The rains on 7–9 August were very strong monsoon rains that lasted for three days. The intensity was increased by two tropical storms passing to the North of the Philippines. Comparing with historical data over 1947–2010, the amount of rain on a single day was the third largest recorded, implying a return time of around 20 year. However, as the rains lasted three days, the total amount has a return time of more than 600 years. In general, greenhouse warming will produce more intense short-duration rain events, but the contribution of global warming in intensifying this specific event is still an open question.
There is an international network for the exchange of weather information used for weather forecasts called GTS. The observations at the station “Science Garden” in Manila for 7, 8 and 9 August were 323.4, 363.7 and 292.6 mm/day respectively. However, data on GTS is notoriously unreliable. One of the most common errors is decimal point displacement, so that a rainfall event of 800.1 mm (Siliguri, India, 14 July 1951) was in fact only 80.01 mm. To check this we verified that the first two values were also mentioned in a newspaper interview with the Philippine Meteorological Service, PAGASA. Secondly, the values are consistent with 3-hourly values also recorded on GTS. Thirdly, the total amount is similar to the amount observed with the TRMM satellite instrument, see Figure 1. TRMM is most accurate over the ocean and shows amounts over 900 mm on the ocean in front of Manila. To put these amounts in perspective, the 3-day sum is more than the mean annual rainfall in the Netherlands.
On 6 August 323.4 mm of precipitation had already poured down over the Metro Manila. This huge amount is more than half the average rainfall of 504 mm for the entire month of August. This torrential rain resulted from an unusual atmospheric situation of the southwest monsoon (called 'habagat' locally), which was amplified by the tropical storm (TS) Haiku located 200 km away in the North of Philippine (see Figure 2).
The monsoon wind, which is part of the Asian summer monsoon circulation, converged over the South China Sea and was lifted up, resulting in heavy thunderstorms that moved toward Manila. In addition, the geographical situation of Manila and surrounding region intensified the event. The monsoon winds have to pass over the Sierra Madre mountains which makes the rainfall more intense on the western side where Manila is located rather than in the east over the West Pacific ocean, as seen in the satellite image (Fig. 2) or in TRMM data (Fig. 1).
The SACA&D database of daily climate data from South-East Asia contains many series from the Philippine, among them data for the same Science Garden station. The `blended series’ combines data from different series and covers the period from 1946–2010, with a few gaps. In general these data show a wet season peaking in August, with days alternating between dry and very wet, almost every year includes days with more than 100 mm of rainfall.
The wettest day on record was caused by Typhoon Ondoy on 27 September 2009, when the Science Garden station recorded 454.9 mm of rainfall. A useful way to present the data is to plot the wettest day of the year against the position of the year in the record list. The 454.9 of 2009 occurred once in 64 years. An event as large or larger than the second one (2007, 391.9 mm, not associated with a typhoon) occurred twice in 64 years, or once every 32 years. The third wettest day was 2012 (which suggests that there has been an upward trend; statistically the trend is significant at p<0.1). The return time of 2012 in the observations up to 2010 is therefore around 20 years, not very extreme. This is shown by the blue line in the left panel of Fig. 4. The green lines give a theoretical fit assuming a constant climate and that all extremes are of the same type.