Clouds turn into rain and wind ceases to snow. As the saying goes: 'The weather is like a child's face—it will change. When it’s cold, you should wear more clothes; when it rains, you should take an umbrella; and pay attention to safety when it snows... These common weather phenomena are closely related to our lives, which is closely linked to the earth’s atmospheric circulation. It is difficult to 'communicate' with unpredictable weather, let alone this as a research object. However, Professor Shujuan Hu and his team from the College of Atmospheric Sciences of Lanzhou University have been actively exploring the theories of atmospheric circulation for nearly 20 years.

       In the context of global warming, major natural disasters are on the rise, causing serious economic losses and casualties. Due to its special geographical environment, China has been severely affected by meteorological disasters for a long time, among which drought and flood disasters are particularly prominent. Therefore, monthly-seasonal climate forecasts focusing on drought and flood forecasting have always been a major demand for national development. However, the accuracy of the existing climate model forecasts is far from meeting actual needs, and the fundamental reason for the inaccurate climate prediction lies in the lack of basic theories of climate dynamics.

       At present, the prevailing international atmospheric circulation theories either only consider large-scale horizontal motion in mid-high latitudes and ignore local vertical motion, or only consider vertical motion in low latitudes and ignore horizontal motion. However, for climate prediction, it is necessary to consider the interaction between mid- and high-latitude atmospheric circulation and low-latitude atmospheric circulation, as well as horizontal and vertical movements, and it is urgent to develop a dynamic theory for global atmospheric circulation.

       In August 2020, the Hu Shujuan team published a paper on the high-level academic journal SCIENCE CHINA: Earth Sciences with the topic of 'Theory of three-pattern decomposition of global atmospheric circulation', systematically introducing their significant progress in research on the theory of global atmospheric circulation dynamics. For the first time, their research quantitatively puts forward new definitions of global horizontal, meridional and latitude circulations, originally developed a three-type decomposition model of global atmospheric circulation, and constructed the theory of global large-scale circulation dynamics equations. A complete new theoretical framework of climate dynamics is provided by them, which makes up for the deficiencies in traditional research in artificially dividing mid- and high-latitude atmospheric dynamics and low-latitude atmospheric dynamics, and lays a theoretical foundation for improving and enhancing my country’s climate prediction on the monthly-seasonal time scale.

       For 20 years, Hu Shujuan and her team worked silently while bowing their heads, while looking up at the 'sky', portraying their 'city in the sky' step by step.

 

 

       Hu Shujuan is a native of Lanzhou University. In 1993, she was admitted to the Department of Mathematics of Lanzhou University, majoring in computational mathematics. After graduating with a master's degree in 2000, she worked in Lanzhou University. Since 2001, she has been transferred to the College of Atmospheric Sciences to study for a doctorate degree with the famous meteorologist Niu Jifan. Talking about how she embarked on the path of meteorological research, Hu Shujuan's memory goes back to 20 years ago. At that time, she had a plan for her future Ph.D.: 'I wanted to do some applied mathematics.' After knowing her idea, professor Ding Fangyun, a master tutor, recommended to her an expert in meteorological research, Academician Niu Jifan. At that time, Academician Niu Jifan was considering how to carry out research on the dynamic theory of the unified description of global atmospheric circulation, and was looking for students in related majors.

       'At that time, I didn't even dare to think that I could study with Mr. Niu Jifan for a Ph.D.' Hu Shujuan said. Mr. Chou Jifan is one of the few scholars in the field of atmospheric sciences in China and the world who specialize in atmospheric theory and make important contributions. He has profound academic attainments and many of his works are groundbreaking.

       Hu Shujuan was anxious and full of expectation to write a letter to Mr. Niu in Beijing, explaining her own situation and intention to study, and soon Mr. Niu's reply was coming. 'Teacher Niu's reply surprised me and moved me.' This letter became an important node in Hu Shujuan's destiny. She set the itinerary to study for a PhD in Beijing and officially embarked on the path of meteorological research.

       There were many difficulties and ups and downs in the days of Ph.D. Not only I had to quickly adapt to the new environment and new life, but also to overcome the knowledge gap caused by the leap from mathematics to meteorology. Fortunately, Mr. Niu gave generous help. Hu Shujuan is still grateful to that period: 'Teacher Niu helped me a lot. He provided accommodation for several of our students in life. Not only did he help me in inter-professional courses, but he also guided me on scientific research and so on, which allowed me to study and work in Beijing with peace of mind.” Soon, Hu Shujuan's study and life were on the right track, and all this was inseparable from the guidance and help of Mr. Niu.

       In the process of communicating and learning with the teacher, Hu Shujuan gradually understood Mr. Niu's ideas on atmospheric circulation research. The traditional two-dimensional circulation decomposition representation method often divides the global atmospheric circulation artificially into mid- and high-latitude atmospheric circulation and low-latitude atmospheric circulation, which leads to the insufficient division of mid- and high-latitude atmospheric dynamics and low-latitude atmospheric dynamics. Developing a unified description of the global atmospheric circulation is an important problem faced by meteorologists.

