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Typhoon Ondoy's Heavy Rainfall Explained

A lot of theories and speculations in the unusual heavy rainfall brought about by typhoon Ondoy, that cause massive flooding in Metro Manila and nearby areas. Here is the analysis of the Tropical Rain Measuring Mission (TRMM), a joint NASA and JAXA mission to monitor and study tropical rainfall.

Tropical Storm Ketsana Brings Record Flooding to Northern Philippines

The latest tropical system to the hit the Philippines, Tropical Storm Ketsana (known locally as "Tropical Storm Ondoy"), has resulted in record flooding in the capital of Manila on the island of Luzon in the northern Philippines. So far 140 people have died in the resultant flooding and mudslides with at least 32 more still missing. Ketsana began as a tropical depression in the west-central Philippine Sea on the morning of the 25th of September 2009 about 500 miles (~800 km) east-southeast of the central Luzon coast.

The system was upgraded to a minimal tropical storm on the evening of the 25th (morning of the 26th local time) as it tracked westward towards Luzon. Ketsana maintained minimal tropical storm intensity as it crossed central Luzon on the afternoon of the 26th (local time). The main deluge in the Manila area, located on the western side of Luzon, began around 8:00 am local time (00 UTC 26 September) even though the center of Ketsana had yet to make landfall on the eastern side of the island.

A record 13.43 inches of rain fell in Manila in the 6 hours between 8:00 am and 2:00 pm local time, which is equivalent to about a month's worth of rain for the area. The reason for the enhanced rainfall over on the Manila-side of the island as the storm approached was the interaction between Ketsana's low-level circulation and the seasonal southwest monsoon. The southwest monsoon comes about from the summertime heating of the Asian landmass. As warm air rises over the continent it induces low pressure as the surface, which draws air in from surrounding regions. The southwest monsoon typically runs from June to September in the Philippines and draws warm, humid air up from the southwest across the South China Sea and into the islands where it can interact with the topography. Ketsana's counterclockwise circulation enhanced the effect, which resulted in the torrential rains.

Typhoon Ondoy ll 21-28 Sep 09

Armed with both a passive microwave sensor and a space-borne precipitation radar, the primary objective of the Tropical Rainfall Measuring Mission satellite (better known as TRMM) is to measure rainfall from space. For increased coverage, TRMM can be used to calibrate rainfall estimates from other additional satellites. The TRMM-based, near-real time Multi-satellite Precipitation Analysis (TMPA) at the NASA Goddard Space Flight Center is used to monitor rainfall over the global Tropics. TMPA rainfall totals are shown here for the 7-day period 21 to 28 September 2009 for the northern Philippines and the surrounding region. The highest rainfall totals occur in an east-west band over central Luzon, including Manila, south of storm's track (indicated by the solid black line). Amounts in this region are on the order of 375 mm (~15 inches, shown in dark yellow) to over 475 mm (~19 inches, shown in orange). The highest recorded amount near Manila was 585.5 mm (almost 24 inches).

After passing through the Philippines, Ketsana intensified into a minimal typhoon over the central South China Sea and is expected to make landfall along the central coast of Vietnam.

TRMM is a joint mission between NASA and the Japanese space agency JAXA.
Images produced by Hal Pierce (SSAI/NASA GSFC) and captions by Steve Lang (SSAI/NASA GSFC)

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