Does the Sierra Madre Mountain Range in Luzon Act as a Barrier to Typhoons?
Gerry Bagtasa1* and Bernard Alan B. Racoma1,2
1Institute of Environmental Science and Meteorology, University of the Philippines, Diliman, Quezon City, Philippines
2Department of Meteorology, University of Reading, Reading, United Kingdom
Tropical cyclones (TCs) making landfall in Luzon weaken due to the surface friction of Luzon’s mountainous terrain and the reduced ocean heat, momentum, and moisture fluxes over the landmass. Landmass also influences TC rain distribution by orographic enhancement or blocking of a TC’s moisture-laden circulation. In this study, we investigated the influence of the effects of the Sierra Madre and the Cordillera Mountains Ranges (SMMR and CMR) on TC-associated wind and rainfall hazards to answer the question of whether the SMMR or the CMR mitigates TC hazards. We used the Weather Research and Forecasting (WRF) model with modified SMMR and CMR terrains to disentangle the effects of the orography with flat land. Results show that Luzon-passing TCs maintain their intensities at landfall regardless of the frictional effects of the mountain ranges, but the CMR inhibits the re-intensification of westward-moving TCs emerging from landmass after traversing Luzon. The SMMR reduces wind exposure and basin-wide rainfall of the Cagayan Valley. Hence, the SMMR can be considered a barrier for that region. In addition, the weakening effect of the SMMR reduces the wind exposure of the island of Catanduanes and eastern Bicol the most. However, for the rest of Luzon, the SMMR enhances rainfall, which will likely compensate for the slight decrease in wind exposure – especially considering that most TC-related damages are water/rainfall related.
The CMR, overall, has a larger hazard-mitigating effect than the SMMR. In any case, we believe that shifting the discourse to these mountains’ biodiversity conservation and restoration – rather than their purported TC mitigating effects – will be more strategically constructive.
Keywords: Cordillera mountain, rainfall, Sierra Madre mountain, tropical cyclones, wind
*Corresponding author: [email protected]
INTRODUCTION
In recent years, intense tropical cyclones (TCs) have affected the Philippines with increasing economic costs (Cinco et al. 2016). In fact, the top 50 costliest TCs have all happened since 2009, and seven of the top 10 occurred in Luzon (EMDAT 2023). Typhoon (TY) Mangkhut (Ompong 2018), TY Rammasun (Glenda 2014), and TY Vamco (Ulysses 2020) are some of the TYs in recent memory that brought damages to crops and infrastructure,
severe flooding and landslides in various regions of Luzon, and more than 330 fatalities for those three events alone (NDRRMC 2018, 2021; EMDAT 2023). During TC passages in Luzon, a common theme that emerges in public conversations from both traditional and social media is the role of the Sierra Madre Mountain Range (SMMR) in tempering the hazards associated with Luzon- passing TCs (Mangosing and Andrade 2022). Local folklore has it that Sierra loves and is protective of her two sons, Iloco and Tagalo. Sierra had a suitor, Bugsong Hangin (king of the easterly winds), who often visited ISSN 0031 - 7683
Date Received: 19 Jan 2023
the eastern coastal regions and made his presence felt by Sierra and her two sons. However, Sierra was in love with another warrior, Lusong, who died from Bugsong Hanging’s attacks and had her promise to protect their two sons. With this, Sierra lay on her side facing her two sons (west) to protect them with her back towards Bugsong Hangin (east) (Gonzales 2022). Hence, the belief that the SMMR mitigates (at least some) the effects of extreme weather disturbances such as TCs.
This is not to say that this belief is without a physical basis.
We can consider a TC as a quasi-axisymmetric system whose motion above the boundary layer is in a gradient wind balance (i.e. pressure gradient force = centrifugal force) (Emanuel 1989). Inside the boundary layer where most air-surface interactions occur, however, surface friction is a significant energy sink that can induce an imbalance in the gradient wind (Kepert 2010). To briefly explain how surface friction influences TCs, we follow Holland and Merrill (1984) in their definition of TC intensity and strength – where intensity is the maximum wind speed (MWS) and strength is the average relative angular momentum within the inner TC circulation. In TCs, intensity and strength can evolve independently of each other. Balanced vortex dynamics show a radial inflow of angular momentum is induced by a warm TC central core due to diabatic heating from intense eyewall convection (Shapiro and Willoughby 1982). TCs intensify by the quasi-conservation of angular momentum as they radially flow inward toward the TC center, which also gives rise to the contraction of the radius of maximum winds. On the other hand, the TC wind field strengthens by a substantial inward flow of angular momentum into a TC circulation (Schubert and Hack 1982; Holland and Merrill 1984). Dissipation due to surface frictional loss, especially in the boundary layer with rugged terrain like mountain ranges, reduces the inflow of angular momentum, thus prohibiting its conservation. This process weakens the maximum and/or tangential winds associated with TCs (Heng and Wang 2016).
