Mu Mu, academician of the Chinese Academy of Sciences, explains in detail how weather forecast errors occur

    ·Numerical weather prediction is actually a combination of mathematics, physics, chemistry, etc., and solves it with mathematical and physical equations describing atmospheric motion.

    One of the things that the atmospheric science community is doing is a seamless forecasting system. It is hoped that changes such as tornadoes with a time scale of only a few minutes or ten minutes and typhoons with a time scale of several days can be seamlessly forecasted .

    What exactly does atmospheric science research focus on? How does atmospheric science research affect human production and life? Where are the sources of errors in weather forecasts? What aspects of research is being done by the atmospheric science community to improve compensation for errors?

    On November 19, Mu Mu, an academician of the Chinese Academy of Sciences, an atmospheric dynamicist, and a distinguished professor of the Department of Atmospheric and Oceanic Sciences of Fudan University, discussed "the atmosphere and the habitability of the future earth" in the 2022 Fudan University School of Management Science and Technology Week series activities. He delivered a speech on the topic and shared his thoughts and views on the above-mentioned issues. The following is the full text of the speech.

    Atmospheric science mainly does three things

    What I'm going to talk about today is roughly divided into three parts, the first is the preface. The object of our atmospheric science research is the atmosphere, and of course we must also consider its interaction with other circles, such as oceans, land, and so on. Atmospheric science is mainly to study the dynamical problems, physical problems, chemical problems and so on that occur in the atmosphere. So, we are a discipline built on the foundations of mathematics, physics, chemistry, and computer science. At the same time, atmospheric science still has a very good tradition. Once a new technology is born, such as computer, radio technology, radar technology, satellite technology, and now very popular AI, big data, etc., this discipline will be very timely. New technologies are applied to it.

    What does atmospheric science mainly do? Let me sum it up for you in a nutshell. The first thing, we have to look at the "face of God", which mainly means that we have to make observations and design the observations. The second thing, we need to "feel God's temper", that is to know and understand the atmosphere. The third is "guessing God's mind", which mainly means that we need to do numerical simulations and forecasts at the same time.

    What does atmospheric science mainly do? This can be divided into two aspects. On the one hand, we can make valuable forecasts for high-impact sea-atmosphere environmental events, such as typhoons, tornadoes, smog, climate events ENSO, and storm surges on the ocean, etc. Another very important aspect is how to deal with climate change, including all walks of life, national and government policy formulation, and now we are concerned about carbon neutrality, carbon peaking, etc., all of which are atmospheric sciences to build a livable earth. important content.

    Human beings are very life-saving. The atmosphere on the earth is the umbrella of our life. Without this atmosphere, the average temperature of our surface is about minus 18 degrees. The average temperature of the surface in 2020 is 14.9 degrees. In addition, the atmosphere is equivalent to 4.5 meters thick concrete, which can block cosmic rays, charged particles, ultraviolet rays, etc. from space. We often describe the atmosphere with changing winds and clouds and myriad weather conditions. You may not know that there are 1,800 thunderstorms on the earth at any time, with an average of 4,000 thunderstorms per day, and the energy released by a moderate-intensity typhoon in 24 hours is equivalent to the annual energy demand of the United Kingdom and France. Of course, unfortunately, we are not yet able to use this energy.

    Mark Twain famously said: Everyone talks about the weather, but few do anything about it. From the current level of scientific understanding, he seems to be half right-now, humans have affected the weather and the climate. In the future, we humans can make our own contribution to the weather and climate.

    The atmosphere is the most active layer in the complex weather and climate system. It interacts with the ocean, land surface, etc., so it constitutes our current weather and climate system. Here I especially emphasize the ocean, because the ocean drives the atmosphere with its huge heat energy, and its density is much higher than that of the atmosphere. To give an example, if the temperature of one liter of seawater drops by one degree, the heat emitted by it can raise the temperature of 3600 liters of air by one degree. This is why we study the ocean while studying the atmosphere. Weather and climate change are related to the fate of human beings on the earth in the future. With climate change, the intensity of tropical cyclones will increase, and global warming will melt the cryosphere, which will also bring more floods, desertification and other problems. The casualties and property losses caused by the torrential rain in Zhengzhou on July 20 last year were very huge.

    This year, we have experienced the high temperature and heat waves in the northern hemisphere in summer in the Yangtze River Delta and the Yangtze River Basin. On the live map of the highest temperature from July 21 to August 30, the northwest and southeast above the diagonal line are high temperature heat waves that last for a long time. The highest places in the Yangtze River Delta region and the Yangtze River Basin even exceeded 30 days. Other places in the northern hemisphere, such as Europe and South America, were also affected by the heat wave. The British Met Office issued the first red warning for abnormal high temperature in history on July 18, and the temperature in some areas exceeded 40 degrees for the first time. Everyone should know that Britain is the place where many wealthy people from Arab countries go to spend their summer vacation. This year, their wish has largely come true.

