Category 3 Chan-hom: One of Shanghai's Strongest Typhoons on Record?

Category 3 Typhoon Chan-hom is headed northwest at 14 mph towards China, and appears poised to make landfall as one of the strongest typhoons on record for a portion of the country unused to strong typhoons. Of particular concern is Chan-hom's storm surge, which has the potential to bring the highest water levels ever observed into Shanghai, China's most populous city, with 23 million people in the metro area. In their 5:45 am EDT Friday advisory, the Japan Meteorological Agency (JMA) put Chan-hom's central pressure at 935 mb; in their 5 am EDT Friday advisory, the Joint Typhoon Warning Center (JTWC) rated Chan-hom's top winds at 125 mph. A storm of this magnitude is sure to pile up a large storm surge, particularly since Chan-hom is a very large typhoon with tropical-storm force winds that extended outwards up to 310 miles from the center. This storm surge will pile up throughout the Yellow Sea, from China to the Korean Peninsula. Since the Yellow Sea is shallow and enclosed on three sides, the potential exists for some of the highest water levels ever recorded along portions of the coast south of Shanghai, to the right of where the center makes landfall. Chan-hom is likely to weaken significantly as the storm approaches landfall, due to cooler waters, higher wind shear, and interaction with land. JTWC and JMA were forecasting on Friday morning (U.S. EDT) that Chan-hom would make landfall between 06 - 09 UTC Saturday (2 am - 4 am EDT, or 3 - 5 pm JST.) In their 5:45 am EDT Friday forecast, JMA predicted that Chan-hom would have a 950 mb pressure and sustained 10-minute average winds of 90 mph at landfall. In their 5 am EDT Friday forecast, JTWC predicted that Chan-hom would be a Category 1 storm with 90 mph winds (1-minute average) at landfall. Even at this lowered intensity, Chan-hom would still be the strongest landfalling storm to hit within 200 miles of Shanghai in at least 35 years. On Friday morning, the typhoon was undergoing an eyewall replacement cycle, where the inner eyewall collapses and is replaced by a larger-diameter outer eyewall. While this process weakens the peak winds near the center, it spreads the typhoon-strength winds over a larger area, increasing the size of the storm surge.


Figure 1. Typhoon Chan-hom as seen by radar on Okinawa at 7:45 pm EDT Thursday (08:45 JST Friday, July 10), 2015. At the time, Chan-hom was a Category 4 storm with 130 mph winds. Image credit: JMA.

Strong typhoons hitting near Shanghai: a rare occurrence
China gets hit by about nine tropical cyclones (tropical depression, tropical storms, or typhoons) each year (Chen, 2000), but these strikes occur primarily in the southern portion of the country. The Jiangsu Province where Shanghai lies received only seven landfalls in the 50-year period 1947 - 1999, so the region does not have a lot of typhoon experience. Since 1979, no typhoon with winds in excess of about 85 mph (75 mph winds using a 10-minute averaging time) has made landfall within about 200 miles of Shanghai (Figure 2.) Historically, the strongest typhoon to affect the city in the past century may be Typhoon Gloria of July 24 - 25, 1949, whose storm surge overwhelmed the city's flood walls and left much of Shanghai a flooded ruin, with over 250,000 people homeless (See David Longshore's Encyclopedia of Hurricanes, Typhoons, and Cyclones). Note that Typhoon Wanda of 1956 was at Category 3 strength when it hit the coast of China near where Chan-hom is predicted to strike. Wanda killed 2000 people in China. China has had four typhoons that have killed at least 37,000 people each--most recently in 1975, when torrential rains from what had been Super Typhoon Nina caused the Banqiao Dam to fail, killing 90,000 - 230,000 people.


Figure 2. Tracks of all typhoons with at least 75 mph winds (10-minute average winds as rated by the Japan Meteorological Agency) to pass within a 230-mile diameter circle (light shaded region) near Shanghai, China. Typhoon Winnie is labeled in white. Ten-minute average winds of 75 mph are roughly equivalent to 85 mph winds for the one-minute averaging time winds used for the U.S. Saffir-Simpson scale. All of the storms in this plot had sustained 10-minute average winds of 75 mph or less when they made landfall. Image credit: NOAA.

A historical analogue: Typhoon Winnie of 1997
The largest storm surge observed at the coast in Shanghai since 1921 was 5.9 feet (1.81 meters) during Typhoon Emma of 1956. However, Emma's maximum surge did not occur at high tide (the difference between low tide and high tide in Shanghai is about 7.2 feet or 2.2 meters, so it makes a big difference when the maximum storm surge arrives, relative to high tide.) The highest storm tide (water level) in Shanghai came during Typhoon Winnie of August 1997. Although Winnie was only a Category 1 storm with 85 mph winds when it made landfall, and the storm struck relatively far from Shanghai, about 180 miles to the south, the storm surge from Winnie was only 5.5" (14 cm) below the top of the 19.2-foot (5.86 meter) Suzhou Creek floodgate that protects downtown Shanghai on the Huangpu River, which flows through the center of town. This floodwall was rated to protect against a 1-in-200 year flood, and was overtopped by about one foot (30 cm) along a 8.5 mile (13.7 km) section inland from the downtown area, flooding over 400 homes (source: Flood probability analysis of the Huangpu barrier in Shanghai, M.S. thesis by Qian Ke.) According to EM-DAT, Winnie killed 240 people and did $2.7 billion (1997 dollars) in damage to China. The floodwall protecting downtown Shanghai has been raised by 3.4 feet (1.05 meters) since then, giving the city protection against a 1-in-1000 year flood. Thus, it will take a much stronger storm than Winnie to flood the city. Chan-hom is predicted to be a much stronger storm at landfall than Winnie was, and is forecast to hit closer to Shanghai. The newly raised floodwalls of Shanghai may see their highest water levels in history when Chan-hom makes landfall, depending upon whether or not the peak storm surge occurs near high tide.

