The Unique Features of Typhoon Rai (2021): An observational study

doi:10.21203/rs.3.rs-4620200/v1

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The Unique Features of Typhoon Rai (2021): An observational study

https://doi.org/10.21203/rs.3.rs-4620200/v1

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Tropical Cyclone (TC) RAI (2021) made a devastating landfall in the Siargao-Dinagat Islands of the southeastern Philippines on December 16, 2021, causing about USD 1.05B in damage, 405 reported dead and 52 missing. This TC reached a maximum sustained wind speed (MSW) of 105 kts (194.5 kph) and 915 hPa mean sea level pressure (MSLP) according to the WMO-IBTrACS. When compared with Philippine landfalling TCs from 1979 to 2020, this TC, among super typhoons (STYs), ranked second in terms of MSW and translational speed. Moreover, the TC had an unusual westward movement, faster translational speed, larger radius, and greater intensity when compared to seven other TCs that made landfall in the same month and region. The environmental factors along the path of TC RAI that may have contributed to its intensification include, but are not limited to, the above normal SST (+ 0.5 to 1.5°C) and ocean heat content, high low-level relative humidity (RH), and high specific humidity. These factors resulted in strong convergence and intensification until landfall. Composite analysis of and comparison with the seven TCs reveal that the atmospheric conditions during TC RAI had a consistently higher near-surface RH_{850hPa − 500hPa}, which helped sustain its movement across the central Philippines. Moisture from the Philippine Sea was also drawn into central Philippines, which received at least 125–150 mm of rainfall. The extension of the western North Pacific Subtropical High along 20°N and strong easterly flow may have facilitated the TC’s unusual straight and westward movement.

Rai (2021)

tropical cyclone

Odette

Philippines

landfalling tropical cyclone

The Philippines is situated along the Tropical Cyclone (TC) corridor over the western North Pacific (WNP). On average, about 9 out of 20 TCs that enter the Philippine Area of Responsibility (PAR) annually make landfall over the Philippines (Cinco et al. 2016; Santos, 2021). Majority of these TCs originate from the Pacific Ocean, yet the influence of those TCs originating from the west of the Philippines cannot be neglected. Although TCs may bring rainfall that can replenish the hydrological resources of the country, they often bring catastrophic damages to agriculture and infrastructure, and often result in the loss of human lives. Hence, characterizing these TCs is essential.

Several studies have examined the important characteristics of landfalling TCs over the Philippines in terms of trends (Fudeyasu et al., 2014; Takagi and Esteban, 2016), induced rainfall (Bagtasa, 2017), and changes in intensity, translational speed, and direction of movement (Petilla et al., 2023). Increasing trends in the number of landfalling TCs between 1945 and 2013 have been found in Leyte, central-southeastern Philippines (Takagi and Esteban 2016). This is the same area that was devastated by Super Typhoon HAIYAN (2013) in November 2013 (henceforth, TC names are spelled in capital letters with its corresponding year enclosed in parenthesis), the strongest and fastest typhoon on record (Takagi and Esteban, 2016). TCs affecting the southern Philippines during December have also been increasing in recent decades (Basconcillo and Moon, 2021). Several studies have identified the environmental factors that contribute to TC development and intensification in the WNP. For example, Kuroda et al., (1998) explained that when a TC encounters a sea surface temperature (SST) greater than 29°C for 1 to 2 days, it often intensifies. A weak environmental vertical wind shear is a primary contributor to TC life, wherein a weak vertical wind shear prevents the destruction of the vortex’s vertical profile (Wong and Chan, 2004). A vertical wind shear higher than 12.5 ms^− 1 impedes the TC’s development (Zehr, 1992; Flatau et al. 1994). Moreover, a higher environmental moisture content along the path of the TC also contributes to a favorable condition for its development (Wu et al., 2015).

