Taipei--In pounding Taiwan over the past few days, Typhoon Nesat and Tropical Storm Haitang both pulled surprises, following quirky tracks that defied forecasts and left some parts of the country unprepared for their arrival.
Nesat was supposed to make landfall in eastern Hualien County or Taitung County and cut across southern Taiwan. Haitang was first predicted to run up Taiwan's western coast, then later expected to head up the island's eastern coast.
That the storms did none of those things showed the limitations of modern weather forecasts, which suffer especially from the lack of access to weather conditions at sea, forecasters and experts said Monday.
Taiwan's Central Weather Bureau (CWB) did a satisfactory job in giving the most precise and up-to-date forecasts possible for the two storms, based on objective international standards, said Wu Sheng-yu (???), a weather analyst with WeatherRisk Explore Inc.
But there remains crucial information from the sea that is mostly out of reach to forecasters worldwide, Wu said.
That gap can lead to mistakes.
"Forecasters nowadays depend heavily on the global numerical weather prediction system to give them a blueprint for short-term weather development, but the data are insufficient because not enough telemetry instruments are released at sea," Wu said.
The challenge isn't just faced by the Central Weather Bureau. Even the best-known weather models that Taiwan often relies on --the National Centers for Environmental Prediction Global Forecasting System and the European Centre for Medium-Range Weather Forecasts -- are vulnerable to these blind spots, he said.
The problem is difficult to solve, as front-line information contributed to the systems by weather stations across the world is mostly gathered on land.
The nearest weather stations that provide data for Taiwan are located in Banqiao in northern Taiwan, Hualien in eastern Taiwan, and the Japanese island of Ishigaki about 230 kilometers east of Yilan County in the Pacific Ocean, according to Wu.
That's not ideal when storms are approaching Taiwan from the Philippine Sea or the South China Sea.
"We miss out on both key information about a storm in its early stages and any real-time weather developments at sea that could hugely affect a storm's impact on Taiwan," he said.
The one solution used at present for obtaining data at sea is through the CWB's "Wind Chasing" missions.
During those missions, meteorologists and former Air Force pilots fly above and even into the very heart of the storms to drop thermos-shaped devices, called "dropwindsondes," to detect data such as humidity, wind direction and velocity.
The frequency of those missions is limited, however, by budget constraints. The last time the CWB launched such a mission was back in September 2016 for Typhoon Megi; none were conducted to learn more about Nesat or Haitung, creating even greater uncertainty.
In such cases, the information gap is typically bridged by forecasters, who hold meetings to compare different systems and introduce other variables by checking how storms sharing similar weather patterns have developed in the past, said CWB forecaster Chen Chien-an (???).
That explains why different countries with their own accumulated experience and history come up with different forecasts for the same weather pattern, and why it is at this stage when "errors" can be made, he said.
In the case of Nesat, the CWB, along with most weather bureaus, forecast on July 26 that it would not land on Taiwan.
That forecast was abruptly changed a day later when Nesat was expected to make landfall between Hualien and Taitung. But the typhoon eventually hit Taiwan in northeastern Yilan County on July 29, at odds with the predictions of weather bureaus around the world.
For Yilan green onion farmers, that offers little consolation. Having seen the forecasts that the worst of Typhoon Nesat would be in southern Taiwan, the farmers didn't bother to harvest their crops prior to the storm's arrival, and suffered heavy losses as a result.
The forecast had been based on expectations that a Pacific High pressure system would keep the storm further south, but when the system turned weaker than the CWB had predicted based on the global models, Nesat pushed further north, Chen explained.
Adding to the challenge was the complicated interaction between the typhoon's periphery and the island's terrain before the eye of the storm made landfall, Chen said.
"It's difficult to make predictions, especially in the last minute before a storm's landing, even though we have weather monitors that can feed us local images ranging as far as a few hundred meters with perfect clarity," he said.
Similar uncertainty was seen with Haitang, which forecasters had a very difficult time reading as it headed northward from the South China Sea in Taiwan's general direction.
Then once it landed in Pingtung County at Taiwan's southern tip, it unexpectedly cut through southwestern Taiwan by surprise rather than moving north along the east coast as had been forecast shortly before the storm made landfall.
"A weak storm like Haitang can be easily affected by terrain, and its interaction with nearby Nesat was another variable we had to consider," Chen said.
Still, despite the different challenges facing modern weather forecasts, both Wu and Chen said the CWB's forecasting ability was up to international standards.
Any inaccuracy of less than 100 kilometers within a 24-hour time frame is acceptable by international standards, Chen said.
But with Taiwan only about 450 kilometers north to south and 150 kilometers east to west, even predictions that are off within the acceptable standard can still have a major impact, Chen said, as Taiwan learned over the weekend.
Source: Focus Taiwan News Channel