Four questions about heavy rainfall and abnormal cold air in South China

Cold and warm air confrontation, strong cold air, rare south pressure

  Since May 9, southern Jiangnan and southern China have experienced the strongest rainfall since the flood season this year.

From 08:00 on the 12th to 06:00 on the 13th, there were heavy rains in parts of central and southern Guangdong, southeastern Guangxi, and southeastern Fujian. Heavy rains occurred in Zhuhai, Jiangmen, Shenzhen, Shanwei, Heyuan, and Zhangzhou, Fujian, and Zhuhai, Guangdong. Heavy rains occurred locally in Jiangmen, Shanwei and other places.

Has this round of rainfall exceeded the extreme value?

How does the Bay of Bengal storm affect this rainfall?

Why is the earliest orange warning in the past decade?

In response to these problems, we invite Zhang Tao and Lv Xinyan, chief forecasters of the Central Meteorological Observatory, to interpret them.

A question about the fact: on the 13th, many places in southern China entered the period of the strongest rainfall.  

  On May 13, many places in southern China have entered the strongest period of this rainfall process, and more extensive rainfall will occur.

From the daytime on the 12th to the daytime on the 13th, it was the main period of large-scale rainfall. The rainfall range gradually expanded, and the intensity at a single point increased slightly.

The rainfall in Guangdong Province is currently in a state of "blooming everywhere", with the characteristics of "strong on the spot and uneven distribution on the surface".

The strongest rainfall of this process occurred in the central and coastal areas of Guangdong, and extremely heavy rains occurred in Zhongshan City, Zhuhai City, northern Guangzhou, and southern Qingyuan.

At present, some sites have exceeded the daily rainfall extreme in May, and it is very likely that the ten-day rainfall will exceed the historical extreme value.

  On the morning of May 10, flood control baffles and sandbags were set up at the entrance of Huacheng Avenue of Metro APM Line in Huacheng Square, Guangzhou, Guangdong Province.

Liang Yitao Photography

Second question: The cold and warm air confrontation, the Bay of Bengal storm "double contribution"    

  The confrontation between cold and warm air is the main reason for the formation of this rainfall.

The strong South China Sea monsoon and the southward cold air meet in southern China. The cold air blocks the warm and humid air flow, and the two sides are deadlocked within a certain range, so there is a situation of continuous rainfall in the same area.

The confrontation between the cold and warm air flow is fierce, which greatly increases the probability of rainstorms and increases the extremeness.

  The storm "Asani" in the Bay of Bengal played a role in transporting warm and humid air over long distances for this process.

The impact of the Bay of Bengal storm was two-sided.

One is that during strong periods, the Bay of Bengal storm acts like a "pumping machine", which helps to accelerate the Somali jet stream that crosses the equator, which is conducive to accelerating the onset of the monsoon.

However, at the same time, the abundant water vapor carried by the monsoon was swept and absorbed by the strong Bay of Bengal storm itself, weakening the momentum of the Somali jet stream passing the storm and continuing eastward into the Indo-China Peninsula and the South China Sea.

Second, during the extinction period, the storms in the Bay of Bengal weakened and disappeared, and the eastward monsoon had fewer "blockers" and could develop eastward more smoothly to the South China Sea and the Indo-China Peninsula, which in turn affected the rainfall situation in my country.

This is also one of the reasons why the rain in southern China intensified and the rainfall range further expanded from the 12th to the 13th.

  It is reported that storms in the Bay of Bengal have a unique structure of double cyclone seasons. The two peaks are in May and October-November respectively, which basically coincide with the onset and end of the South Asian summer monsoon. Among them, the generation rate of super cyclone storms is higher in May.

The impact of the Bay of Bengal storm on my country is mainly precipitation. When it moves eastward, heavy rain and heavy snowfall may occur in southern Tibet and southwestern Yunnan.

In addition, the Bay of Bengal storm also provided a long-distance water vapor transport channel for the middle and lower reaches of the Yangtze River and southern China in my country, which enhanced the precipitation in these areas.

Therefore, the impact of the Bay of Bengal storm on my country's precipitation can be extended to the entire southwest region, and even to the south of the Jianghuai River Basin.

Three questions and warnings: On May 11, the Central Meteorological Observatory issued the earliest orange warning for heavy rain in the past ten years

  At 18:00 on May 11, the Central Meteorological Observatory issued the first orange rainstorm warning this year, which is the earliest orange warning issued by my country in the past ten years.

Since the flood season this year, the precipitation in southern China has been significantly less than the same period.

Beginning on May 9, affected by the confrontation between the cold and warm air, the rain belt basically remained in the area of ​​Guangdong, Guangxi, and Fujian, and it will continue until the 15th.

On the afternoon of the 11th, considering that the rainfall lasted for a long time, the range of the falling area was relatively fixed, and the rainfall in the next 24 hours would increase significantly, the local daily rainfall may exceed the historical extreme value for the same period, so the yellow rainstorm warning was upgraded to an orange warning.

  After the 15th, the heavy rainfall process will basically end, and most of southern China will turn to cloudy or light rain.

  On May 12, heavy rainfall occurred in Qinzhou City, Guangxi Zhuang Autonomous Region. The strong short-term rain caused water accumulation in many sections of the urban area. The Municipal Meteorological Bureau launched an emergency response and issued a yellow warning signal for lightning and an orange warning signal for heavy rain.

Li Binxi Photography

Four questions about cold air: abnormal cold air raids southward, breaking through the historical low temperature extreme value in May in many places  

  Affected by the cold air, gusts of magnitude 7 to 9 occurred in Xinjiang, Gansu, Qinghai, central and western Inner Mongolia, and central Shaanxi, with local gusts reaching magnitude 10 to 11; Parts of western Gansu, southern Ningxia, central Shaanxi, southern Shanxi, and northeastern Hebei experienced a temperature drop of 6°C to 10°C.

  The beginning of summer has passed, and the temperature should have gradually risen, but in the first ten days of May, 167 national-level meteorological stations have broken the record for the highest and lowest daily temperature in the same period in history. These places experienced the coldest early May since meteorological records began.

  Many places are experiencing the "coolest early summer".

Taking Yunnan as an example, since last winter, the cold air has been controlled in the Yunnan area for a long time, resulting in low temperature. Tengchong, Yunnan, which should have entered the summer, has broken the minimum extreme value of the daily maximum temperature in early May.

Taking Beijing as an example, the average maximum temperature in Beijing in early May was 24.9 ℃, but the maximum temperature in Beijing on May 8 was only 13.7 ℃.

  According to statistics, in the middle of May last year, the cold air rarely crossed the south of the Yangtze River. Even if it crossed the Yangtze River, it would basically not cross the Nanling Mountains.

This time, the continental cold air mass traveled all the way south to the middle of the South China Sea in mid-May, and even crossed Yunnan to the southwest to Myanmar. This is not only unusual in May, but also very rare in winter when the cold air is strong.

The reason behind it, from the perspective of atmospheric circulation, the direct reason is that the cold air is strong in East Asia, and the high-altitude northeast cold vortex guides the cold air to the south, and it continues to supplement and transport it continuously. The rate of temperature rise.

It is expected that in the next week, the temperature in the north will gradually return to normal due to weaker cold air and more sunshine, but the low temperature in the south will continue, or even further aggravate.

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