Aside from HAIYAN (2013), another catastrophic TC that made landfall in the Philippines is Typhoon (TY) RAI (2021), locally known as Odette. Initially, TC RAI was identified as a Tropical Depression (TD) by the Japan Meteorological Agency (JMA) on December 12, 2021, which intensified into a Tropical Storm (TS) on the following day. On December 16, 2021 at 0600Z, TC RAI made its first landfall as a Category 5 typhoon in northeastern CARAGA region in southern Philippines causing widespread disruption across five Philippine Regions. The estimated damage cost is about USD 1.05B, with 405 reported dead and 52 missing. According to the World Meteorological Organization (WMO), the 10-minute maximum sustained wind speed (MSW) of TC RAI reached 105 kts (194.5 kph), and a minimum sea level pressure (MSLP) of 915 hPa. The TC made its last landfall in the Philippines on December 17 and continued to recurve northwards along the South China Sea (SCS) as shown in Fig. 1. Interestingly, this TC underwent two intensifications over water: one prior to landfall over the Philippines on December 16 and the other when it was west of the Philippines on December 19. As such, RAI (2021) was considered as the only Super Typhoon (STY) in December to have affected the SCS since 1961 by the Hong Kong Observatory (Chan et al. 2022).

Limited studies have investigated the notable features of TC RAI during landfall. A field survey conducted by Esteban et al. (2023) demonstrated that maximum storm surge levels were measured around 2.54m and 4.06m in Cebu and Bohol islands, respectively. In terms of damage to infrastructure, Bolanio et al. (2022) utilized satellite imaging to evaluate the immediate effects of the event using damage maps. In the aspect of marine life, Dolorosa et al. (2022) have investigated the impact of TC RAI on the coral reefs in Palawan Island, western Philippines. However, most of the studies have focused on the impacts of the extreme weather event, and detailed analysis of the characteristics of TC RAI landfall have not yet been explored. More specifically, the unique features of this TC relative to other landfalling TCs in the Philippines and the environmental factors contributing to its development and intensification have not yet been quantitatively identified and compared. To address the above-mentioned research gap, this study aims to build on these previous studies and elucidate the unique features of TC RAI in the context of other landfalling TCs in the Philippines, particularly in the northern CARAGA Region. Environmental factors, such as SST, moisture flux, humidity, wind shear, and composite analysis with TCs having similar track, landfall, and season are investigated in this study. This paper is organized as follows. Section 2 shows the data and methodology used in the study. Section 3 provides detailed observations of TC RAI relative to other landfalling TCs in 1979–2020. A composite analysis was also performed using the seven TCs having similar tracks that made landfall in December. Section 4 summarizes the study's key findings.

This study focused on the intensity, translational speed, and westward movement of TC RAI relative to all landfalling TCs over the Philippines from 1979–2020. Then, similar TC cases in terms of season, landfall location, and track among the climatological TCs were identified and compared with TC RAI. Environmental factors such as SST, moisture flux, and specific, and relative humidities were examined during TC RAI’s approach to the Philippines. Lastly, TCs that specifically made landfall in December in the vicinity of the Siargao-Dinagat Islands in eastern Philippines and having comparable bearing were identified, and a composite analysis was conducted to compare the environmental features of TC RAI.

The naming of the TC positions used in this study were based on Chien and Kuo (2011), which are the landfalling point (LF) and the 6-hourly positions before (LF-6, LF-12, …) and after (LF + 6, LF + 12, ...) the landfall. The intensity, translational speed, and direction of movement of the TCs were computed based on the 6-hourly best track archive of the WMO from the National Centers for Environmental Information’s (NCEI) International Best Track Archive (IBTrACS) (Knapp et al., 2010). The translational speed and direction, henceforth referred to as speed and movement, respectively, at the best track position were computed relative to the previous 6-hourly position. More specifically, the translational speed was computed by finding the difference between the displacement traveled from the TC’s current position and its previous 6-hourly position and dividing the distance by 6 hours. In this study, as long as the raw data are complete, then the TC case would be included in the comparison (i.e., TC location, TC lifetime, and landfalling times are guaranteed to be available from the IBTrACS dataset).