The SMMR is the longest mountain range in the country spanning approximately 600 km from the province of Cagayan in the north to the province of Quezon to the south along the eastern Pacific coast of the main island of Luzon (Rantucci 1994). Its mountains range from heights of 312–1932 m. Since most intense TCs come from the east, SMMR is the mountain range primarily hypothesized to weaken TCs. However, following the physical principle of mountain ranges as energy sinks, the Cordillera Mountain Range (CMR) to the west of the northern SMMR region should also influence the intensity and strength of passing TCs. Thus, we include the CMR in the analysis of the present study. The CMR is a 300 km long and 90 km wide (Rantucci 1994) mountain
range also in the path of most Luzon-passing TCs. The CMR has a higher elevation with peaks reaching 2928 m. The most extensive swaths of rainforests in Luzon – which host highly biodiverse species of plants and animals, many of which are endemic – are found in both the SMMR and CMR regions (Heaney and Mittermeier 1997; Bankoff 2007; Balete et al. 2013). However, deforestation in those regions has significantly reduced forest cover in the past century (Moya and Malayang 2004; Apan et al. 2017), whereas recent reforestation efforts seem ineffective (Perez et al. 2020). The SMMR is also home to the Agta people, semi-nomad hunter- gatherer descendants of the first island settlers between 35,000–60,000 yr ago (Bellwood 2005). They number around 10,000 individuals and have survived frequent destructive TYs by appropriately using their indigenous knowledge system and practices that have been enriched by their traditional knowledge of their surroundings and environment. This is illustrated by their choice of materials and locations for their shelters. They use natural materials and avoid using galvanized iron, which can be hazardous during intense TY passages. Settlements are built away from large trees or near dry riverbeds. They also build small typhoon shelters, called “kubo-kubo,” within areas of weaker winds such as enclaves of trees and small hills (Buenafe-Ze and Telan 2018).
Annually, around nine TCs make landfall in the Philippines with most TCs hitting the island of Luzon, particularly the northeastern region, followed by the rest of eastern Luzon (Cinco et al. 2016). The eastern Luzon region has recorded not only the highest TC landfall frequency but also the highest in the calculated population-normalized TC hazard index, a measure of exposure to different TC categories. The east of Luzon shows the highest exposure to TC wind hazards (Tierra and Bagtasa 2022). In addition, heavy rainfall associated with TCs also contributes most to the rainfall of Luzon, with extreme precipitation within 250 km of a TC center frequently affecting northeastern Luzon (Bagtasa 2017). These findings from historical TC data show that many TCs do indeed traverse the SMMR.
However, there are limited studies on the influence of the SMMR on passing TCs. Brand and Blelloch (1973) showed that TYs crossing the Philippines had an average intensity drop of 33% by analyzing 30 events from 1960–
1970. Using a more recent and longer TC dataset, Adelino and Bagtasa (2023) showed that mean TC intensity drops of 6.0–7.8 and 11.1–12.0% were observed during TC landfall in Luzon using the Japan Meteorological Agency (JMA) and Joint Typhoon Warning Center (JTWC) best track data, respectively. Kaplan and DeMaria (1995) found that TC intensity drops on landfall are proportional to the TC landfall intensity, where the constant of proportionality – called the decay constant – describes the roughness of the underlying land surface. TC intensity decreases when it
interacts with land due to surface friction and the reduction of ocean heat, momentum, and moisture fluxes (Wu and Choy 2015; Racoma et al. 2016). In addition, warm and dry air descending on the lee side of mountains underlying TCs reduces the moisture supply of TCs and also weakens them (Bender et al. 1985). However, we cannot disentangle the weakening effects of surface friction from changes in the surface fluxes from empirical studies. In terms of TC-induced precipitation, the interaction of the moisture-laden TC circulation and the orography leads to enhanced and/or suppressed rainfall depending on the location of a TC relative to the SMMR (Racoma et al.
2016). TCs that move in the northern (southern) region of the SMMR will induce orographic rainfall on the western (eastern) side of the SMMR along the southern (northern) flank of the TCs. This is due to the shifting in the location of the windward side between the TC circulation and the SMMR slopes. In another study (Racoma et al. 2023), they found that orographic precipitation is proportional to the product of the normal wind and mountain slope;
albeit, the data was calculated from orographic rainfall along the CMR. Compared to the higher Central Mountain Range of Taiwan, which peaks at 3997 m, Tang and Chan (2014) found that the TC tracks of TCs moving towards Central Taiwan deflect northward as a result of changes in local environmental flow due to gyres induced by Taiwan’s Central Mountain Range. However, the same idealized experiment they did for Luzon did not show track deflection owing to the lower mountain heights and moister environment of Luzon.
