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Typhoon over Manila

On Wednesday 16 July 2014, typhoon Glenda / Rammasun hit the northern Philippines and proceed to pass almost straight over the capital, Manila.

Typhoons are not rare there: the Philippines get more hurricanes than any other country in the world. This typhoon hit the shore with category 3 winds (178-208 km/hr), but weakened over land to category 1, still with 119-153 km/hr wind over Manila. Fortunately the course was just South of Manila, so that the main winds were off-shore and most rain fell on the back of the mountains. The damage was therefore much less severe in Manila than it would have been with a course further to the north, which was the most likely one in the weather forecasts.

Track of typhoon Glenda / Rammasun on July 16. Source: Tropical Storm Risk
Track of typhoon Glenda / Rammasun on July 16. Source: Tropical Storm Risk

Climatology

Tropical cyclones (hurricanes) are called typhoons in this part of the world. They also often have two names. There is one list used by most countries, in this sytem this typhoon is called Rammasun. However, the Pilipino weather service uses its own list, in which it is Glenda. The north-western Pacific has the most numerous and most powerful hurricanes in the world, caused by the warm sea water and little wind shear. These typhoons first move to the west, then curve to the north and later north-east. Many of them pass over the central and especially northern Philippines.

The probability that a tropical cyclone (hurricane, typhoon) passes within 160 km. Source: KNMI
The probability that a tropical cyclone (hurricane, typhoon) passes within 160 km. Source: KNMI

Most damage of hurricanes is due to water: the storm surge of seawater piled up by the wind and the copious rainfall that accompanies these storms. As the wind turns counter-clockwise around the centre (on the northern hemisphere), the storm surges are largest on the west coast and north of the track. On the east coast smaller surges occur to the south of the centre of the storm.
The highest daily precipitation in Manila was caused by a relatively weak tropical storm Ondoy (Ketsana) in 2009. This passed to the north of Manila and the resulting onshore winds dumped up to 450mm of rain on the city, causing widespread flooding

Meteorology

The course of Glenda looks more like Milenyo (Xangsane, 2006), although the first reports indicate that the damage is not so severe as durng that storm. The track of Glenda was predicted well a few days in advance by weather centres such as the ECMWF. Early warnings made a big difference in the storm's impact, as citizens, government, and civil society took early action to prepare for landfall. 530,000 people were evacuated prior to the storm, and emergency response stocks were prepositioned to allow for immediate response.

Over land the typhoon followed a slightly more southerly course than the most likely predicted one, although within the uncertainties of the forecast. This made a big difference to Manila. The strongest winds were now off-shore and most rain fell on the back of the mountains, so that there was little flooding in the city. To the south the impact was more severe. Overall, the good preparations have paid off: officials expressed relief that Rammasun had come and gone with fewer casualties and less damage than feared.

Weather forecast of 14 juli 2014 for the track of typhoon Glenda / Rammasun from the European Centre for Medium Range Weather Forecasts (ECMWF).
Weather forecast of 14 juli 2014 for the track of typhoon Glenda / Rammasun from the European Centre for Medium Range Weather Forecasts (ECMWF).

Climate Change

According to the recent climate report of working group 1 of the IPCC (2013), a significant change in the number of tropical stroms or hurricanes cannot be detected anywhere on earth. There are changes in these numbers, but these are also caused by natural fluctuations of sea surface temperatures and wind patterns, and by air pollution by aerosols. Before the satellite era weak and short-lives storms were often not detected. The natural decadal variability mean that trends can only be shown in long time series, which are not reliable enough.
Climate models project a possible decrease of the number of storms over the rest of the century, combined with an increase in the strongest storms. However, in these models the trend up to now is much smaller than the natural variability, so that it is not yet possible to attribute a larger number of heavy storms to anthropogenic influences. (Sections 2.6.3, 10.6.1.5, 14.6.1)

Projected chenges over the next 80 year in the number of tropical storms storms (I), the number of Category 4 and 5 storms (II), the maximum intensity (II) and the precipitation intensity during the maximum (IV). Source: IPCC AR5, www.climechange2013.org,
Projected chenges over the next 80 year in the number of tropical storms storms (I), the number of Category 4 and 5 storms (II), the maximum intensity (II) and the precipitation intensity during the maximum (IV). Source: IPCC AR5, www.climechange2013.org,
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