    In addition to high temperature and heat waves, the situation of winter blizzard is also severe. You may have seen the movie "The Day After Tomorrow", and the 2018 mini version of "The Day After Tomorrow" is being staged in reality. In addition, there is drought. From September 2019 to February 2020, the drought and fires in Australia in the southern hemisphere displaced animals. Therefore, it is no exaggeration to say that the greenhouse gases emitted by humans on the earth are related to the future destiny of mankind. Research in the 1990s suggested that global warming would melt ice and snow, and that the Earth might eventually turn into a water globe. At the beginning of this century, the movie "The Day After Tomorrow" was released, which believed that the earth might become a world of ice and snow. Last year, there was a very popular movie "Dune", which believed that as the temperature rises, the earth may become a deserted planet. Of course, these are some studies and are not conclusive, but they are related to how humans respond to climate change.

    Three Contributions to Atmospheric Sciences

    The first aspect of the major contribution that atmospheric science has made to our human society is forecasting and forecasting. "Predicting the future" is probably one of the eternal pursuits of human beings. The weather forecast is to guess the mind of God in the future, and the climate forecast also considers the ocean, so we are actually guessing the mind of God and the sea dragon king. There is a similar story in Greek mythology. There are probably two aspects to measure whether a science is mature or not. One is to make valuable predictions about future events, and the second is to make predictions about undiscovered phenomena that can be confirmed by future observations. . In the history of mankind, astronomy was at the forefront at the beginning, and later oceanography was used to predict tides, and now it is able to predict weather and climate, which is the contribution of atmospheric science research. A report by the National Science Council of the United States rated advances in understanding and predicting the weather as one of the most remarkable successes of science in the 20th century. The World Meteorological Organization also evaluates weather forecasting as one of the most significant technological and social advances in the 20th century.

    Atmospheric science is a small subject, but it can be said without humility that we have made relatively large achievements. The highest level of our national science and technology awards is the National Highest Science and Technology Award. As of 2020, there are a total of 35 winners. Our very small discipline has 2 winners. One is the famous meteorologist Mr. Ye Duzheng, One is Academician Zeng Qingcun. These are also my two teachers when I was working at the Institute of Atmospheric Physics, Chinese Academy of Sciences.

    The history of weather forecasting can be traced back to the first numerical weather forecasting made by the British mathematician Richardson from 1916 to 1918. At that time, it failed because it was calculated by a computer. In the 1950s, Charney and von Neumann successfully carried out the first numerical weather prediction with the first electronic computer. Numerical weather prediction is actually a combination of mathematics, physics, chemistry, etc., and solves it with mathematical and physical equations describing atmospheric motion. The process of making a forecast by a high-speed electronic computer is roughly like this. At the beginning, we need to observe and obtain data, through data assimilation, input it into the model, make a forecast, and finally output the result.

    Here I will talk a little bit about the importance of our numerical weather prediction. This is an example worth mentioning. On May 22 last year, a 100-kilometer mountain marathon was held in Baiyin, Gansu. The weather forecast gave a big forecast, but because the venue of the competition is a mountainous area, we can't make a detailed forecast, but we still have a good forecast of the frontal process. Players with meteorological knowledge prepared jackets and escaped the difficulty. However, runners who don't understand that there must be strong winds to cool down after the front face wear too thinly during the marathon, and many people lose their lives.

    Let us now turn to the contribution of atmospheric science to science, particularly to global warming and climate change. Last year, the Nobel Prize in Physics was awarded to Manabe, Hasselmann and Parisi. Parisi is doing analytical science complexity, and the first two are atmospheric scientists. predicted global warming has made a significant contribution. There is no Nobel Prize for atmospheric science, but what we are doing is physics and chemistry.

    Related to management science is Nordhaus, the 2018 Nobel Prize in Economics. He incorporated climate change into long-term macroeconomic analysis, constructed an economic model, and won the Nobel Prize in Economics.

    In terms of protecting the human ecological environment, we know that high-altitude ozone is good because it can block short-wave ultraviolet rays and protect humans and ecosystems. But the ozone near the ground is not a good thing, because it can endanger human health, and even have a very bad impact on the crop ecosystem. In 1995, the Nobel Prize in Chemistry awarded three prizes in atmospheric chemistry. Their main contribution was to discover that the formation of the Antarctic ozone layer hole is mainly related to the so-called condensing agent Freon emitted by humans, and their work promoted " The signing of the Montreal Protocol, thus eliminating about 99% of ozone-depleting substances. Therefore, their contribution to the development of human society is huge.

