Shulou(Shulou.com)11/24 Report--

Li Cundong

School of Physics, University of Chinese Academy of Sciences

Training unit: Institute of Physics, Chinese Academy of Sciences

It was Lesser Cold two days ago, as the saying goes: Lesser Cold was in 239, and the weather was so cold that it was shivering. Lesser Cold's solar terms are usually about to mean entering 39, so the period of Lesser Cold and Greater Cold is usually the coldest time of the year, but this year Lesser Cold seems to have a warm feeling like spring, as if the pace of spring is approaching. However, as the saying goes: Lesser Cold is not cold, maybe the cold days are yet to come.

We must still remember the severe cold in December last year. In the early winter of this year, the whole country experienced two large-scale cold waves, and many students in Guangdong even turned out their down jackets, so we can't help but wonder: can the warmth of these days last? Is it a cold or warm winter this winter? What is triple La Nina and what effect does it have on the weather this winter?

Big cotton-padded jacket Hey two cotton-padded trousers, inside is sheepskin wrapped in cloth, no matter how cold we are not afraid! | the picture is from Zhao Benshan's sketch "Red Sorghum models" to answer the first question first. The warmth of these days is coming to an end. Next, the Siberian high pressure will reorganize the cold air to the south, and almost the whole country will usher in a cooling weather. The worse news is that as it is so warm these two days, this cooling will be particularly prominent. Large areas of the country will usher in a cooling of more than 10 ℃, and a cooling of 15 ℃ will be very common, and some areas will even have a cooling of more than 20 ℃. Take a look at the following early warning picture to show you what three or nine days are. Everyone can turn out the big cotton-padded jacket and two cotton-padded trousers.

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The first cold wave warning in 2023 is eaten in conjunction with the previous picture. The picture is from the second question of the China Weather Network. Is it a cold winter or a warm winter this year? I don't know. Weather change is a typical complex dynamics. If I had studied it thoroughly, I would have achieved great success if I had won the Nobel Prize in physics. As for the third question, let's answer it at the end of the article, or I'm afraid you won't finish it.

What is a cold wave? the extreme weather that our country faces in winter is mainly a cold wave, which is essentially an extremely powerful cold air.

This will be my living state in two days. As the name implies, the difference between cold air and ordinary air is the temperature, and the air in the low temperature area is cold air, which will lower the temperature in the area. The cold air that affects our country usually comes from Siberia or the farther Arctic. According to the degree of strength, cold air in China is divided into five grades: weak cold air, medium intensity cold air, strong cold air, strong cold air and cold wave. The division principle here is based on two indicators: the decrease of daily minimum temperature and the value of daily minimum temperature in areas affected by cold air.

It is not difficult to see that a cold wave is cold air, but it is the strongest cold air. Cold wave refers to the weather process in which the strong cold air in the polar or high latitudes invades the middle and low latitudes on a large scale in winter, resulting in severe cooling and northerly winds, sometimes accompanied by rain, snow, rime or frost.

The standard of cold wave is that the daily minimum temperature of a place drops more than or equal to 8 ℃ within 24 hours, or 10 ℃ within 48 hours, or 12 ℃ within 72 hours, and the daily minimum temperature drops to 4 ℃ or less.

China also has a corresponding meteorological early warning system for cold wave disasters. Cold wave warning signals are also divided into four levels, which are indicated by blue, yellow, orange and red respectively:

Cold wave blue warning signal. Standard: within 48 hours, the minimum temperature will drop by more than 8 ℃, the minimum temperature will be less than or equal to 4 ℃, and the land average wind will reach more than 5; or it may have dropped more than 8 ℃, the minimum temperature will be less than or equal to 4 ℃, the average wind will reach more than 5, and may last.

Yellow warning signal for cold wave. Standard: within 24 hours, the minimum temperature will drop by more than 10 ℃, the minimum temperature will be less than or equal to 4 ℃, and the land average wind will reach more than 6; or it may have dropped more than 10 ℃, the minimum temperature will be less than or equal to 4 ℃, the average wind will reach 6 or more, and may last.

Orange warning signal for cold wave. Standard: within 24 hours, the minimum temperature will drop by more than 12 ℃, the minimum temperature will be less than or equal to 0 ℃, the land average wind can reach more than 6; or it has dropped more than 12 ℃, the minimum temperature will be less than or equal to 0 ℃, the average wind will reach more than 6, and may last.

Red warning signal for cold wave. Standard: within 24 hours, the minimum temperature will drop by more than 16 ℃, the minimum temperature will be less than or equal to 0 ℃, and the land average wind will reach more than 6; or it may have dropped more than 16 ℃, the minimum temperature will be less than or equal to 0 ℃, the average wind will reach 6 or more, and may last.