While the studies presented above show the effects of the mountain ranges on certain TC characteristics and some of its associated hazards, to the authors’ knowledge, there is no literature that has investigated the purported hazard- mitigating effects of the SMMR and the CMR. Here, we aim to answer whether the SMMR and/or the CMR mitigate the detrimental effects of TCs using the WRF model to simulate the impacts of these two major mountain ranges. We performed three numerical simulations with [1]
both mountain ranges present (Control run), [2] flattened SMMR (SMMR run), and [3] flattened CMR (CMR run). In this way, we are able to distinguish the effects of topographic friction against surface fluxes. We do note that the changes done were purely on the height of the mountains alone and no modifications were made to the land use categories in the flattened domains. Furthermore, to answer the question – Does the SMMR in Luzon act as a barrier to typhoons? – we distinguish between wind- and rainfall-related TC hazards in the present study. Then, we looked at how the mountain ranges – both the CMR and the SMMR – influence the characteristics of passing TCs, as well as the spatiotemporal distribution of the wind- and rainfall-related hazards.
DATA AND METHODS
The WRF model (Skamarock et al. 2008) is a non- hydrostatic numerical weather prediction model developed by the National Center for Atmospheric Research (NCAR), used for atmospheric research and operational forecasting. A suite of physical parameterization schemes that represent processes such as hydrometer microphysics, cumulus convection, radiative transfer, PBL, and land surface are available. The Advanced Research WRF (WRF-ARW) solver uses the Arakawa-C grid as the computational grid and the Runge-Kutta 3rd-order time integration schemes (MMML-NCAR 2019). In this study, the WRF model domain was configured with a two-way nested domain of 25 and 5 km to dynamically downscale the 1°-resolution FNL reanalysis data. TC intensity, wind strength, and rain are derived from the output of the inner (5 km) model domain. The microphysics and cumulus schemes used are the WSM-6 single-moment (Hong and Lim 2006) and the Kain-Fritsch (Kain 2004) schemes, respectively, with spectral nudging turned on. The rest of the model setting used the Yonsei University PBL scheme (Hong et al. 2006), the Rapid Radiative Transfer Model (RRTM) longwave and the Dudhia (Dudhia 1989) shortwave radiation schemes, and the Unified Noah Land Surface Model scheme. These followed the configurations in previous TC simulation studies (Delfino et al. 2022;
Bagtasa 2021). Table 1 presents all the Luzon-traversing TCs from 2000–2020 that were simulated in this study, together with their corresponding landfall times, category, and locally designated names. A total of 45 TCs were included in the simulations where each simulation started from 2.5 d prior to the TC landfall until 2.5 d after landfall with one day allotted to the model spin-up. TC accumulated rain was taken to be rain from 24 h before to 24 h after landfall, following the method of Racoma et al. (2021). Figure 1 shows the track of the selected TCs from the JMA best track data and the extent of the WRF model nested domains.
To simulate the impacts of the SMMR and CMR on passing TCs and their associated hazards, we designed three numerical experiments. The first experiment is the control (CTRL) run, where the model terrain was unmodified (Figure 2A). The second and third experimental runs used a terrain with flattened SMMR and CMR (Figures 2B and C), respectively. The mountains were individually flattened to match the height of the surrounding plains. In the case of the SMMR, the flattened terrain connected the eastern Cagayan Valley plain to the eastern Luzon coast following a constant slope. For the CMR, the western Cagayan Valley plain was connected to the coastal lowlands of Region I also with a constant slope. Figure 2 shows the model topography used in the simulations. The simulated TCs in the three experimental runs had inherent
Table 1. Category, international and local names, and landfall times of all simulated TCs.