This study also used the Saffir-Simpson scale to classify the TCs based on their maximum sustained wind speed (MSW): Tropical Depression (TD) at < 33 kts, Tropical Storm (TS) from 34 to 63 kts, and Typhoon (TY) with Categories 1 to 5 at > 63 kts based on the Regional Specialized Meteorological Center (RSMC) (Choi and Moon, 2012). Moreover, when winds are 105 kts and above, a special classification of super typhoon (STY) is used (Chan et al., 2022). The IBTrACS dataset does not record TCs with intensities less than 30 kts; hence, these TCs are assumed to be in the TD category. This study also examined the TC radius using the long and short 30-kt TC radius, derived from the WMO-IBTrACS dataset from RSMC Tokyo.

The accumulated cyclone energy (ACE) was used to estimate the energy released by TC RAI throughout its lifetime (Han et al., 2022), which is given by:

$$ACE=1{0}^{-4 }\sum {{V}^{2}}_{max}$$

where V_max is the sustained near-surface wind speed during its lifetime. The landfalling dissipation rate (LFDR) of the TC was also computed using the ACE, 24 hours before (i.e., LF-24, …, LF) and after landfall (LF, …, LF + 24). The LFDR is given by:

$$LFDR=1-\frac{AC{E}_{+24hr}}{AC{E}_{-24hr}}$$

where ACE_− 24hr is the ACE from LF-24 to LF, and ACE_+ 24hr is the ACE from LF to LF + 24. This metric describes the TC’s energy dissipation, wherein a higher (lower) LFDR indicates more (less) rapid TC weakening. In some instances, the LFDR becomes negative if the ACE after landfall is greater than the ACE before landfall. In this case, the TC is noted to be intensifying.

For the composite analysis, TCs having similar track, landfalling month, and landfall location within the 1° radius of the Siargao-Dinagat Islands were identified. To synchronize the timing of the landfalling dates of these TCs, the landfall time and dates were tagged as LF. Then, the atmospheric conditions (i.e. humidity, winds, geopotential height, water vapor) were retrieved from European Center for Medium-Range Weather Forecasts (ECMWF) reanalysis 5 (ERA5) hourly data at multiple pressure levels with 0.25 × 0.25° horizontal resolution (Hersbach et al., 2020). The mean conditions of similar landfalling TCs were retrieved and compared with that of TC RAI. Previous studies have revealed that the ERA5 dataset tends to underestimate TC events (e.g., Dulac et al. 2024), particularly their intensity and wind speed. As such, we compared the results with the Japanese 55-year reanalysis (JRA55) dataset to check the robustness of the results (Kobayashi et al., 2015). Comparison with the JRA55 data set confirms the similarity of the findings from the ERA5 dataset (i.e. existence of a humid environment, higher integrated water vapor and convergence, and vertical wind shear). Therefore, to save space, only the results using the ERA5 are shown in the succeeding sections.

The daily Group for High Resolution Sea Surface Temperature (GHRSST) global Level 4 sea surface temperature dataset at 0.25° × 0.25° horizontal resolution dataset was used for the SST values during the TC event (Reynolds et al. 2007). The SST anomaly (SSTa) was computed using the baseline from 1986 to 2005. The monthly ocean heat content (OHC) anomaly was retrieved from the JMA's global ocean heat content data (Ishii et al., 2017). The computation for the OHC entails integrating the heat content for each level from the depth of 0 to 2000m. The data has a monthly temporal resolution with a 1° × 1° spatial resolution. The vertical wind shear (200–850 hPa) was computed using the formula:

$$\sqrt{({u}_{200}-{u}_{850}{)}^{2}+({v}_{200}-{v}_{850}{)}^{2}}$$

where u₂₀₀ and v₂₀₀ are the u and v winds at 200 hPa, respectively, and u₈₅₀ and v₈₅₀ are the u and v winds at 850 hPa, respectively, as defined by Cao et al. (2016).

