's large-scale topography is generally low in the east and high in the west, and there are a large number of east-west mountain ranges in the country. When the southeast monsoon and southwest monsoon prevail in summer, the humid air is lifted by the topography, and rainstorms mostly gather on the windward slopes on the east or south side of the mountains.

For example, Beijing is located in the North China Plain, with the Yanshan Mountains in the north, the Taihang Mountains in the west, and plains in the southeast. Summer rainstorms often occur on the hillsides in the west and north.

In view of the different amounts of rainstorms and the damage and disasters they may cause, my country currently uses rainstorm warning signals to divide rainstorms into four levels: blue, yellow, orange and red.

If forecasters determine that rainfall will reach more than 50 mm within 12 hours, or that more than 50 mm of rainfall has already occurred, a blue warning will be issued.

If the rainfall reaches 50 mm or more within 6 hours or even 3 hours, a yellow or orange warning will be issued.

If the rainfall reaches 100 mm within 3 hours, or sustainable rainfall of more than 100 mm has occurred, a red rainstorm warning signal should be issued immediately to remind the public and relevant departments to take preventive measures to avoid disasters.

Extremely heavy rain is a type of disastrous weather that can cause floods and soil erosion. In mountainous and rural areas, it can easily lead to mountain torrents, dike breaches and flooded crops. In cities with poor drainage systems, it can also easily lead to waterlogging, traffic paralysis and even personal and property losses.

Therefore, it cannot be overemphasized how important it is to pay attention to the warning of heavy rain.

Why do heavy rain warnings sometimes feel inaccurate?

The first is that people’s subjective feelings vary.

As we said before, issuing a rainstorm warning must depend on the accumulated precipitation over a period of time. Although some short-term heavy rainfall has a large amount of rainfall, it does not last long, only a few minutes, and may not even reach the level of heavy rain.

Sometimes the rain falls gently and the accumulated rainfall is large, but you don’t feel it deeply, and you will think you have encountered a fake rainstorm.

In addition, heavy rain warning is a problem worldwide and is much more difficult than you think.

Heavy rain is a complex weather phenomenon caused by the interaction of different time scales and spatial scales. If you want to make an accurate warning of heavy rain, you need to conduct a comprehensive analysis of all conditions and data related to the heavy rain.

Conventional high-altitude observation systems are insufficient for small and medium-scale observations. Heavy rain is like someone pouring a basin of water from the sky onto the ground. The water will not fall evenly on the ground. Some places are wetter and some places are drier. It is very difficult to know the extent of wetness at each point.

With the advancement of technology, the spatial and temporal resolution of the numerical models used in my country to predict heavy rain is getting higher and higher. The GRAPES global numerical forecast system has even become a national weapon.

In addition, there is global monitoring of refined intelligent grid precipitation warnings using meteorological satellite remote sensing, which can increase the spatial resolution to 5 kilometers with an hourly time interval, and can continuously improve the accuracy of precipitation forecasts through rolling updates.

Although nature is always full of unpredictability, I believe that with the development of technology, we will be able to give full play to our subjective initiative and deal with "unpredictable situations" in the future.

References:

1. National Disaster Reduction Network: Conditions for the Formation of Heavy Rain

Link: http://www.ndrcc.org.cn/qxswzh/7905.jhtml

2. National Meteorological Information Center

Link:

https://data.cma.cn/article/getLeft/id/320/keyIndex/3.html

3. Xinhuanet: Misunderstandings behind weather forecasts

Link:

http://www.xinhuanet.com/tech/2020-08/25/c_1126408204.htm

This article is produced by the Science Popularization China-Creation Cultivation Program. Please indicate the source when reprinting