In recent days, severe convective weather has been frequent in southern China. Since 6 a.m. on March 30th, the China Meteorological Administration has issued severe convective weather warnings for four consecutive days. In the early hours of March 31st, severe convective weather struck Nanchang, Jiangxi Province, with maximum winds reaching force 11, resulting in the deaths of four people and injuries to more than 10 others.
So, what exactly is severe convective weather? Can it be accurately forecasted? And how reliable are the warnings?
Severe convective weather refers to catastrophic weather phenomena such as short-term heavy precipitation, thunderstorm gusts, hailstorms, and tornadoes, typically characterized by small scale, rapid development, and fast movement. These weather events can cause immense destruction, often accompanied by qualifiers like "localized" and "momentary" in weather forecasts.
According to the National Committee for Disaster Reduction and the Ministry of Emergency Management, in 2023, China experienced a total of 33 episodes of severe convective weather, with 26 recorded tornadoes, including 9 strong ones. The associated disasters, including thunderstorm gusts, hailstorms, and tornadoes, affected 6.053 million people to varying degrees, resulting in 57 deaths, affecting 1,174.5 thousand hectares of crops, and causing direct economic losses of 11.73 billion CNY.
Thunderstorms, the genesis of convective weather, arise from cumulonimbus clouds, which develop rapidly and can reach great heights. Unlike typical thunderstorms with only rain showers, severe thunderstorms, which involve strong winds, hail, and tornadoes, require more intense atmospheric heat, momentum, and external lifting forces. In terms of the direct disastrous weather system caused by thunderstorm gusts, small-to-medium-scale convective systems are significant.
Weather systems can be categorized into large-scale, medium-scale, and small-scale based on their spatial and temporal scales. Large-scale weather systems, like cyclones and anticyclones, have spatial scales ranging from hundreds to thousands of kilometers and lifetimes of one to several days. Their dynamics and thermodynamics can be described using classical fluid mechanics equations, making their generation, development, and dissipation signals relatively easy to capture with conventional meteorological observation instruments.
Severe thunderstorms belong to medium-to-small-scale weather systems, with horizontal scales ranging from tens of meters to two to three hundred kilometers and lifetimes ranging from a few minutes to several hours. The physical processes of brewing, occurring, developing, and dissipating medium-to-small-scale convective systems are influenced by the evolution of large-scale systems and the environmental conditions, making them either derived from large-scale system evolution or from local medium-scale circulation evolution.
Because the physical conditions for short-term heavy precipitation, thunderstorm gusts, hailstorms, and tornadoes overlap significantly, even if severe convective weather is predictable, distinguishing between them remains very challenging. Once severe convective weather occurs, it drastically alters the atmospheric conditions and interacts with the surrounding atmosphere, potentially triggering new thunderstorm genesis or dissipation. Therefore, studying the mechanisms of the direct disastrous weather systems caused by thunderstorm gusts and the complexity and regional specificity of the resulting hazardous weather remains a global scientific challenge.
Severe convective weather can be seen as an aggregation of large, medium, and small-scale systems influencing each other. Monitoring and predicting the internal structural evolution and movement of these systems and their genesis and dissipation is difficult, while the spacing between meteorological stations is larger than the spatial scale of medium-to-small-scale systems such as severe thunderstorms and tornadoes, akin to catching fish with a net with holes slightly too large, resulting in the omission of significant events. Hence, weather forecasting is a probabilistic science with no definitive answers.
So, how can we judge the reliability of warnings for hazardous weather?
From the perspective of severe convective weather forecasting, it is a scientific problem influenced by both large-scale and medium-to-small-scale interactions, corresponding to potential forecasting and classification/proximate forecasting, respectively. Potential forecasting, provided by the China Meteorological Administration, forecasts the evolution of large-scale systems and issues weather forecasts several hours to several days in advance, indicating the approximate time, region, type, and intensity of severe convective weather. Although highly reliable, it lacks precision.
Classification/proximate forecasting, due to the short lifespan of medium-to-small-scale weather systems, ranging from a few minutes to several hours, is issued independently by provincial, municipal, and county-level meteorological agencies, focusing on various hazardous weather phenomena associated with severe convective weather and rolling out blue, yellow, orange, and red warning signals.
Issuing an orange warning for strong winds indicates that the average wind force can reach 10 or more within 6 hours, or gusts of 13 or more. This implies that the region may experience winds akin to a fast-moving train, although not necessarily at every moment or location within the area. Misinterpretation of this could lead to the false perception of a "crying wolf" scenario.
Although forecasting severe convective weather such as thunderstorm gusts is extremely challenging, advancements in satellite, radar, encrypted ground observation, and the application of high spatiotemporal resolution numerical forecasting models have rapidly improved China's forecasting capabilities, prevention and warning levels, and monitoring and forecasting standards for severe convective weather. For the general public, dealing with severe weather requires understanding warning signals and identifying safe shelters. When local-level alerts are issued, it's crucial to strictly adhere to defense guidelines. For governments, especially in regions prone to severe weather and during peak seasons, it's essential to coordinate emergency management plans in advance. Public education on defense strategies should be prioritized, alongside efforts to enhance warning capabilities, ultimately reducing casualties and property losses. (Authored by a professor at the School of Atmospheric Sciences, Nanjing University of Information Science & Technology)
(Original title: How to Evaluate the Credibility of Severe Weather Warnings - China Science Daily, April 3, 2024, Front Page News)