3.1. Observations of TC RAI

TC RAI is the 15th TC that entered the PAR and the final TC of the 2021 Pacific typhoon season (Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA), 2022). Figure 1 shows the TC trajectory and the Saffir-Simpson Scale as color coded dots. Based on the IBTrACS dataset, this weather system was formed on December 11, 2021 1200Z at about 2500 km from the east coast of the Philippines (Knapp et al., 2010; Chan et al., 2022). This weather system later developed into a TD southeast of Palau on December 12 0300Z (National Disaster Risk Reduction and Management Council (NDRRMC); https://ndrrmc.gov.ph/attachments/article/4174/Final_Report_for_Tropical_Cyclone_ODETTE_2021.pdf). It moved westward towards the Philippines and was declared as a TS on December 13, 2021, 0600Z. Then on December 14 1200Z, the TC entered the PAR, and the first TC bulletin was released by the PAGASA, the Philippines’ meteorological agency. Six hours later, the TC further developed, reaching Typhoon (TY) intensity (> C1 on the Saffir-Simpson Scale) on December 14 1800Z (shown as yellow dots in Fig. 1). The speed during this period was about 10–15 kts with a maximum sustained winds (MSW) of 70 kts and a mean sea level pressure (MSLP) of 970 hPa (WMO data). Furthermore, TC RAI rapidly intensified (Chan et al., 2022) on December 15 and developed into a STY the following day.

The TC center made its initial landfall in Siargao Island, northeastern Mindanao on December 16 0600Z with 105 kts MSW and a central pressure of 915 hPa MSLP (Fig. 1c). Within 24 hours, the TC has made 8 landfalls. This is a result of the almost westward track, which allowed it to traverse on to Dinagat Island (2nd), followed by multiple landfalls in central Visayas; more specifically, 3rd and 4th in Southern Leyte, 5th and 6th in Bohol, 7th in Cebu, 8th landfall in Negros Oriental (Fig. 1c; NDRRMC, 2022; PAGASA, 2022). During its movement across central Philippines, field surveys from Esteban et al. (2023) indicate maximum storm surge heights of 3.66, 2.54, and 4.06 m in the coastal areas of Leyte, Cebu, and Bohol islands, respectively, as the eye of the TC passed over these provinces.

It made its 9th and final landfall in Palawan Island on December 17 and exited the Philippines on December 18. During its movement across central Philippines, TC RAI was able to maintain its strength of around 80 kts with a translational speed of around 14–16 kts. Figure S1 shows the 6-hourly accumulated rainfall throughout the TC event from the GSMaP gauge adjusted hourly rain, version 6 (Kubota et al., 2020). At least 100–150 mm of rainfall was received by areas along the path of the TC. Afterwards, the TC continued to traverse west of the Philippines and re-intensified into a STY reaching 105 kts and 915 hPa MSLP on December 18 1800Z, and gradually recurving northwards towards Hainan Island and Southern China. The TC ultimately weakened before dissipating on December 21 near southwest of Hong Kong (Chan et al., 2022).

3.2. Unique Features of TC RAI

This section further examines the characteristics of TC RAI in terms of translational speed, intensity, minimum sea level pressure, TC radius, and lifetime with respect to similar landfalling TCs in the Philippines to identify potential unique features of TC RAI.

3.2.1. Translational Speed, Intensity, and Direction of Movement

We analyzed TC events that made landfall in the Philippines from 1979–2020. We identified the TCs that made landfall in Luzon, Visayas, and Mindanao to represent the northern, central, and southern areas of the Philippines, respectively (Figure S2). Table 1 shows the count of all TCs at different categories upon landfall in these three areas.

Table 1

Tropical Cyclone Category of Philippine TCs (1979–2020) upon landfall using WMO-IBTrACS.
Category	Philippines	Luzon	Visayas	Mindanao
TD	123	49	46	28
TS	88	53	28	7
C1	33	27	6	0
C2	31	23	6	2
C3	8	5	3	0

As mentioned previously, since the IBTrACS dataset only records MSW > 30 kts, TCs that do not have any MSW entry in the dataset were assumed to be in the TD category and were excluded, resulting in 160 TCs that were used in the analysis. Next, the MSLP, translational speed, and MSW were compared as shown in Figs. 2a and 2b to ascertain any trends. Based on the results, a strong negative correlation between MSLP and MSW was found (r=-0.9561; p < 0.0001), while no linear relationship between the translational speed and MSW could be established (r=-0.02; p = 0.8011). This observation agrees with the findings from Choi et al. (2016), in which MSW and MSLP were found to have an inverse relationship, while no particular relationship between translational speed and MSW (Takagi and Esteban, 2012) could be confirmed. Upon closer analysis of the TCs, TC RAI had the second highest MSW, tied with BETTY (1987) and HAIMA (2016), and the 3rd lowest MSLP from the landfalling TCs, after HAIYAN (2013) and BETTY (1987) (see color codes in Fig. 2). One key finding is that among the TCs in the STY category upon landfall, TC RAI had a speed of 16.7 kts, which is the second fastest landfalling TC next to HAIYAN (2013).