Low tide in Shanghai is at 07:07 UTC Saturday, about the time that Chan-hom is forecast to make landfall. That is potentially good news for the coastal region near the landfall point. However, high tide is at 12:48 UTC Saturday, at a time when the center of Chan-hom is predicted to be over land but just south of the city, so the counter-clockwise circulation around the center will be pushing water into the city. Fortunately, this high tide is not a very high one--high tides late next week will be more than two feet higher than this. Though Chan-hom will be weakening as it approaches Shanghai during Saturday's high tide, JTWC is predicting the storm will still be at Category 1 strength. If Chan-hom follows the JTWC track and intensity forecast, it will be capable of pushing a record-size storm surge into the city during this 12:48 UTC Saturday high tide, potentially challenging the 1-in-200 year water levels observed during Typhoon Winnie of 1997. I've read several studies explaining how storm surge propagation in the Yellow Sea is extremely complicated, so I am unsure just how the great the risk is from this storm without seeing data from a sophisticated real-time storm surge model, though.


Figure 3. Typhoon Winnie as it passed just south of Okinawa on August 17, 1997, at 11:36 UTC. Note that the small inner eyewall of the typhoon had become completely surrounded by a concentric 230-mile diameter eyewall. Image credit: Navy Research Lab, Monterey.

Why was Typhoon Winnie's storm surge so high?
If Typhoon Winnie was only a Category 1 storm, and its center crossed the coast relatively far away from Shanghai (180 miles), why did it bring such a large storm surge to the city? Well, Winnie was a freak. According to the Joint Typhoon Warning Center (JTWC) 1997 Annual Tropical Cyclone Report, as Winnie moved toward Okinawa on August 16, 1997, a large outer rain band began to encircle the inner eyewall. By the time the typhoon passed over Okinawa, the rain band had become a complete, 230 mile (370 km) diameter concentric outer eyewall, possibly the largest ever observed (tied with 1960's Typhoon Carmen, which also had a 230-mile diameter outer eyewall.) Whenever an intense tropical cyclone forms concentric eyewalls, the peak winds of the inner eyewall fall significantly, but the hurricane-force winds of the storm spread out over a wider area that encompasses the outer eyewall, increasing the size of the storm surge. Winnie's strong winds were able to pile up a massive mound of water into the relatively shallow waters of the Yellow Sea off the coast of Shanghai. Since the Yellow Sea is enclosed on three sides, with the Korean Peninsula blocking the flow of water to the northeast, this extra water had nowhere to go except up onto land when the center of Winnie pushed inland.


Figure 4. The 22.6-foot (6.9 meter) Suzhou Creek floodgate that protects downtown Shanghai from storm surges coming up the Huangpu River, which flows through the center of town. This floodwall is rated to protect against a 1-in-1000 year flood. Image credit: Dorothy Tang.

Sea level rise and Shanghai
Sea levels have been rising globally by about 3.3 mm per year over the past few decades. According to a 2015 study, Sea level change and city safety—The Shanghai as an example, sea levels have been rising a bit slower in Shanghai--about 2 mm/year, but the land has been sinking at more than double that rate, due to compaction of soil and groundwater pumping to support intensive urban development. As a result, the relative rise of sea level in the city has been about 7 mm/year, which is a huge concern for a city whose average ground level is already below the average high tide level. The authors predicted that over the next twenty years, Shanghai will see the relative sea level rise by 10 - 16 cm (3.9 - 6.3 inches), which will make storm surges from typhoons like Chan-hom more dangerous. With sea level rise likely to accelerate due to increased melting of the Greenland and Antarctic ice sheets, Shanghai will be increasingly hard-pressed to keep the ocean at bay this century using ever-higher flood walls. A new approach, called the Yangtze River Delta Project (YDRP), offers a more innovative way to manage Shanghai's increasing flood risk due to the steadily rising seas. The YRDP research group develops "soft" infrastructural strategies (as opposed to "hard" floodwalls) to respond to sea level rise and storm surge. For example, the team is studying the flood control techniques devised by Yu the Great (2200-2100 BCE, founder of China's Xia Dynasty), who created a system of irrigation canals that channeled river floodwaters into agricultural fields, building low earthen dikes to guide the water’s flow. Other studies undertaken by the research group include projects for the transformation of New York and New Jersey’s Upper Harbor and a land-building sediment diversion proposal for the Mississippi River Delta.

The new Japanese Himawari satellite has some spectacular imagery of Chan-hom (Sector 4 in Band 3=visible, and Sector 6 in Band 13=IR.)

Jeff Masters