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The Change in Rainfall from Tropical Cyclones Due to Orographic Effect of the
Sierra Madre Mountain Range in Luzon, Philippines

Bernard Alan B. Racoma1,2*, Carlos Primo C. David1,
Irene A. Crisologo1, and Gerry Bagtasa3

1National Institute of Geological Sciences, College of Science,
University of the Philippines, Diliman, Quezon City, Philippines
2Nationwide Operational Assessment of Hazards,
University of the Philippines, Diliman, Quezon City, Philippines
3Institute of Environmental Science and Meteorology, College of Science,
University of the Philippines, Diliman, Quezon City, Philippines

ABSTRACT
This paper discusses the Sierra Madre Mountain Range of the Philippines and its associated influence on the intensity and distribution of rainfall during  tropical cyclones. Based on Weather and Research Forecasting model simulations, a shift in rainfall was observed in different portions of the country, due to the reduction of the topography of the mountain. Besides increasing the rainfall along the mountain range, a shift in precipitation was observed during Tropical Storm Ondoy, Typhoon Labuyo, and Tropical Storm Mario. It was also observed that the presence of the Sierra Madre Mountain Range slows down the movement of a tropical cyclones, and as such allowing more time for precipitation to form over the country. Wind profiles also suggest that the windward and leeward sides  of mountain ranges during Tropical Cyclones changes depending on the storm path. It has been suggested that in predicting the distribution of rainfall, the direction of movement of a tropical cyclones as well as its adjacent areas be taken into great consideration. While the study shows high amounts of variation in the characteristics of different tropical cyclones with respect of the Sierra Madre Mountain Range, the results of this study can provide insights to pre-disaster operations before tropical cyclones approaches land. The decrease in tropical cyclones speed introduced by the Sierra Madre Mountain Range can be used to identify the possible areas that can experience prolonged rains due to the mountain range. Disaster management authorities can also prepare in advance by identifying which locations can experience orographic enhanced precipitation. However, due to the lack of available data and resources, further studies are recommended due to the study presenting limited cases.

Key words: geomorphology, numerical weather prediction, orographic effect, precipitation, tropical
                 cyclones, weather and research forecasting modelling

INTRODUCTION
The Philippines is in constant exposure to tropical cyclones (TCs) that originate from the West Pacific Ocean (WPO). Out of all storms that form in the WPO,  an  average of 30.3% of these TCs make landfall in the Philippines (David et al. 2013). Correspondingly, every year as much as 19.4 TCs enter the Philippine Area of Responsibility, with 9 of these TCs actually making landfall (Cinco et al. 2016). Packing both strong winds and bringing extreme rainfall, most of these cause direct and indirect effects to the country, resulting to loss of lives and large damage to property. While the Intergovernmental Panel on Climate Change (IPCC) projects a decrease in the number of TCs over the West Philippine Sea (a part of the WPO) in the future, it conversely projects an increase in activity near subtropical Asia (Kirtman et al. 2013; Wang et al. 2011). Similarly, a significant increase in number of Category 4-5 TCs is expected between the years 2000 to 2050 in the neighboring southwest Pacific Ocean Basin (Leslie et al. 2007).

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