Figures 2c and 2d further illustrate the landfalling TCs in terms of days until landfall and TC lifetime. In this study, days until landfall is defined as the duration of TC activity from genesis to landfall in the Philippines, while lifetime is defined as the duration of TC activity from genesis to dissipation. Based on Fig. 2c, TC RAI had 4.75 days from genesis until its landfall in the Philippines, which is comparable to HAIYAN (2013), BETTY (1987), and HAIMA (2016). Also, based on Fig. 2c, TCs make landfall in the Philippines about 4.3 ± 3.2 days from genesis, and that TC RAI is within the 56th percentile among the TCs considered. These findings are comparable with locations such as Japan, where Nayak and Takemi (2023) observed that TCs from 2006–2019 take about 5 to 8 days to make landfall in Japan. In terms of TC lifetime from genesis to dissipation, TC RAI had about 10 days, which is comparable to HAIYAN (2013) and BETTY (1987). TCs that make landfall in the Philippines are active in about 8.6 ± 3.5 days, with TC RAI within the 71st percentile. On average, TCs make landfall in less than 5 days with a lifetime of less than 8 days during the 1979–2020 period (Figure S3). So far, there are no clear trends, yet there are peaks in the time to reach land and TC lifetime in 1997 and 2018.

While TC radius is usually not included in other climatological studies, the estimated radius of gale-force wind (in km), R30, of all landfalling TCs in the Philippines are examined in this study. R30 is often used to determine TC impact and assess disaster risk (Kim et al., 2022). Figures 2e and 2f show the long and short TC radius of the 30 kt wind, respectively, during landfall wherein TC RAI has a 240 km long and 150 km short radius for its 30 kt winds. In terms of other TCs, the average landfalling TC radii are about 188 ± 78 km for long and 157 ± 68 km for short radii. The long (short) TC radius for TC RAI is within the 83rd (64th) percentile. Based on the aforementioned findings, TC RAI’s lifetime, days until landfall, and its radius fall within the usual characteristics of a typical landfalling TC in the Philippines. When comparing TC size, it is worth noting that HAIYAN had a 250 (180) km long (short) 30 kt radius, while the largest TC in terms of radius was ZEB (1999) having both 450 km long and short 30 kt radii.

Further examining the intensity and temporal distribution of the landfalling TCs, Fig. 3 shows the frequency distributions of the 160 TCs per year from 1979–2020 and intensity distributions according to MSW at 5 kt intervals. Although there were 17 and 14 TC landfalls in 1993 and 1995, respectively, there are no significant trends in the peaks of annual TC count from 1979–2020 (r = 0.017; p = 0.92) (Fig. 3a). This agrees with Takagi and Esteban (2016), who found no significant trend in most areas of the Philippine archipelago except along 10°–12°N. Moreover, for Fig. 3b, using Kruskal-Wallis (Hollnader and Wolfe, 1973) and the Anderson-Darling test (Thode, 2002), the data is not normally distributed. Albeit, based on Fig. 3b, it will be noted that TC RAI is near the rightmost tail of the distribution, which is around the 96th percentile.

To identify the unique features of TC RAI, we examine December TCs having similar track, area of landfall, and direction of movement, using the IBTrACS data set and found seven TCs: NORRIS (1986), MARGE (1986), NELL (1993), AXEL (1994), KAJIKI (2001), WUKONG (2012), and JANGMI (2015). Figure 4 shows the track of these TCs with their corresponding genesis and dissipation points. The tracks of these TCs differ in their genesis points, yet resemble a similar track west of 130°E. It can also be seen that TC RAI formed closer to the Philippines and at a lower latitude at 5.3°N.