TC name (local name) – Landfall time in UTC
1 TY Bebinca (Seniang) – 02 Nov 2000 12Z 24 TY Krosa (Vinta) – 31 Oct 2013 12Z 2 TY Imbudo (Harurot) – 22 Jul 2003 06Z 25 TY Rammasun (Glenda) – 15 Jul 2014 12Z 3 TY Krovahn (Niña) – 22 Aug 2003 12Z 26 TY Kalmaegi (Luis) – 14 Sep 2014 12Z 4 STS Melor (Viring) – 01 Nov 2003 0Z 27 STS Linfa (Egay) – 05 Jul 2015 00Z 5 TY Nanmadol (Yoyong) – 02 Dec 2004 12Z 28 TY Mujigae (Kabayan) – 01 Oct 2015 18Z 6 TY Tembin (Ondoy) – 10 Nov 2005 12Z 29 TY Koppu (Lando) – 17 Oct 2015 18Z 7 TY Prapiroon (Henry) – 31 Jul 2006 06Z 30 TY Sarika (Karen) – 15 Oct 2016 18Z 8 TY Xangsane (Milenyo) – 28 Sep 2006 06Z 31 TY Haima (Lawin) – 19 Oct 2016 18Z 9 TY Cimaron (Paeng) – 29 Oct 2006 18Z 32 STS Pakhar (Jolina) – 25 Aug 2017 12Z 10 TY Chebi (Queenie) – 11 Nov 2006 00Z 33 TY Doksuri (Maring) – 12 Sep 2017 00Z 11 STS Lekima (Hanna) – 29 Sep 2007 00Z 34 TY Khanun (Odette) – 12 Oct 2017 18Z 12 TY Peipah (Kabayan) – 04 Nov 2007 12Z 35 TS Haikui (Salome) – 09 Nov 2017 18Z 13 TY Mitag (Mina) – 25 Nov 2007 18Z 36 TY Mangkhut (Ompong) – 14 Sep 2018 18Z 14 TS Higos (Pablo) – 01 Oct 2008 06Z 37 TY Yutu (Rosita) – 30 Oct 2018 00Z 15 TY Ketsana (Ondoy) – 26 Sep 2009 00Z 38 TS Podul (Jenny) – 27 Aug 2019 12Z 16 TY Mirinae (Santi) – 30 Oct 2009 18Z 39 TY Vongfong (Ambo) – 15 May 2020 06Z 17 TY Conson (Basyang) – 13 Jul 2010 12Z 40 TS Nuri (Butchoy) – 11 Jun 2020 12Z 18 TY Megi (Juan) – 18 Oct 2010 06Z 41 TY Saudel (Pepito) – 20 Oct 2020 12Z 19 STS Nock-Ten (Juaning) – 27 Jul 2011 00Z 42 TY Goni (Rolly) – 01 Nov 2020 00Z 20 TY Nesat (Pedring) – 27 Sep 2011 00Z 43 TY Vamco (Ulysses) – 11 Nov 2020 18Z 21 TY Nalgae (Quiel) – 01 Oct 2011 06Z 44 STS Ma-on (Florita) – 23 Aug 2022 00Z 22 TY Utor (Labuyo) – 11 Aug 2013 18Z 45 STY Noru (Karding) – 25 Sep 2022 06Z 23 TY Nari (Santi) – 11 Oct 2013 12Z
Figure 1. Tracks of the TCs from the JMA best track data included in the study and the domain of the WRF model (add domain delineation).
The red dashed boxes delineate the region of [N] north-, [C] central-, and [S] south-tracking TCs.
errors in terms of deviation from observed tracks and/or intensity biases. The average (and standard deviation) 6-hourly direct positional error of all simulated TC tracks is 67.5 ± 48.7 km, with values ranging from 0.4 km up to 233.0 km deviation occasionally found in weak systems.
Simulated peak intensities had a mean bias of –12.3 m/s, mean absolute error of 12.7 m/s, and root mean square error of 16.3 m/s, as compared with the peak pre-landfall intensities from the JMA best track dataset. About one- third of TCs had biases of ≤ 5 m/s but larger for more intense TYs such as in the cases of Megi (2010), Haima (2016), Mangkhut (2018), and Goni (2020), exceeding 25 m/s in intensity bias. Overall, the simulated TC tracks and intensities were still able to approximate the observed TC.
These errors will not be further discussed as the goal of the study is not to model the best representation of TCs with the least amount of track and intensity errors but to simulate the influence of the mountain ranges on passing TCs, even if some of the TCs did not exactly follow their observed tracks. Nevertheless, we checked and observed that while there were some considerable track errors – especially for the weak TCs – all TCs still traversed Luzon, and their circulations interacted with either the SMMR, the CMR, or both.
The TC center positions of the simulated TCs were determined by getting the centroid of the region of weak winds in the center of TC circulations that overlapped with the minimum central pressure inside that weak-wind region. The MWS was then estimated by getting the wind maxima within a 3° distance (around 330 km) of the selected TC center position. By getting the difference between the (no-terrain) SMMR or the CMR runs and
the (with terrain) CTRL run, we can derive the frictional influence exerted by the mountain ranges on the passing TCs. Moreover, all differences in TC hazards presented in this study are calculated to be statistically significant in the 95% confidence interval level using the Student’s t-test.