       Hu Shujuan said: “We want to solve the theoretical framework of climate dynamics for global atmospheric circulation that is needed for climate prediction.” The three-type decomposition theory of atmospheric circulation emphasizes the use of a global perspective to look at actual atmospheric motion problems, not only to see different latitudes. For regionally dominant and typical large-scale circulation movements, the important role of other circulations in various latitudes should also be considered. It is like observing a flowing river, not only paying attention to the horizontal movement of the surface, but also seeing the vertical movement of the undercurrent inside it.

       The 'three-type decomposition of global atmospheric circulation' was produced under such circumstances, and no one abroad has touched this field. Hu Shujuan said: 'The name of the'three types' was derived from Teacher Niu.' The 'three types' in the 'three types of decomposition theory of global atmospheric circulation' are horizontal, meridian and latitude respectively. The horizontal circulation describes the horizontal movement in the atmospheric circulation, which is mainly concentrated in the middle and high latitudes. The meridional circulation and the latitude circulation respectively express the large-scale circulation dominated by vertical movement in low latitudes. The actual atmospheric circulation can be regarded as the sum of these 'three-type' circulations.

       The theoretical ideas are put forward, and a quantitative set of mathematical equations needs to be established, and rigorous calculation verification is required. This important task fell on Hu Shujuan, who was a mathematics major. Hu Shujuan said: 'What I want to do is to give a rigorous mathematical description process to verify the 'three types' of physical thinking.' As an inter-professional student who is new to meteorological knowledge, facing a new atmospheric theoretical framework, Shujuan felt uneasy after receiving the assignment, but she was inspired by Mr. Niu’s determination and perseverance to make something with Chinese characteristics, and decided to take up the research task. Hu Shujuan said: 'Teacher Niu thinks this is a very important, meaningful and innovative work. He has always advocated that theories should not be led by foreign countries, and should with Chinese characteristics.'

       In 2001, Hu Shujuan, who did not want to disappoint her mentor, took on the important task of theoretical research on the 'three-type decomposition of atmospheric circulation'. 'Teacher Niu has profound thoughts and I am very encouraged. I feel honored to follow him to do something down-to-earth. 'So, she took this 'sprouting' theory into her arms and began to cultivate it slowly, and has been studying it till now.

 

 

       The establishment and verification process of the 'three-type decomposition theory of global atmospheric circulation' is difficult and arduous. It requires not only reasonable physical thoughts, but also a solid mathematical foundation. At the beginning, Hu Shujuan and her team had no clue on how to quantify and how to construct mathematical models. “There is no three-type theory construction work abroad, so there is no reference. I didn’t think of a method at the time, and I really didn’t know what to do. 'Facing with the inability to start the theory, she fell into deep anxiety, but when she thought that the research work of the 'three-type decomposition theory of global atmospheric circulation' is a scientific research project that the teacher wanted to do, Hu Shujuan felt that she could not fail. Just insist on exploring.

       Hu Shujuan said: 'It is easier to express theoretical ideas in qualitative language, but it is more difficult to express them in quantitative mathematical equations. How to combine atmospheric circulation ideas with mathematical model definitions, and how to combine them to derive a reasonable set of equations, which are the problems need to be solved urgently.'

       There are many difficulties in the research road, but Hu Shujuan did not give up. If she can't understand the problem, she will discuss with teachers and classmates. 'Teacher Niu will give guidance in a general direction.' She will write down all the inspirations that burst out in the discussion, and then continue to ponder.

       'Teacher Niu's guiding ideology is profound, and it is very difficult to realize it. I remember a junior who said vividly at the time that when we went to find teacher Niu, most of them entered the door in frustration, went out happily, and continued to be full of frustration soon afterwards.' Hu Shujuan felt that one of the most difficult tasks was to establish the dynamic equations of the three-type circulation decomposition. At that time, there was no suitable idea, and coupled with the complexity of the atmospheric motion equations themselves, 'the draft paper of the calculation process was always piled up all over the floor.'

       'In the end, I went to discuss with teacher Niu to solve the problem. Teacher Niu suggested that I don't derive term by term. Instead, try to use the form of operator equations to establish a system of equations from a macro perspective of the nature of the operator. The teacher's words inspirited me suddenly. In the end, following the path of Niu's 'operator equation', Hu Shujuan found a suitable method and broke through the formation of equations.

       This is a long journey of research that can't be seen at a glance. Hu Shujuan said frankly: 'To be honest, I have always been ashamed, and I feel sorry for the teacher, because the progress of the project is too slow.' The advancement of the 'three-type decomposition theory of global atmospheric circulation' requires not only profound theoretical thinking, but also a large amount of calculation derivation, algorithm design and programming implementation, so it is difficult to produce results in a short period of time. Although this principle is known, the reality is still difficult. Hu Shujuan will inevitably be discouraged. Whenever she communicated with her teacher, Hu Shujuan was embarrassed about her slow progress. But teacher Niu did not urge her to blame, but always patiently encouraged her: 'Don't worry, do it slowly.'

