The dog days are over, are cooler days coming?

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On August 24, the 40-day dog ​​days finally came to an end. The high temperatures this year have "melted" many people's yearning for summer. Will the high temperatures continue after the dog days end? Will the weather become cooler? Will there be another "autumn heat"?

Let us put the conclusion first: the day when high temperatures completely come to an end is not far away.

Seeing this conclusion, you may wonder, is the current weather forecast really accurate? Looking back at the hot summer weather, many people paid attention to the weather forecast in the hot summer, hoping that there would be a heavy rain to bring a short coolness. But every time the heavy rain in the forecast approached, it suddenly disappeared, or only a symbolic light rain fell, which made people very disappointed.

Why is this? Is the weather forecast wrong? Maybe you don't really understand the weather forecast at all.

01

Uncertainty in weather forecasts

Weather forecasts are uncertain . Modern weather forecasts and climate predictions are based on numerical weather forecasts. The uncertainty of such weather forecasts comes from at least three major aspects.

The first is the problem of initial values ​​of numerical forecasts, which includes whether the actual observations can fully represent the current state of the atmosphere, and whether the actual observations can be well utilized in the mathematical equations of weather forecasting. This is the initial error .

The second is whether the established mathematical physics equations can truly simulate the various physical processes in nature that affect the atmosphere. For example, how the sun's radiation is transmitted to the atmosphere through ground heating; how the moisture in the atmosphere is converted between water vapor, cloud water, and cloud ice, etc. This is the model error .

Finally, the most fundamental factor affecting forecast uncertainty is the nonlinear problem or the chaotic problem. For a complex chaotic system like the atmosphere, randomness is natural. No matter how precise the mathematical and physical equations are , they cannot fully simulate the complex atmospheric system.

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In short, numerical weather forecasts are approximate simulations of future weather. The accuracy of forecasts based on them is bound to be relative, and the uncertainty is absolute . In general, the larger the range of weather, the more trend-oriented the forecast, the higher the reliability; while the smaller the scale of the weather system, the more detailed the forecast, the lower the reliability. Similarly, the closer the forecast is to the time, the smaller the uncertainty; the longer the forecast is, the greater the uncertainty.

High temperatures in summer are typical of large-scale weather, and the reliability of trend forecasts is very high, but there are still uncertainties in the details. For example, when predicting the high temperature weather trend this week last week, it was found that the high temperature in the north of the Yangtze River would end, and the high temperature in the south of the Yangtze River would also significantly weaken. But today, due to the lack of cold air intensity, the high temperature in the south of the Yangtze River has weakened slowly. However, the trend that large-scale high temperatures will end next week is basically certain.

As mentioned earlier, the uncertainty of weather caused by small and medium-scale weather systems such as rainfall, especially thunderstorms, is even greater.

02

Now that the dog days are over, will the temperature drop?

After being tortured by the long and hot dog days this year, many people are looking forward to the cooler weather after the dog days. We have just learned about the uncertainty of weather changes, and the dog days are only an approximate range. We cannot judge the temperature changes in specific times and regions by "entering the dog days" or "exiting the dog days".

However, from the temperature change chart over the past two days, the cold air has moved southward from the east and crossed the Yangtze River. Although the cold air has weakened after crossing the river, it is enough to alleviate or even end the high temperature in the middle and lower reaches of the Yangtze River and the areas north of the river.

Image source: China National Meteorological Administration

On August 24, the cold air moved southwards. Although it was getting weaker and weaker, it could still further reduce the high temperature south of the Yangtze River. Since the cold air moved southwards from the east, the easternmost region, Zhejiang, where the cold air was the strongest, also saw a significant reduction in the high temperature, or even an end to it. After crossing the Yangtze River, the cold air gradually weakened and degenerated. Except for central and northern Zhejiang, most areas of Jiangnan, such as Hunan, Jiangxi, and Fujian, still maintained high temperatures. Similarly, the cold air was also very weak in entering the Sichuan Basin from the west, so the high temperatures in Sichuan and Chongqing remained the same.