RESULTS AND DISCUSSION
TC Characteristics
All of the selected TCs formed in the WNP region from the east of the country and then moved westward over Luzon. Figure 3A shows the evolution of the mean TC intensity (or MWS) from 30 h prior to landfall, to the moment of landfall, and up to 40 h after landfall from the three experimental configurations. Apparent differences in mean TC intensity are seen at –18 to –12 h, wherein the CTRL points have the lowest values among the model runs. However, comparing the intensity difference at the time of landfall of all TCs between the SMMR run and CTRL and the CMR run and CTRL, we found that there are no significant differences in the simulated intensities between the no-terrain and with-terrain runs (p = 0.80 for SMMR vs. CTRL and p = 0.73 for CMR vs. CTRL). Once the TCs emerge from land to the South China Sea or West Philippine Sea (hereinafter referred to as outgoing TCs) from +12 h onwards, CMR run TCs are significantly (p
= 0.04) more intense compared to CTRL while moving away from Luzon. The results indicate that there are no climatological changes in TC intensity before landfall and while traversing land regardless of the presence of
Figure 2. Topography in the WRF model of CTRL (left), SMMR (center), and CMR (right) experiment runs. CTRL run used real topography, SMMR run has flattened SMMR terrain, and CMR run has flattened CMR terrain. Red boxes indicate location of modified/flattened terrain.
the SMMR or the CMR. However, TCs emerging from land in the West Philippine Sea region are more intense if the CMR is removed (green line in Figure 3A, indicating that the CMR inhibits the intensification of (westward) outgoing TCs.
Figure 3B shows the mean TC translational speed of the three experimental runs. TCs in the Philippine Sea accelerate toward Luzon as they intensify but tend to slow down right before (6 h) making landfall. TCs appear
to speed up on land as the TC centers mostly become obscured over land and then appear to re-emerge in the western region of the CMR or Central Luzon. The CTRL run seems to slightly slow down on landfall, indicating that both the SMMR and CMR slow down TC movement speed. The translational speed differences from –6 h to +6 h are marginally significant as TCs move faster with p = 0.08 for the SMMR vs. CTRL and p = 0.06 for the CMR vs. CTRL run. Once the TCs leave the landmass, the CTRL run accelerates the fastest away from land, followed by the SMMR then the CMR runs. Finally, Figure 3C shows that the intensity change of the simulated TCs is linearly dependent on the intensity at landfall, consistent with the literature (Brand and Blelloch 1973; Kaplan and DeMaria 1995). The linear trend lines of the three model runs show limited variations between runs, indicating no significant differences between the runs in the decay of TCs on landfall. In addition, while there were some deviations in the track of the TCs between the different experiments, no systematic deviations from the CTRL track were found beyond the uncertainty in track determination.
TC Wind Field
TCs are classified according to their intensities, which are based on their MWS. On the other hand, wind-related threats, which are the primary consideration in terms of hazards of incoming TCs are based on the wind strength along a TC’s circulation. The effect of TC winds on structural loads is a very complex process that depends on a myriad of factors such as turbulence-induced buffeting vibration, wake buffeting from nearby obstacles, surface roughness, and even surrounding topographic features.
Moreover, most structural damages from intense TC winds are due to debris impacts rather than the wind itself (Crosbie 1997). While MWS is an important parameter in TC classification and warnings, it does not accurately convey the destructive potential of a TC. This led to the use of other metrics such as the Accumulated Cyclone Energy (ACE) (Bell et al. 1999) or the Power Dissipation Index (PDI), which are measures of wind energy and power, respectively. Other wind-related indices, including the Integrated Kinetic Energy (IKE) (Powell and Reinhold 2007) and the Hazard Index of Kantha (2006), are more related to wind energy or power rather than wind speed.
Energy is important in the threat assessment of extreme wind because it is a better measure of stress load on houses and/or roofs (Crosbie 1997). In the previous section, we showed the presence of the SMMR or CMR has a minimal effect on the intensities of landfalling TCs.
Notwithstanding, the distribution of hazardous wind affecting landmass will change due to the changes in surface roughness.