       Hu Shujuan was inspired and moved, and strengthened her determination to 'sit on the bench for ten years.' To continue to advance the research, the team has devoted a lot of time and effort. They are not afraid of the slowness of theoretical research, even if they can only move forward a small step, they will not give up.

       After more than ten years of hard work, the 'three-type decomposition theory of atmospheric circulation' finally has blooming.

       The team constructed a mathematical model to quantitatively express the global atmospheric circulation. They added the role of vertical circulation in the traditional mid- and high-latitude circulation representation process, and added the role of horizontal vortex circulation in the representation of low-latitude circulation. The quantitative description process of vertical circulation was extended to mid-high latitudes, and the description of horizontal vortex circulation in mid-high latitudes was extended to low latitudes, thereby more accurately reflecting the operation of the entire earth’s atmospheric circulation, and to some extent compensated for the deficiencies of traditional research methods.

       Seeing that the influence of 'three-type decomposition theory of atmospheric circulation' has continued to expand domestically and internationally, Hu Shujuan said with satisfaction: 'It is gradually being paid attention and recognized by everyone, and many industry experts and scholars are discussing with us on this theory.'

       Hu Shujuan and his team have a clear plan for future research directions: 'In addition to using data and new models to study new characteristics of atmospheric circulation, we also want to do some research on the dynamics of equations to further improve the dynamics theory of this equation system.' Hu Shujuan is not satisfied with the status quo, and still maintains a sincere and tough attitude, and continues to promote the research on the 'three-type decomposition of global atmospheric circulation'.

 

 

       The 'three-type decomposition of global atmospheric circulation' theory can help people more accurately describe the operation of the actual atmospheric circulation from a global perspective. Hu Shujuan said: 'If the circulation field can be simulated well, precipitation can be predicted well.'

       The major climatic events of summer drought and flood in China are often closely related to the abnormal evolution of the western Pacific subtropical high. Applying our circulation decomposition theory to the description of Xitai subtropical high, this paper reveals the relationship between the continuous precipitation from the middle and lower reaches of the Yangtze River to the Jianghuai basin in June and July 2020 and the continuous anomaly of the Xitai subtropical high.' This summer, they compared the theory with the actual climate, precipitation and other changes, and got the results consistent with the actual situation.

       In the work report of the 2020 mid-term inspection of the key special project of the national key research and development plan 'Monitoring, Early Warning and Prevention of Major Natural Disasters' in which the team participated, the theoretical work of the Hu Shujuan team was affirmed by experts. Multi-model optimal ensemble prediction scheme plays an important role.” This means that the three-type decomposition theory of atmospheric circulation can help us prevent meteorological disasters related to major climate events and minimize the damage it brings. Although the 'three-type decomposition theory of atmospheric circulation' has not been seen to directly serve social life, it is indispensable.

       Hu Shujuan said: 'Atmospheric theory research is very important and meaningful, but it is very difficult to apply it. A theory is applied, it takes a long process.' Therefore, it will take some time to put the 'three-type decomposition theory of atmospheric circulation' into practice. I hope this beautiful wish can be realized soon.

       'The theory has been developed, and the practical application is now at the beginning stage. My graduate students can gradually do a little work in this area, coupled with the cooperation and support of other teachers, so I believe this work can be developed better.' , Hu Shujuan will also infiltrate some content related to the 'three-type decomposition theory of atmospheric circulation' to undergraduates and graduate students of other teachers, and encourage everyone to use this theory for meteorological analysis and research.

       She attaches great importance to cultivating students’ theoretical thinking and is willing to listen to their ideas. “Students’ abilities and potentials are really limitless. With a little guidance, they can hit the sparks of thought.” Hu Shujuan meets with students regularly every week, including two group meetings, one work progress report, one document explanation. She encourages doctoral students to actively participate in international conferences and requires master students to have the experience of attending a domestic conference and giving an academic report every academic year. 'If necessary, I will bring the students to Beijing to discuss problems with teacher Niu. She always does her best to help the students.

       To enable the graduate students to better understand the basic theory, so that they can gradually understand and apply mathematical formulas, Hu Shujuan installed a blackboard on the wall of the graduates' study room, which is convenient for discussion with students at any time and derive the theoretical formula. With the increase in the number of students in the study room, this blackboard has been used less frequently, but it is still the only blackboard in the graduates' study room of the College.

       'It's a very meaningful job. We will steadily continue to do this work. Now, I invite teachers and experts inside and outside the College to study with my students. As the theory is slowly accepted by colleagues, they are full of confidence in the future path of the 'three-type decomposition of atmospheric circulation'. Hu Shujuan looks forward to the future development of the 'three-type decomposition of atmospheric circulation', and hopes that more people in this area will pay attention to this issue and join in the exploratory research and application.