Typhoon Bolaven Strikes Okinawa; Heads for South Korea: AIR Analysis

August 28, 2012

According to catastrophe modeling firm AIR Worldwide, Typhoon Bolaven made landfall on northern Okinawa around 12:00 UTC on August 26, as a strong category 3 typhoon, as calculated by the Japan Meteorological Agency (JMA).

It also reported minimum central pressure at landfall of 920 mb and 10-minute sustained wind speeds of 175 km/hr [110 mph]. However, the lowest observed sea level pressure reading was 935.1 mb, at Nago, located on the west central part of the island directly within the eye.

According to AIR, Bolaven’s impact on Okinawa, Japan, was “less severe than expected and buildings withstood the storm well; most insured residential structures on Okinawa are built of concrete. In fact, government authorities have reported that no major damage was inflicted by Bolaven on Okinawa as of Monday morning. Damaged buildings have been reported on the nearby Amami Islands, but only limited damage information is available at this time. High winds resulted in power outages to 17,500 households and felled trees. As of Monday morning, 9,800 households in Okinawa are still without power.”

Jason Butke, senior scientist at AIR Worldwide, explained: “Based on the radar at landfall, Bolaven was likely undergoing an eye wall replacement cycle at the time of landfall. Dry air was entrained into the core as intense convection was restricted to the small, 20 km [12.5 miles] diameter eye wall and to isolated feeder bands encircling the storm. This would explain why weaker-than-expected winds were observed on Okinawa; that is, the high wind speeds aloft were not being efficiently transferred to the surface.”

AIR said the “strongest 10-minute sustained wind speeds actually recorded on Okinawa were 25.1 m/s, or 105 km/hr (1-minute sustained), at Naha. The strongest wind gust was 38.5 m/s, or 140 km/hr, also at Naha. 100 km [60 miles] to the north of landfall at Okinoerabu, a 10-minute sustained wind speed of 29.4 m/s, or 120 km/hr (1-minute sustained) and a wind gust of 42.7 m/s, or 155 km/hr was observed.”

The typhoon is currently crossing the East China Sea with a minimum central pressure estimated at 960 mb and 10-minute sustained wind speeds estimated at 140 km/hr [87.5 mph], according to the JMA. Bolaven is forecast to continue to weaken in an increasingly hostile environment of cooler sea surface temperatures and higher wind shear.

Butke added: “Bolaven is forecast to continue moving north, towards the Korean Peninsula, and make landfall tomorrow over southwestern North Korea. The forecast landfall intensity is 975 mb and 10-minute sustained wind speeds of 110 km/hr [68.75 mph], or very weak category 1 typhoon intensity. If this forecast holds, Bolaven would be one of the strongest tropical cyclones to affect this northerly region of South Korea in the last 60 years. After making landfall in North Korea, Bolaven is forecast to recurve to the northeast and transition extra tropically into a mid-latitude cyclone.”

Although storm surge was expected to be a major problem for Okinawa—more than 400,000 people live at elevations less than 50 meters—this threat did not materialize.

According to AIR, Bolaven “may cause more extensive damage when it makes landfall on the northern Korean Peninsula. Minor damage to non-engineered wood frame buildings is expected, particularly to roof envelopes. Engineered structures could suffer minimal damage to windows and cladding due to wind-borne debris.”

Source: AIR Worldwide

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