To determine changes in TC wind strength due to topography, here, we define the accumulated wind-field
Figure 3. [A] Mean TC intensity prior, on, and after landfall (gray box is on land); [B] mean TC translational speed prior, at, and after landfall (gray box is on land); [C] intensity change on landfall vs. peak intensity right before landfall with the corresponding linear trend lines. Note that the x-axis for A and B show time to landfall from right to left, where t < 0 denotes hours prior to landfall, and t >
0 denotes hours after landfall.
energy (AWE) as the sum of the squares of the 6-hourly simulated wind speed along the TC circulation that is greater than gale force wind (> 34 kts), multiplied by a factor 1 x 10–4, to represent exposure to strong TC winds. This is similar to the calculation of ACE (Bell et al. 1999); however, AWE includes all wind within the TC wind field region and not just the MWS. Figures 4A–C show the maps of the total AWE for all simulated TCs for the CTRL, SMMR, and CMR runs. The highest values of AWE in the CTRL run (Figure 4A) are found on the eastern side of the SMMR and the summit of CMR, as their high altitude exposes their summits to strong TC winds. The removal of the SMMR in Figure 4B shows more inland penetration of high AWE and higher overall
AWE in the northern half of Cagayan Valley. On the other hand, the removal of the CMR also results in higher inland AWE in the Cagayan Valley despite the existence of the SMMR. Figure 5 shows the AWE anomalies, in percentage change, of the SMMR and CMR runs against the CTRL. Figure 5A shows a decreased AWE of around 3-5% on the eastern slopes of the SMMR and an increase of 10–16% AWE for the rest of Luzon. In Figure 5B, there is up to a 30% increase in parts of Regions I, II, and III, as well as a visible AWE drop along the summit regions of the CMR. From the results, it can be seen that model runs without the mountains increased the overall AWE around the TC wind fields without allowing TCs to become more intense. This suggests that the increase
Figure 4. Simulated total accumulated TC wind field energy (AWE) for all 45 Luzon-passing TCs from the [a] CTRL, [b] SMMR, and [c]
CMR model runs.
Figure 5. Accumulated wind field energy (AWE) differences between [a] SMMR-CTRL and [b] CMR-CTRL in percentage change.
in AWE is more in the domain of mesoscale interactions between the TC wind field and terrain – and not entirely due to a more intense TC. Surface friction also appears to reduce the wind energy of TCs, hence the more inland penetration of higher AWE in the no-terrain runs. In addition, the higher AWE anomalies along the western section of Luzon (Region I) in the CMR run are due to the higher intensity of TCs when they emerge from land and move over the West Philippine Sea region, resulting in an overall higher increase in AWE across the island of Luzon. This is consistent with the results of Section 3.1.
TC weakening due to surface friction depends on terrain height (Wu and Choy 2015). Because of the varying terrain in the different regions of Luzon, we separated the simulated TCs according to three regions of landfall – namely, the northern region (e.g. Mangkhut, Kalmaegi, Melor), central region (e.g. Megi, Pakhar, Utor), and southern region (e.g. Rammasun, Vamco, Ketsana).
This is to delineate TCs traversing both the SMMR and CMR (north), SMMR only (central), and the relatively flat terrain of southern Luzon (south), as illustrated in Figure 1. We then referred to the TCs as north-, central-, and south-tracking TCs. North-tracking TCs in Figure 6A tend to have higher anomalous AWE increases in the CMR run and more or less similar increases, albeit with varying distribution for the central-tracking TCs (Figure 6B). For the south-tracking TCs (Figure 6C), the CMR run also shows higher AWE anomalies in the Central and southwest Luzon regions. It is interesting to note that AWE in the southeastern Bicol region and Catanduanes show the highest increase in AWE anomalies for south-tracking TCs in the SMMR run. This result suggests that the presence of the SMMR disrupts the balanced gradient wind of south- tracking TCs, which results in the considerable weakening (~ 30%) of TC wind field strength over the eastern parts of the Bicol region and the island of Catanduanes.
TC-induced Rain
Figures 7A–C show the total accumulated TC rainfall for the CTRL, SMMR, and CMR model runs, respectively.
The SMMR can act to either block or enhance rainfall depending on the location of a TC depending on which side of the SMMR serves the windward slope (Racoma 2016, 2023). Figure 7A shows the highest rainfall amounts found along the eastern slopes of the northern SMMR, followed by the slopes of the CMR. The removal of SMMR in Figure 7B shows reduced rainfall along the eastern coastal regions and more inland penetration of precipitation, particularly in the Cagayan Valley region (Region II). There is also reduced rainfall in the southern region of the SMMR, including Metro Manila. Racoma et al. (2016) reported that in rainy TCs such as in the case of Ondoy, extreme precipitation occurs in the southern hemisphere of TCs, as the SMMR orographically lifts
the incoming (eastward) TC wind, enhancing rainfall to the west of the SMMR. This explains why the removal of SMMR results in lower rainfall in Metro Manila. On the other hand, the removal of CMR in Figure 7C shows an overall reduction in rainfall across most parts of Luzon but higher overall rainfall on the western slopes of the whole of SMMR, also including Metro Manila. The absence of the CMR terrain will result in stronger westerlies from the TC circulation (as outgoing TCs tend to be more intense), which leads to more orographically-enhanced rainfall along the western slopes of the SMMR.
