Why is strong convective weather the most difficult to report？
In the flood season, lightning, hail, strong winds and short-term heavy rainfall occurred one after another. Even on June 23 this year, Yancheng, Jiangsu Province suffered a tornado and hail disaster, which all meant the arrival of a high incidence of strong convective weather in China. As the saying goes, "hail hits one line", "rain crosses another road" and "sunrise in the east and rain in the west" all illustrate the characteristics of convective weather.
Comic "Sunrise in the East and Rain in the West"
Extremely destructive and local.
The reason why the forecast of severe convective weather is the most difficult of all weather types is to start with what it is and how it happens.
Strong convective weather refers to severe convective weather that occurs suddenly, changes dramatically and is extremely destructive, often accompanied by thunderstorms, hail, squall lines, tornadoes and short-term heavy precipitation. Strong convective weather is easy to cause disasters, which is destructive and influential. In addition, it has strong locality, small spatial scope and short duration or life span, but it can release powerful forces in a short time. This feature makes every weather phenomenon in the strong convective weather family have extremely strong destructive power and disaster.
In China, strong convective weather often occurs in spring and summer, and the impact of strong convective weather in these two seasons on cities and rural areas is also different. In spring, hail and strong winds often appear, which are easy to damage buildings and even cause casualties; For agricultural production, it will destroy crops and destroy agricultural facilities. Strong convective weather mostly occurs in summer, during which the temperature is high and the humidity is high, which provides favorable temperature stratification and water vapor conditions for convection. Convective weather often forms in the afternoon to the evening in a day, and sometimes lasts until the first half of the night, which is closely related to the daily change of temperature. In summer, short-term heavy rainfall, thunderstorms and strong winds are often visited. Cities with poor drainage facilities may cause waterlogging, while in areas with weak geological structure, natural disasters such as mountain torrents, landslides and mudslides are prone to occur. Short-term heavy rainfall will also cause rivers and lakes to rise rapidly in a short time, causing floods, threatening people’s lives and property safety.
Severe convective weather forecast is a worldwide problem.
The weather system with small scale and rapid development is a difficult point in meteorology. Under the current observation conditions, not only China, but also all countries in the world do not have a comprehensive grasp of its information, so there are still some limitations in the understanding and understanding of this weather. The unpredictability of severe convective weather is a common understanding of experts and scholars engaged in meteorological prediction and scientific research at home and abroad.
At present, the predictions of possibilities in all countries in the world are only potential forecasts, that is, in a relatively long period of time, convective weather may occur in a certain area. Because of the long time period, only a large-scale forecast can be given comprehensively, which is not particularly targeted. Therefore, in order to provide targeted services, countries all over the world adopt the approach of approaching early warning to strengthen services and guidance.
Short-term heavy precipitation is a kind of weather phenomenon which is easy to predict in strong convection family. In summer, a lot of rainfall is a mixture of stable precipitation and convective precipitation, and convective precipitation often has a large amount of precipitation in a short time, which also has a great impact. In recent years, the forecast accuracy of short-term heavy rainfall in China has been steadily improved year by year, while the forecast of hourly rainfall intensity has improved the fine level and service ability of severe convective weather forecast.
Lightning disaster is one of the main disasters caused by severe convective weather. Because of the rapid formation of lightning, it has brought great difficulties to its early warning and forecasting. Researchers and professionals at home and abroad have done a lot of research on short-term lightning forecasting, using mesoscale observation systems, radars, satellites and lightning positioning systems to obtain observation data and numerical forecasting model products, and carried out the development and business operation of lightning weather approaching forecasting technology. For example, the United States can now give the cumulative point probability products after 3 hours. The research and development of lightning early warning and forecasting technology and methods in China has just started in recent years, mainly focusing on lightning approaching forecasting products and lightning early warning and forecasting systems. However, due to the lack of understanding of the physical process of lightning itself, there is no very mature business system so far.
Tornadoes are small-scale weather systems or small-probability events with strong randomness in time and space. At present, most meteorological detection equipment and detection systems cannot effectively monitor and track the occurrence and development of tornadoes. Therefore, even in the United States, although tornado forecasting and monitoring have been carried out for many years, the distance between meteorological monitoring stations is decreasing year by year, and the radar network is becoming more and more perfect, it is still difficult to accurately forecast and the early warning time is still limited.
There are also great difficulties in forecasting hail. As long as the atmospheric conditions are suitable, there will be hail everywhere. At the same time, the characteristics that indicate the appearance of hail are easily concealed by weather characteristics such as heavy precipitation, thunderstorm and strong wind, which makes the forecast more difficult. At present, numerical weather forecast can give some meteorological factors related to hail formation, but it can’t directly give the final forecast result of hail size. The forecast of hail is mainly obtained by the forecaster’s grasp of various weather elements and comprehensive analysis based on historical data. At present, the focus of hail forecast is to predict the size of hail, which involves more factors and more complicated mechanisms. At present, it can only be a rough estimate and cannot be applied to business. Meteorologists try to identify the size of hail particles by radar echo, and judge the probability of hail of various sizes and the probability of strong hail by hail detection algorithm, and estimate the maximum hail diameter. This method can be used to forecast the weather within one hour, but it is not mature at present, and its effect needs to be tested. In the hail forecast, at present, we can only rely more on Doppler radar for continuous monitoring, and try our best to find the hail characteristics near the strong convective weather, and release them to the public in time through early warning.
A variety of strong weather forecast and early warning products
Weather forecast can be divided into immediate forecast, short-term forecast and short-term forecast from the time limit of forecast service. The near forecast is the weather forecast within 0-2 hours, the short-term forecast refers to the weather forecast within 2-6 hours, and the weather forecast over 6 hours is classified as short-term forecast. There are forecast and early warning products for severe convective weather in these three kinds of time-limited forecasts.
At present, the Central Meteorological Observatory can provide all kinds of severe convective weather forecast products for up to 72 hours, issue severe convective weather warning before severe convective weather occurs, and give early warning of possible severe convective weather according to the time and scope. Meteorological departments at all levels will also issue early warnings for a certain weather phenomenon, such as lightning warning, hail warning, gale warning, etc.
Aggregation forecasting products help forecast and early warning.
Because the system causing severe convective weather belongs to small and medium-scale weather system, it is difficult to be captured by conventional meteorological observation network, so unconventional observation data (automatic station, radar, satellite, lightning global positioning system, wind profiler radar, etc.) and their fusion, assimilation data and mesoscale numerical model data are the main data basis for short-term and imminent forecast of severe convective weather. At present, the United States, Britain, Canada, Australia, France, Japan, South Korea and China have established short-term forecast systems for severe convective weather.
Ensemble forecasting products provide forecasters with more possibilities for their reference. Convection-scale ensemble forecasting system has higher temporal and spatial resolution, can describe the fine structure of weather system and assimilate more information, and the physical process is closer to the actual situation. Therefore, this system can better simulate the local disastrous weather on a smaller scale and accurately forecast the convective weather system with a larger vertical spatial scale and more vigorous development. In the early warning of severe convective weather, the rapidly updated geostationary meteorological satellite image as an important auxiliary means of monitoring, combined with the radar image as the main monitoring tool, can help forecasters release early warning information for a longer time in advance.
Note: This article is based on the relevant articles of China Meteorological Network and the interview with Liu Xinhua, a senior engineer of the Strong Weather Center of the National Meteorological Center. It is adapted and planned by Su Jiexi.