Volcanoes magnify Metro Manila's southwest monsoon rains and lethal floods

Abstract

Many volcanoes worldwide are located near populated cities that experience monsoon seasons, characterized by shifting winds each year. Because of the severity of flood impact to large populations, it is worthy of investigation in the Philippines and elsewhere to better understand the phenomenon for possible hazard mitigating solutions, if any. During the monsoon season, the change in flow direction of winds brings moist warm air to cross the mountains and volcanoes in western Philippines and cause lift into the atmosphere, which normally leads to heavy rains and floods. Heavy southwest monsoon rains from 18-21 August 2013 flooded Metro Manila (population of 12 million) and its suburbs paralyzing the nation's capital for an entire week. Called the 2013 Habagat event, it was a repeat of the 2012 Habagat or extreme southwest monsoon weather from 6-9 August, which delivered record rains in the mega city. In both the 2012 and 2013 Habagat events, cyclones, the usual suspects for the delivery of heavy rains, were passing northeast of the Philippine archipelago, respectively, and enhanced the southwest monsoon. Analysis of Doppler data, rainfall measurements, and Weather Research and Forecasting (WRF) model simulations show that two large stratovolcanoes, Natib and Mariveles, across from Manila Bay and approximately 70 km west of Metro Manila, played a substantial role in delivering extreme rains and consequent floods to Metro Manila. The study highlights how volcanoes, with their shape and height create an orographic effect and dispersive tail of rain clouds which constitutes a significant flood hazard to large communities like Metro Manila.

Enhancement of Summer Monsoon Rainfall by Tropical Cyclones in Northwestern Philippines

“…In addition, the compounding effects of mesoscale orographic features (e.g., mountain range, volcanoes, etc.) that augment precipitation amounts (Lagmay et al 2015) and modulate the local spatial and temporal distribution of heavy rainfall are difficult to quantify.…”

“…During the "2013 Habagat event", Tropical Storm Trami (locally named "Maring") was similarly located to the east of Taiwan, strengthening the southwest monsoon flow which also led to widespread flooding, damaged properties, and loss of life in the Manila metropolis. Lagmay et al (2015) discussed the influence of two volcanoes to the west of Manila on the spatial and temporal distribution of heavy rainfall in both the 2012 and 2013 Habagat events.…”

118‐year climate and extreme weather events of Metropolitan Manila in the Philippines

“…The equatorial waves induce low‐level westerlies in SCS that overlap and strengthen the prevailing monsoon westerlies. In addition, two volcanoes ~70 km to the west of Metro Manila (Mount Natib and Mount Mariveles) are known to influence the intensity and spatial distribution of TC‐enhanced summer monsoon rainfall in the metro (Lagmay et al ., ). The confluence of synoptic and mesoscale factors make Metro Manila constantly susceptible to heavy precipitation from TCs.…”

“…Extreme weather events in the recent years, particularly heavy precipitation from tropical cyclones (TCs), have resulted to thousands of lives lost and damages to properties in the city (Abon et al ., 2011; Lagmay et al ., ). Most heavy precipitation events along western Luzon, including Metro Manila, are due to TCs enhancing the monsoon flow in Southeast Asia (Bagtasa, ).…”

Using ground radar overlaps to verify the retrieval of calibration bias estimates from spaceborne platforms

“…inhabitants. This region of overlap regularly experiences torrential rains from monsoon and typhoons extending for several days (Heistermann et al, 2013a;Lagmay et al, 2015).…”