The volume of rain far beyond the leeward side of a volcano can be amplified by the presence of it as reported by A.M.F. Lagmay et al in Frontiers in Earth Science .
Most of us are familiar with volcanoes posing as a threat to our lives and our livelihood. Lahar flows, ash falls, frequent earthquakes are just examples of the hazards living near a volcano. One such hazard that is now just observed is that volcanoes can also enhance rains and cloud trails with intense rainfall. These rain-laden clouds can travel far beyond the volcanoes’ slopes. A. M. F. Lagmay et al report these in Frontiers in Earth Science.
The team composed of geologists and atmospheric scientists from the University of the Philippines Diliman have detected that there were cloud trails of intense rain after passing volcanoes based on their Doppler data. Moreover, their simulations suggest that these volcanoes were the ones responsible for the accumulation of these rain clouds.
The figure above shows the Doppler data obtained during August 6-10, 2012 and August 18-22, 2013. The authors said that “plume-like patterns of high accumulated rainfall estimates are observed to emanate from the regions starting from near the summit of the Natib and Mariveles volcanoes towards its leeward side for both cases.” These patterns indicate heavy accumulated rainfall at these areas during these time intervals.
The authors then simulated two scenarios. In the first scenario, they used the topology with the volcanoes while in the second, the volcanoes were removed.
“Simulations of the accumulated water mixing ratio using the WRF corroborate the observations from Doppler radar showing that the two volcanoes are major sources of rain clouds during the Habagat 2012 and 2013 events.” This is shown in the figure below.
Without the volcanoes, there were no trail of clouds with intense rainfall (see c and d of figure below).
The authors wrote, “It is unclear as to why the two volcanoes in the southern part of the mountain range generated more rain-laden clouds that move out to Metro Manila.”
They suggest however, that the presence of these thousand-meter high volcanoes causes the moisture to lift from low elevation to a higher elevation (known as orographic lift) [wiki]. Once at higher elevation, the moist air are then pulled by the presence of a typhoon causing a trail of rain-laden clouds. The monsoon data were obtained just when there was a typhoon northeast of Luzon.
This study highlights two things: 1) that “orography and associated dispersion of cloud trails” should be considered a volcano-associated hazard. There are a lot of places in and outside the Philippines where monsoon rains that are potentially enhanced by volcanoes can generate massive floods and cause destruction. These places should be studied according to the authors.; and 2) that we can learn more about hazards, therefore know how to avoid disasters, by employing new scientific tools and equipment.
 A.M.F Lagmay, G. Bagtasa, I.A. Crisologo, B.A. B. Racoma and C.P.C. David (2015). Volcanoes magnify Metro Manila’s southwest monsoon rains and lethal floods, Frontiers in Earth Science, 2 DOI: http://dx.doi.org/10.3389/feart.2014.00036