Figure 8 shows the TC-induced rainfall anomalies for the SMMR and CMR runs. As mentioned, a reduction of up to 50% rainfall along the slopes of SMMR and an increase in inland precipitation in the Cagayan Valley and on the eastern slopes of CMR of up to 60% is observed. The CMR runs show reduced rainfall in the CMR region and
Figure 6. Accumulated wind field energy (AWE) differences between SMMR-CTRL (left) and CMR-CTRL (right) for [a] north, [b] central, and [c] south tracking TCs (in percentage change).
an increase across the rest of Luzon. We also looked at TCs that made landfall in the northern, central, and southern regions of Luzon. However, while the rainfall anomaly distributions (Figure S1) had variations, the anomaly regions are more or less similar to the total anomalies shown in Figure 8.
The complex distribution of TC-induced rainfall over the island of Luzon is the result of several factors such as local terrain (i.e. mountains, plains) (Racoma et al.
2016, 2023), TC characteristics (i.e. intensity, translation speed, etc.) (Racoma et al. 2021; Bagtasa 2021), and synoptic environmental conditions (i.e. monsoon season) (Bagtasa 2017, 2020). TC rain distribution from TCs with comparable tracks traversing Luzon can vary depending on whether it occurs during the southwest or the northeast
monsoon season (Bagtasa 2021). This dependence of TC rain on monsoon is due to the interaction between the prevailing environmental flow and the circulation of the TC itself (Bagtasa 2020). Accordingly, we also separated TCs to the southwest and northeast monsoon seasons (Figure S2), in addition to landfall regions, and found almost the same rainfall anomaly distributions in both monsoon seasons compared to the total rain anomalies shown in Figure 8. This implies that the changes in the TC rainfall distribution are mainly changes due to the TC rainbands within its circulation rather than rain from TC-monsoon interaction. Furthermore, most of the damages attributable to TC rain are not due to the rainfall itself but from the runoff and the resulting flood inundation of precipitated water (e.g. flooding, landslides).
Figure 7. Simulated total accumulated TC rainfall for all 45 Luzon-passing TCs from the [a] CTRL, [b] SMMR, and [c] CMR model runs.
Figure 8. Rainfall differences between [a] SMMR-CTRL and [b] CMR-CTRL in percentage change.
However, the modification of terrain in the experimental runs would also result in different river systems and watershed regions arising from the new terrain. Thus, running hydrological models that simulate runoff of the new rainfall amounts would be ineffective. Nevertheless, the numerical experiments in this study show us the influence of the mountain ranges while still simulating the changes in surface fluxes due to the landmass of Luzon.
We summarize our findings in the next section.
SUMMARY AND CONCLUSION
The Philippines is frequently affected by intense TCs.
Recent numerous landfalling intense TCs in Luzon has brought up conversations on the mitigating effects of the SMMR in traditional and social media. The idea that the SMMR weakens passing TCs is not new. Local mythology has stories that liken the Sierra Madre Mountains to a protective and caring mother to her sons against the wrath of the king of easterly winds who pursues her. Also, there is existing literature (Brand and Blelloch 1973; Wu and Choy 2015; Racoma et al. 2016, 2023; Adelino and Bagtasa 2023) on the interaction between TC and terrain (or topography), resulting in the weakening of TCs.
This is due to the surface friction and reduced surface moisture, momentum, and heat fluxes that feed energy to TCs. However, there is yet an analysis on disentangling the weakening effects of the mountain ranges of Luzon – including the SMMR and CMR – due to land and orographic features. In the present study, we used a state- of-the-science numerical weather prediction model called WRF to simulate the influence of the SMMR and CMR on 45 Luzon-passing TCs from 2000–2020 by having three numerical experiments: [1] a control run with real topography, [2] a run with flattened SMMR, and [3] a run with flattened CMR terrain. By simulating TCs with and without the orography of SMMR and CMR, we are able to simulate the effects of land and mountain in the control runs and the effects of land only in the SMMR and CMR runs. Consequently, we can isolate the effects of the mountains by taking the difference between the no-terrain SMMR/CMR runs and the with-terrain control runs.