    Opportunities and Challenges of Atmospheric Science

    The third aspect is the opportunities and challenges faced by atmospheric science. Although our models have made great achievements and even won Nobel Prizes, with global warming, extreme weather and climate events have occurred frequently around the world in recent years, and our forecasts have frequently made mistakes.

    Human history has experienced many extreme weather disasters. On May 18 last year, the highest heat wave was observed in the Arctic, and Moscow also set a record. The Australian fire just mentioned also brought huge ecological disasters. However, we frankly say that our forecasting level is still not up to the accuracy required by us humans. Our country is also a high-incidence area of extreme weather and climate events. For example, last year's heavy rain in Zhengzhou, its fall area and intensity, we all predicted unsatisfactory. Although we can predict a wide range of weather conditions, we have not predicted where such a heavy rain will fall. Of course, in October 2021, the rainstorm in Shanxi was predicted by our meteorological department quite well. The Yangtze River Delta is more concerned about typhoons. Although the average 24-hour typhoon track forecast is probably less than 100 kilometers, there are still many examples of very poor forecasts. So there is a joke, the common people say, fortunately we don’t live in a “partial” place, otherwise we would have rain every day, because our previous forecast said that there would be showers sometimes in some areas.

    Why do our forecasts have errors? There are three aspects. One is that there are errors in observations, and any instrumental observations have errors. The second is that our model has errors, and our understanding of the natural world is not comprehensive and sufficient. The third is that the weather and climate system is a very complex and nonlinear system. Speaking of complexity and non-linearity, it is necessary to mention Lorenz's butterfly effect, which refers to the flapping of a butterfly's wings in Texas, which may affect a storm or tornado in New York. And when you study climate, you have to factor in the ocean, which doesn't really play that big of a role, but anyway, it illustrates the importance of these non-linear components.

    The surface of the earth is very complicated, and the clouds are also very complicated. It is very difficult to describe these systems in our forecast models. Among the geoscience world's conundrums, the first question is, can we stop global climate change? Connected to that is where we can store the excess carbon dioxide. The third is whether we can make good forecasts. The fourth is also related to future climate change: If the ice in the North and South Poles and the glaciers on the earth melt, how high will our sea level rise? How much impact will it have on coastal cities like Shanghai? ... These are very challenging questions, but they are related to the survival of human beings and whether the earth is still livable.

    Atmospheric science is closely connected with mathematics and physics. For example, there is a very famous NS equation problem in the mathematical puzzle. The solution of this problem is closely related to whether the weather and climate can be predicted well and how long it can be predicted. So we also expect mathematicians to contribute.

    Now, one of the things that the atmospheric science community is doing is a seamless forecasting system. We hope that the time scale of tornadoes is only a few minutes or ten minutes, the time scale of typhoons is a few days, and then the climate is half a year. Annual and decadal changes and climate predictions can be seamlessly forecasted. Of course, this is a very challenging subject that needs to be worked on slowly.

    In this regard, the research group I lead is probably doing two things.

    We always have uncertainty in forecasting, so what should we do about this uncertainty? Considering that our observations have errors, we have to use another method, which is to use probability forecasting. Now everyone can see, for example, the probability of precipitation tomorrow is 40%, 25%, and the probability of how high the temperature is, all of which are made using probability forecasting. The way to achieve probability forecasting is called ensemble forecasting. How to do ensemble forecasting? Our research group is working hard to do this work. We proposed a method called CNOP (Conditional Nonlinear Optimal Perturbation, Conditional Nonlinear Optimal Perturbation) method. In addition, we can do target observation, which is also a way to improve typhoon forecasting. The several typhoon target observations we made in 2020, 2021 and 2022, the effect is relatively ideal, and we have published articles about its effect.

    On the other hand, we are also paying attention to the Arctic. This is one of the key ways to improve the weather forecast for 15 days to 30 days, or even longer. Many studies have found that the Ural blocking phenomenon has a great impact on our extremely cold winter weather. Our research has found that in the Arctic region, the conditions of sea ice in the three regions of the Greenland Sea, the Barents Sea and the Sea of Okhotsk have a great impact on us. Congestion forecasts have a large impact. Therefore, we hope that our work can also provide scientific guidance for future observations in the Arctic.

    (The author, Mu Mu, is an academician of the Chinese Academy of Sciences, an academician of the Academy of Sciences for the Developing World (TWAS), a member of the Chinese Society of Industrial and Applied Mathematics, and a distinguished professor of Fudan University. He is currently a member of the Executive Committee of the International Association of Meteorology and Atmospheric Sciences (IAMAS), China Chairman of the IAMAS Committee, member of the Eighth Expert Advisory Committee of the Earth Science Department of the National Natural Science Foundation of China.)


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