It is obvious that the picture comes from the cold wave of Hubei Meteorological Bureau, which has the characteristics of large cooling range, wide range of influence, serious disaster and so on, and does great harm to China's industrial and agricultural production and daily life.

For agricultural production, cold waves may kill some overwintering crops, and direct exposure to wild plants may cause irreversible damage to plants due to the freezing of physiological water in the body due to drastic cooling. Plants planted in greenhouses may also be frozen to death because of strong winds or snowfall. Days of precipitation may also reduce the yield of some plants in critical periods such as flowering and fruiting.

For transportation, the cooling and precipitation brought by the cold wave may cause road snow, road icing, low visibility and other phenomena, hinder driving and affect traffic.

For electric power transportation, the freezing rain caused by cold wave will cover the power lines with rime and increase the load on power lines and towers, which may break the power lines and towers in conjunction with the windy weather, resulting in interruptions in power transmission and network communication.

For people, the cold wave is extremely easy to cause colds, while cooling will also cause vasoconstriction, easily lead to lung disease and cardiovascular disease.

When the trend of cold wave in China in recent years refers to climate change, the first thing that comes to mind is global warming. indeed, it is certain that the global temperature is accelerating, and this is an indisputable fact that we have to face. In the latest sixth assessment report of the Intergovernmental Panel on Climate change (Intergovernmental Panel on Climate Change, referred to as IPCC), it is clearly pointed out that the global surface temperature is about 1 ℃ higher than that before the Industrial Revolution, and the global warming trend is significant. In the IPCC "Special report" (IPCC, 2018), it is pointed out that due to the impact of global warming, the global temperature will rise by 1.5C in the middle of the 20th century, or even accelerate. The climate change caused by it is also very severe.

It is true that winters have become warmer and warmer in China over the past few decades because of global warming. But for Eurasia or more limited, for China, winter climate change can not be summarized in a word of warming.

The mean temperature in winter in China shows obvious Interdecadal variation characteristics. From the 1960s to the mid-1980s, the winter in China was generally in a cold period, and the East Asian winter monsoon was more active. From the mid-1980s to the beginning of this century, the East Asian winter monsoon weakened obviously, China was in a warm period, the winter average temperature increased and the extreme weather decreased. However, in this century, especially after 2008, the warming trend of winter temperature in China has slowed down, and it can even be said that extreme weather has become more frequent in the cold period.

In 2008, China experienced a large-scale cooling weather of rain and snow once in 50 years, and the cooling range in some areas was rare in a century, which brought great disasters to our country. A "overlord" cold wave occurred in China from January 21 to 25, 2016. During this severe cold wave, the temperature dropped sharply, and the lowest temperature in many parts of the country fell below the historical extreme. More recently, people may be more impressed. At the end of 2020 and the beginning of 2021, including the past December 2022, China has visited the cold wave weather all over the country, and the impact of these cold wave weather has gone deep into the vast south of the Yangtze River, as far as Guangdong.

Right now, the United States on the other side of the world is experiencing a severe winter storm, which has been raging since the end of last year, sweeping almost the whole of the United States. If you search for relevant terms, you will find that the United States also experienced nationwide winter storms last year, and last year's winter storms even put Texas into a state of emergency. In fact, due to factors such as topography, extreme weather in winter is more frequent in the United States than in East Asia.

The picture comes from Xinhuanet, but in fact, global warming is not a lie. The warming rate in the Arctic is much faster than that in other regions, which leads to a significant reduction in the area of Arctic sea ice, affecting the heat exchange between the polar ocean and the atmosphere, and indirectly affecting the climate of Eurasia. Some scholars have proposed that in recent years, Eurasia shows the mode of "Arctic warm, Eurasian cold". Arctic climate change may be the internal reason for the frequent occurrence of extreme weather in Eurasia. of course, there is still a debate about the mechanism of Arctic warming and extreme weather in the middle latitudes of the Northern Hemisphere.

The Siberian cold high belongs to the typical monsoon climate in China. In winter, the subtropical high weakens, and the winter monsoon is mainly affected by the Siberian high.

Let's first take a look at the Siberian high, which is a direct factor affecting the cold air in winter in China.

Speaking of Siberian high pressure, in fact, the cold air here does not originate from Siberia, but from the Arctic. Arctic cold air tends to gather in Siberia, so it is called Siberian cold high.

In high school geography, we should all have learned that three circles of circulation will cause cold air to sink in the polar regions, forming cold high pressure at low altitude and cold low pressure at high altitude, and polar cold air will form a circulation with the subpolar low pressure belt. But when winter comes, the polar temperature drops, the cold air sinks more strongly, and the circulation stops near the ground by mountains, and the polar cold air rotates to form an Arctic vortex. However, due to the difference in thermal properties between land and sea, the center of the Arctic vortex in winter is often not in the Arctic region, but eccentric.