Schematic diagram of cold air and high temperature situation at 8:00 on August 24. Image source: Author

In the rest of this week, the cold air will weaken and replenish. Although the high temperature will remain strong this week, the subtropical high pressure that causes the high temperature will weaken day by day as the cold air and Typhoon "Ma'an" move from the north to the south.

According to the latest meteorological data, the high temperature weather in the north of the Yangtze River will come to an end this week, and the high temperature weather in the south of the Yangtze River is also expected to end around next Tuesday, when a strong cold air and large-scale precipitation process will end. It can be seen that the day when the second round of high temperatures ends is not far away .

03

What is the status of the "autumn tiger" this year?

With the end of this round of the strongest high temperatures in history, we can assume that the high temperatures this summer have come to an end, but there is still a possibility of periodic high temperature weather in the southern region in the future.

According to the short-term climate trend forecast from September 5 to September 20 released by the National Climate Center on August 22, except for the central and southern parts of Northeast China, most of North China, the eastern part of Northwest China, northern Central China, and northern Northeast China where the temperatures are lower than the same period of previous years, the temperatures in most other parts of the country are close to or higher than the same period of previous years.

This conclusion means that most parts of the south are significantly warmer than the same period in previous years, and being warmer means that the probability of the "autumn tiger" is relatively high .

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Take Jiangxi and Zhejiang as examples. In mid-September, the annual average temperature in most areas of the two provinces was 24℃-26℃, the annual average maximum temperature was 29℃-32℃, and the extreme maximum temperature exceeded 37℃, and even exceeded 40℃ in some areas. The average temperature is 1℃-2℃ higher, and the maximum temperature is likely to be 3℃-4℃ higher or even more. This means that there is a high probability that the maximum temperature will be above 35℃, which means that "autumn tiger" weather will occur. However, even if "autumn tiger" occurs, its high temperature is not comparable to the high temperature in midsummer. The high temperature of "autumn tiger" is mainly dry heat, and it is mainly hot during the day, and it will not be too torturous at night .

04

Meteorological drought is developing

Although the high temperature is about to end, the meteorological drought caused by high temperature and low rainfall will not necessarily end with it, and may even develop.

The Central Meteorological Observatory began issuing this round of continuous high temperature warnings on July 21, and upgraded them to the maximum high temperature red warning on August 12. However, the first meteorological drought warning was not issued until August 18, one week after the high temperature red warning. Is this a sign of "slow response" by the meteorological department?

The delay in issuing drought warnings compared to high temperature warnings is not due to the slowness of the meteorological department, but due to the complexity of meteorological drought, which includes the lag in its formation relative to the weather .

Meteorological drought monitoring released by the Central Meteorological Observatory on August 23. Image source: Central Meteorological Observatory

Drought disasters have been a well-known natural disaster since ancient times. From the perspective of modern science, drought problems are very complex and involve a wide range of aspects. They can generally be divided into meteorological drought, agricultural drought, hydrological drought, and economic and social drought.

Meteorological drought

Focuses on measuring drought in terms of a combination of weather factors such as precipitation and evaporation;

Agricultural drought

Focus on the actual drought conditions of crop growth;

Hydrological drought

It focuses on the degree of drying up of rivers and reservoirs. Among them, meteorological drought is the cause of other droughts, but meteorological drought does not necessarily lead to other droughts immediately. It is also affected by many factors, and there is a complex relationship between them.

Since meteorological drought is closely related to precipitation and evaporation over a period of time, the weakening and end of high temperatures does not necessarily mean the end of meteorological drought . As far as this round of meteorological drought is concerned, it is still developing. According to the short-term climate forecast for early and mid-September, the meteorological drought area may not only have high temperatures, but also continue to have little rainfall.

However, it is worth being happy that the high temperatures are coming to an end. We can finally enjoy some cool weather. (This content is produced by Science Popularization China - Creative Cultivation Program)

Author: Zhang Tao, Chief Forecaster of China National Meteorological Center

Reviewer: Ai Wanxiu, Chief Expert Researcher, Climate Service Office, National Climate Center

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