How to understand a typhoon warning？

Expert: Gao Shuanzhu, chief forecaster of the Central Meteorological Observatory
Interviewer: Zhang Yibo, intern reporter of China Meteorological News.

　　Every typhoon season, the circle of friends will always be screened by a typhoon warning, and the moving path and real-time changes of the typhoon will affect everyone’s heart. However, do you really understand the typhoon warning of hundreds of words? Let’s take the typhoon "Mikla" warning issued by the Central Meteorological Observatory at 6: 00 on August 11th as an example to learn about the information that may be ignored.
　　How to determine the location and intensity of typhoon?
　　A typhoon warning often starts with live information. "This year’s No.6 typhoon’ Mikla’ has been upgraded from a strong tropical storm to a typhoon level at 6 o’clock this morning (11th). Its center is located on the sea surface of the Taiwan Province Strait about 85km south of Xiamen, Fujian Province. The maximum wind force near the center is 12, and the lowest pressure in the center is 980 hectopascals. The radius of the seven-level wind circle is 100-150km, and the radius of the ten-level wind circle is 20-30km."
　　"Typhoon center" is the lowest value of typhoon pressure, which is generally used to indicate the location of typhoon. The reason to describe "the maximum wind force near the center" is because the place with the maximum wind force is not in the center of the typhoon, but near it. "Seven-level wind circle radius" and "Ten-level wind circle radius" are used to represent the typhoon volume. Generally speaking, the center of the typhoon is very small, theoretically occupying zero space, but the influence range of the typhoon can reach four or five hundred kilometers on average, and even 1000 kilometers. Obviously, "Mikla" belongs to the smaller one in the typhoon family.
　　How do forecasters determine this information? In principle, to determine the minimum typhoon pressure, we have to measure it with numerous barometers, but in the face of typhoons coming from far away from the sea, we obviously don’t have such observation conditions, so forecasters mainly determine the location of typhoons by looking at satellite cloud pictures. If there are no measurement conditions on the ocean, how can forecasters know the typhoon intensity and its changes? In fact, forecasters can be described as "judging the typhoon by its appearance"-determining the intensity of the typhoon according to the appearance of the typhoon on the satellite cloud map. Forecasters have established a set of CI indices according to cloud types, and each index represents a different typhoon intensity. For example, the index is 2.5, which generally means that the typhoon intensity reaches 18 meters per second.
　　Among them, there must be a big error, just as we don’t know someone’s date of birth and have to judge their age according to their appearance, which can provide reference, but most of them will be different. When judging the typhoon intensity by satellite cloud images, the error is less than 5 m/s in most cases, and it is larger in a few cases. Some meteorologists think that the maximum error can reach 30 m/s in some cases. In this case, it is possible to regard the typhoon as a strong tropical storm level or a strong typhoon level.
　　Where does the typhoon go? Where to land?
　　"It is expected that Mikla will move to the north at a speed of about 25 kilometers per hour, with little change in intensity. It will land in the coastal area from Zhangpu to Jinjiang in Fujian this morning (28~33 meters per second, 10~12, strong tropical storm or typhoon level), and the intensity will gradually weaken after landing."
　　Besides the live information, the forecast of typhoon track and intensity is an important part of early warning. If you read it in conjunction with the path probability forecast map for the next 24 hours released by the Central Meteorological Observatory, you will find a colored line in the forecast map, which is the route of the typhoon center. Among them, different colors represent different intensities, such as yellow for tropical depression, blue for tropical storm, green for strong tropical storm, orange for typhoon, pink for strong typhoon and red for super typhoon, with increasing intensity. There will also be a shadow area around this line, indicating that 70% of the typhoon center will appear in this range. Note that the scope of typhoon influence is often much larger than this.
　　Typhoon landing sites have always been a topic of great concern to the public. When the center track of the typhoon crosses the coastline, we think that the typhoon has landed, and the crossing point is the landing point. Although the landing time mentioned in the early warning is "morning" and the landing place is "coastal area from Zhangpu, Fujian to Jinjiang", in fact, when the typhoon does land, we can use a variety of monitoring methods to accurately land the time and place to the town and village level.
　　But why is the statement in the forecast "vague"? There are two reasons. First, because of the observation error and the uncertainty of forecast, a small error will cause a very big gap with the reality. Second, the pursuit of meticulous forecasting of typhoon landing sites is likely to lead people in the surrounding areas to lower their defenses and cause unnecessary losses. Therefore, forecasters choose lots and time periods to express more possibilities. The reason why the landing intensity is described as a range of "28~33 m/s" is also because of uncertainty.
　　However, foreign meteorological departments generally do not specialize in forecasting the landing location and time, but only provide the extent to which strong wind and rain may occur along the coast.
　　How big is the impact of wind and rain?
　　"From 08: 00 on the 11th to 08: 00 on the 12th, there will be 6-7 winds in the South China Sea … Fujian coastal areas and Zhejiang coastal areas, among which there will be 8-10 winds in the south-central coastal areas of Fujian and the southern part of the Taiwan Province Strait. The winds in the nearby sea areas or areas where the’ Mikla’ center passes are 11-12, and the gusts can reach 13-14." Wherever a typhoon goes, it will inevitably bring strong winds, and the intensity of the typhoon is precisely determined by its wind power. However, due to the limitation of ocean observation conditions, wind power is often overestimated or underestimated.
　　Compared with other rainstorms, the accuracy of rainstorm forecast brought by typhoon will be higher, but there are still some problems, such as large deviation of rainstorm area forecast, small or large total rainstorm, empty report or missing report, especially for more than 5 days. If the rainfall forecast map brought by typhoon is compared with the actual map, it will be found that the rainstorm range in the forecast is a smooth curve, and there are many discrete points in the actual scene, which means that some extreme rainstorm points are still unpredictable. This is also a difficult problem that forecasters are facing and need to overcome.

　　(Source: China Meteorological News, September 18, 2020, Fourth Edition Editor: Wang Meili)