The results show that despite traversing Luzon and interacting with both the SMMR and CMR, the intensity of TCs does not significantly weaken on landfall and during the first 6 h traversing land regardless of the mountains’
presence. The CMR, however, limits the re-intensification of TCs emerging from land moving westward over the West Philippine Sea. In addition, both the SMMR and CMR slightly slow down the translational speeds of TCs before and at landfall, which means that the mountains’
presence may be able to increase the exposure duration to
TC hazards such as wind and rain. This increased exposure is further supported by mapping the accumulated TC wind-field energy or AWE, which was used as a measure of TC wind-field strength. Removal of the terrains resulted in higher AWE overall (up to 30%) across the whole of Luzon Island. Higher AWE values were found along western and eastern Luzon for the CMR runs as more intense outgoing TCs affect the western side of the island. The results suggest that over Luzon, the CMR has a higher weakening effect on TC wind strength than the SMMR. TC winds are strongest near the path taken by TCs. Therefore, TC wind distribution will depend on where a TC traverses. Distinguishing between TCs that crossed both the SMMR and CMR, SMMR and Central Luzon, as well as the relatively flat southern Luzon region, we found that SMMR reduces wind field strength more uniformly across Luzon than the CMR, where wind strength reduction is more concentrated along the path of the TCs. This is expected as the SMMR covers almost the whole eastern coastal portion of Luzon and its removal exposes the whole island. Interestingly, the SMMR has the most significant mitigating potential for the island of Catanduanes and the southeastern parts of Bicol in the case of TCs traversing the southern Luzon region. The CMR shows a higher weakening effect in terms of AWE exposure in the northern and Central Luzon regions. On the other hand, in terms of rainfall, the SMMR increases rain on both its east and west sides and essentially blocks rainfall in the western regions of the Cagayan Valley. The enhanced orographically-induced rainfall by the CMR is constrained only around its slopes.
Therefore, to answer the question: does the SMMR (and the Cordillera Mountain Range) in Luzon act as a barrier to typhoons? it depends on the hazard involved and the location in question. For wind hazard exposure, SMMR reduces 1–13% and up to 20% in the summit of CMR, whereas the CMR reduces 8–30% AWE in the Cagayan Valley and in the western and Central Luzon. In terms of rainfall, SMMR increases rainfall along its western slopes from 23–48% (and from 25–55% in Metro Manila) and reduces rainfall in Cagayan Valley by 10–59%, especially on the eastern slopes of the CMR as the SMMR serves to block the westward moisture flow. On the other hand, CMR increases rainfall around it by up to 58%. Overall, the CMR shows a more retarding effect than the SMMR and reduces intense wind exposure better, whereas its orographic enhancement of rainfall is constrained around its immediate vicinity.
From the results, it appears that the SMMR is an effective barrier for the Cagayan Valley, as it is able to reduce both wind exposure and basin-wide rainfall; despite this result, it is important to note that Cagayan Valley is still vulnerable to extreme TC events. For other areas like
Metro Manila, the increase (of 25–55%) in rainfall threat will most likely compensate for the reduced (3–8%) wind exposure. Therefore, with the consideration that more TC-induced damages are water related and not due to wind, relying on mountain ranges as barriers to TCs is flawed, inaccurate, and potentially dangerous as this may lead to complacency. Whether the SMMR or the CMR mitigates TC hazards is beside the point as changing the mountain ranges’ orographic features to the extent that there would be effects on TC intensity, wind strength, associated rainfall, and/or movement speed is presently impossible, except for the eventual geologically-driven changes that will occur eons from the present time.
Disastrous TC-related events have and will always occur in the Philippines, many in Luzon, due to the country’s geographic location and the steering mechanisms of TCs in the WNP basin (Bagtasa 2017, 2020).
We believe it is more crucial to highlight the fact that both the SMMR and the CMR are key biogeographic regions, housing rich and unique flora and fauna that are of significance to the Philippines and the world (Biag and Alejandro 2021; Cruz and Afuang 2018; Ong et al. 2002).
Unfortunately, these mountain ranges are continuously threatened due to irresponsible logging and mining, swidden agriculture, and other human activities (van der Ploeg 2011; Apan et al. 2017; Perez et al. 2020). Rather than focusing our conversations on the mountain ranges
“protecting” us from the impacts of TCs, we should instead shift the discourse to protecting these mountain ranges from anthropogenic impacts.
ACKNOWLEDGMENTS
This study is supported by the DOST-PCIEERD (Department of Science and Technology–Philippine Council for Industry, Energy and Emerging Technology Research and Development) funded project titled
"Analysis of tropical cyclone rapid intensification in the Philippines: its characteristics, impacts, and future projections" (1211131). The second author is supported by a scholarship under the Commission on Higher Education’s Joint Development of Niche Programmes agreement with the British Council. We also thank anonymous reviewers for their inputs in improving this study.
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APPENDICES
Figure S1. Rainfall differences between SMMR-CTRL (left) and CMR-CTRL (right) for [a] north, [b]
central, and [c] south tracking TCs (in percentage change).
Figure S2. Rainfall differences between SMMR-CTRL (left) and CMR-CTRL (right) for the [a] southwest and [b]
northeast monsoon (in percentage change).