Arctic eddies usually have two centers, one on Baffin Island in Canada and one in northeastern Siberia, which control the high-latitude currents in the northwest and northeast hemispheres, respectively. When the center of the Arctic vortex develops southward, it often means that cold air is coming.

There are three sources of cold air affecting China, namely, to the west, east and south of Sindi Island, in which the cold air on the ocean surface to the west and east of Sindi Island comes from the center of the Arctic vortex in northeastern Siberia, while the cold air on the south ocean surface of Sindi Island comes from the center of the Arctic vortex on Baffin Island in Canada. When I say this, you may think it is very close, but you can see why it is said to be the three places of origin by looking at the map.

The picture is drawn from the cold air of the Hubei Meteorological Bureau, starting from these three sources and entering the key cold wave areas through three paths:

Northwest route: the cold air to the west of Sindi Island enters China through the Barents Sea and the European region of Russia. It is the cold air that affects the most times (about 50%) and reaches the intensity of the cold wave, and it is also the cold air path that often affects Xinjiang in winter.

The superpolar path: the cold air from the ocean east of Sindi Island enters China through the Kara Sea, Tamil Peninsula and Central Siberia. It occurs less frequently (about 20%), but the temperature is low and it is easy to reach the intensity of the cold wave. It is common in winter and spring

West route: the cold air from the ocean south of Iceland enters China through southern Europe in Russia or the northern part of the Mediterranean Sea, Black Sea and Caspian Sea. The frequency of cold air is about 30%. Because the temperature of this source is higher than that of other sources, it generally reaches the intensity of cold wave, but it can reach the intensity of cold wave after confluence with cold air of other sources in the process of eastward movement. It is common in spring and autumn.

The key cold wave area is located in central Siberia, about 70-90 °E, 43-65 °N, 95% of the cold air will enter this area, and will accumulate and strengthen here. When the cold air accumulates to a certain extent, it will continue to go south. There are four main paths for cold air entering our country: East Road, West Road, Middle Road and East Jiaxi Road.

Extreme low temperature events are often associated with multiple atmospheric circulation systems. anomalies in a single atmospheric circulation system are very common, and they can only cause limited impact under such anomalies. The formation conditions of extreme low temperature events require multiple atmospheric circulation systems to appear anomalies and form cooperation at the same time. In addition to the well-known Siberian high, the atmospheric circulation anomalies related to extremely low temperature weather include polar eddies and blocking high.

Let's talk about the polar vortex first. in fact, it has already been introduced earlier, that is, the Arctic vortex. The polar vortex here should actually be a polar vortex, or polar vortex for short. The polar vortex is a cyclonic vortex around the north and south poles of the earth, which exists in both the north and south poles, but the Antarctic vortex obviously does not have a significant impact on China, so the weather in winter in China still depends on the face of the Arctic vortex.

The stable Arctic vortex in mid-November 2013 is derived from the polar vortex of the National Meteorological Science data Center, which is located in the middle and upper troposphere to the stratosphere. Its shape, intensity, area and other characteristics can show the intensity of cold air affecting China. It is found that after the strong polar vortex event occurred in the stratosphere in 2007 and 2008, the planetary wave extends abnormally southward, which is beneficial to the transport of cold air to the south of Eurasia, resulting in the freezing disaster of low temperature rain and snow in the south in January 2008. According to statistics, among the 171 cold waves affecting China in 10 winter and half a year, 102 are persistent polar eddies over Asia, and 6 strong cold waves are obviously related to the southward position of the polar vortex over Asia.

One interesting phenomenon that I find interesting is that when the polar vortex is weak, the probability of cold air eruption in the middle and high latitudes of the northern hemisphere increases by 50%. When the polar vortex is weak, the subtropical air mass will move northward, pushing the Arctic vortex to the south, and the cold air will go south, hitting Eurasia and North America. As for this subtropical air mass, it is also an old friend of everyone, which is usually the North Atlantic warm current.

The picture is from the National Meteorological Science data Center, and the blocking high comes from the prevailing westerly belt.

The westerly belt is the main average horizontal circulation in the mid-latitude troposphere, which surrounds the polar vortex and usually prevents the polar cold air from moving southward. But the westerly belt is not straight, because of the atmospheric vorticity, the westerly belt is actually a wavy flow pattern. On average, the westerly belt has three trough ridges in winter and four in summer. This wavy flow pattern is called westerly fluctuation. The main feature of its fluctuation is the maintenance and transformation of zonal circulation and meridional circulation.

