On the evening of February 20th, heavy snowfall unexpectedly hit downtown Beijing, blanketing the streets in a layer of white. With no prior indication, residents were taken by surprise. To handle potential hazards or emergencies, the Beijing municipal meteorological, emergency, and transportation departments took swift measures to ensure the safe operation of various systems during the snowy night.
The Beijing Meteorological Observatory issued a yellow warning for road icing at 6:30 p.m. and a blue warning for blizzards at 10:45 p.m. The Beijing Emergency Management Bureau also advised citizens to stagger their travel times, among other recommendations. Over 60 messages were disseminated via the "Beijing Weather" multimedia platform, with a readership exceeding millions. Thanks to timely responses, the urban infrastructure in Beijing operated smoothly, with no significant disruptions to daily life.
How significant was this post-Chinese New Year snowfall in Beijing? According to the Beijing Meteorological Bureau, on February 20th, the city experienced moderate to heavy snowfall, with downtown and southern areas experiencing heavy snow, resulting in a snow depth of 7 to 10 centimeters. From 5 a.m. on the 20th to 6 a.m. on the 21st, the city received an average precipitation of 5.5 millimeters, with downtown areas averaging 7.2 millimeters.
In fact, this snowfall in Beijing was part of a recent nationwide trend of rain and snowfall. Snowfall occurred in Northwest, North, and Northeast China, while areas from the Yellow River to the middle and lower reaches of the Yangtze River experienced sleet or rain, with some regions even experiencing freezing rain. In terms of intensity, the snowfall in Beijing indeed exceeded expectations.
Concerning the discrepancy in the forecast, some netizens raised questions online, sparking media attention and discussion. Whether from the perspective of public perception or meteorological professionalism, this is a topic worthy of exploration, as it aids in understanding weather forecasts correctly and utilizing forecast services reasonably.
First, it's essential to have an objective understanding of the current level of weather forecasting. In recent years, with technological advancements and rapid developments in meteorological practices, China's weather forecasting capabilities have significantly improved, with some indicators reaching international advanced or leading levels. However, despite the progress made, forecast errors still exist, and it's important not to demand beyond our actual capabilities or overinterpret the success or failure of individual forecasts.
Making weather forecasts objective, quantitative, and precise has long been a goal and pursuit. In 1950, meteorologists successfully computed the first numerical forecast result that matched the actual atmospheric evolution by solving sets of physical equations, sparking hope for solving the weather forecasting problem through deterministic solutions. However, this hope was soon challenged. Lorenz's classic paper "Deterministic Nonperiodic Flow" in 1963 revealed the complexity of atmospheric convection and highlighted the sensitivity of initial conditions, leading to the recognition of uncertainty in model calculations. Thus, the focus shifted to improving forecasts within a certain time frame.
According to Lorenz and others, this time frame should not exceed two weeks. Looking at the results from the European Centre for Medium-Range Weather Forecasts, the effective forecast horizon has hovered around 10 days for many years, with accuracy reaching only about 60%. Given the difficulty and uncertainty of forecasting, the explanation for the heavier snowfall in Beijing becomes more understandable.
For any weather forecast, it's essential to understand the changes in relevant elements over time, space, and intensity. This snowfall in Beijing resulted from the interaction of cold air from the north and warm, moist air from the south. It was situated on the northern edge of a nationwide widespread precipitation process, with the precipitation system slowly moving from west to east. Considering the forecast for Beijing's snowfall, at least two issues needed special attention: the speed of eastward movement of the system and the intensity of warm, moist air from the south.
In this case, the system's movement was slower than expected, and the southern warm, moist airflow was stronger than anticipated. These factors, mutually reinforcing, led to the prolonged duration of snowfall in Beijing and exceeded the expected snowfall amount. The multidimensional spatial edges of temporal and spatial variations and element intensity further increase the uncertainty of weather system forecasts, making deviations more likely.
This requires further analysis and summary by meteorological experts to improve forecast accuracy. Additionally, for users of forecast products, it's important to enhance their understanding of the current level of weather forecasting. As forecasters commonly say, we strive for 100%, but weather forecasts cannot achieve 100% accuracy. Recognizing this helps users better judge various meteorological information. A forecast that isn't entirely accurate isn't necessarily useless; it can still be valuable if users consider potential deviations and take appropriate measures in response to weather changes.
Source: China Science News (2024-02-23 Front Page)