The waves in the westerly belt can be divided into three types according to their wavelengths: ultra-long wave, long wave and short wave. The ultra-long wave wavelength is more than 10000 km and is quasi-stationary, which belongs to the medium-and long-term weather process; the long-wave wavelength is 3000-10000 km, and the long wave has a certain moving speed, sometimes quasi-stationary, or even retrogressing to the west. its changes usually lead to obvious changes in general weather systems and weather processes; short-wave wavelengths and amplitudes are smaller and move faster.

In the middle and high latitudes, the wavelength of the long wave can be similar to the radius of the earth, so it is called planetary wave or Rossby wave, and the long wave can be seen from the middle and lower troposphere to the bottom of the stratosphere.

Planetary wave diagram | the picture is derived from the unstable westerly belt of the fruit shell. When the westerly ridge of the westerly belt is significantly strengthened, a closed warm high pressure can often be formed in the long wave ridge, which can become a blocking high pressure, and a closed cold low pressure can be formed on one or both sides of it. It's called cut-off low pressure. The blocking high divides the westerly jet into north and south branches, which are generally maintained for more than 5 days. The southerly air flow prevails in the west, the climate is warm and rainy, and the north air flow prevails in the east, and the climate is fine. Blocking high occurs mainly in the northern hemisphere, such as the Ural blocking high that often occurs in Asia, which can hinder the eastward movement of westerly winds.

However, when the blocking high pressure collapses, it often leads to the rapid outbreak of cold air to the southwest. When there is a westerly trough invading in the west of the blocking high pressure, the blocking high pressure will collapse, because the blocking high pressure is very likely to form a horizontal trough, and when it collapses, it will cause the horizontal trough to turn vertical, so it can make the accumulated cold air go all the way south along the vertical horizontal trough, causing cold wave weather.

A brief summary of the cold wave is that the normal cold air comes from the Arctic and flows south after the accumulation of air from the Arctic in Siberia. Due to subsequent weakness and obstacles along the way, the impact on our country is relatively limited, especially after crossing the Qinling Mountains. It usually does not cause severe harm to the south.

However, when the atmospheric circulation is abnormal, the polar vortex in the Arctic is eccentric and the cold front is pressure to the south, and then guided by the large long-wave trough in the westerly belt-- the blocking high in the Ural Mountains-- cold air will accumulate continuously to Siberia and provide sufficient cold air, which will eventually cause a large-scale and lasting impact on China, that is, the cold wave. At this time, the lower Qinling Mountains are unable to stop the cold air from moving south, and the cold wave will sweep the south of the Yangtze River and even reach Lingnan to the South China Sea. Of course, the formation of cold air generally ends when it reaches Guangdong, and the sea is still dominated by subtropical high.

Other factors, of course, the changes in the weather are still very complex, and the occurrence of the cold wave is far from being explained in a few words. For example, some studies have pointed out that the Arctic Oscillation is associated with cold waves in Eurasia, and we can also monitor extreme events through the Arctic Oscillation Index. The amplification effect of the Arctic weakens the polar vortex and aggravates the splitting of the cold air of the polar vortex to the south. There may also be an internal correlation between ENSO events and extremely low temperature events, where EN refers to El Ni ñ o and SO refers to the Southern Oscillation.

La Nina event is one of the important factors affecting the global climate. The latest monitoring shows that the La Nina event in the equatorial Middle East and Pacific Ocean is still continuing, and it is expected that the La Nina event will last until the winter of 2022 and 2023, which will be the first "triple" La Nina event in the 21st century. The triple La Nina here is not La Nina appeared for three years in a row, but once La Nina lasted for three years.

However, Zhou Bing, chief expert on climate services at the National Climate Center, said that the "triple" La Nina events did not continue to stack year after year, focusing on "big moves" in the third year. Therefore, although there is a cumulative effect to some extent, it does not mean that the climate impact will be doubled, and the possibility of a cold winter this year is very small.

It can only be said that, as a typical complex dynamics, it is very difficult to study the weather thoroughly. Due to the limited space, we can only talk about it this time. Maybe we can have a chance to talk to you again on some issues in the future.

Reference:

[original popular science article] the influence of cold wave and its prevention

The cold wave is coming, you should know that!

Where does the cold air affecting our country come from? Is it really Siberia?

Polar vortex. What does polar vortex mean?

The global warming cold wave is more extreme.

Talking about the westerly trough

Principles of synoptic science in Nanjing university chapter 4 3.ppt

Blocking high. What does blocking high mean?

Liu Ziqi. Interdecadal variation of large-scale extremely low temperature events in China and its weather causes [D]. Lanzhou University, 2022.

Liu Mingxin. Characteristics of two types of extremely low temperature events in China and their causes of atmospheric circulation [D]. Lanzhou University, 2021.

Xu Xinping. Study on the relationship and physical mechanism between Arctic warming and Eurasian winter climate [D]. Nanjing University of Information Engineering, 2020.

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop). Author: Li Cundong

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