In terms of intensity, TC RAI's MSW is comparable to the composite TCs at LF-24 and LF-18 as shown in Fig. 5a. TC RAI further intensified, peaking to 105 kts (194.4 kph) at LF, and decreased its intensity before plateauing at 80 kts at LF + 18 onwards, maintaining its TY category. These observations coincide with TC RAI’s decrease in MSLP starting at 950 hPa at LF-12, and reaching the minimum of 915 hPa at LF. In contrast, the intensity of the composite TCs consistently decreases from LF-24 onwards. In terms of MSLP, TC RAI had a consistently lower MSLP with respect to other TCs across LF-24 to LF + 24 (Fig. 5b). This indicates TC RAI’s unique feature, which shows how it continued to intensify until landfall. The decrease in intensity is expected as a result of the TC encountering topography, and increased friction and loss of latent heat (Wu and Choy, 2015). Unique to the central Philippines is the absence of a mountain range (Fig. 1c) that would heavily affect the intensity and rainfall distribution of the TC. Brand and Belloch (1973) have noted that landfalling TCs in central Philippines are more prone to intensification due to the archipelagic nature of this region. The island-and-sea configuration has lesser effect on TCs due to the smaller surface areas of the land (Brand and Belloch, 1973). Petilla et al. (2023) noted that about 6 (4) TCs passing through central Philippines had increased (maintained) in intensity upon approach and departure. Hence, one probable cause to the sustenance of TC RAI above the TY category may be attributed to the island-and-sea mix in this region (Brand and Belloch, 1973). Additionally, the area of rich moisture along the path of the TC may have allowed it to produce heavier precipitation and more energy, as will be discussed later.

The speed of TC RAI is comparable to that of the composite TCs until LF at which point TC RAI's speed continued to increase until peaking at LF + 6 at around 13.1 kts (Fig. 5c). This behavior has been observed by Petilla et al. (2023) for TCs crossing Mindanao Island and central Visayas. The speed remained consistently greater than 10 kts until LF + 24. At LF + 24, the speed of TC RAI is comparable to the mean of the 7 TCs. More specifically, both the speed of the composite TCs and TC RAI increased starting at LF and peaking on LF + 6. This is in agreement with the observations from Chang (1982) and Bender et al. (1987), where both hypothesized an increase of speed when approaching land, which may be caused by the interaction between the TC and topography.

In terms of direction of movement, TC RAI's north-westward direction is comparable to the composite TCs until LF to LF + 18, where the TC moved westward upon landfall. The significant changes in direction of the TC track may be explained by the minimal amount of mountain ranges along the path of the TC. Figure 1c shows that the island-and-sea mix along the path of the TC is a key feature. The absence of mountain ranges may have influenced the TC’s direction as noted by Maw and Min (2017) who experimented the impact of topography on ROANU (2016), which made landfall in Myanmar. They noted that changes in the altitude of the Rakhine Mountain have an influence on the track of the TC such that the absence (or presence) of the mountain shifted the track away from (into) Myanmar. Additionally, the straight and westward track upon approach also agrees with the hypothesis proposed by Corporal-Lodangco et al. (2016) that TCs formed during a La Nina season often follow a straight-moving track (more details discussed on section 3.2.2). Additionally, Wu and Choy (2015) have suggested that topography is more effective in deflecting TCs with weaker intensity. As such, the westward track combined with fast speed allowed TC RAI to traverse central Visayas and make 8 landfalls within 24 hours as observed by NDRRMC (2022).

The TC 30 kt radius (long) was also examined to understand the evolution and scope of the TC event. As shown in Fig. 5e, TC RAI had a consistent radius of 240 km from LF-24 to LF + 24, and yet the composite TCs show a consistent decrease in size during the same period. Specifically, the radius of TC RAI was still comparable to that of the composite TCs before LF + 6, at which point the composite TC size decreased. The large TC radius contributed to the wider scope of damage caused by TC RAI.

To further quantify the TC’s intensification/dissipation and energy, Table 2 shows the ACE and the LFDR of the composite TCs, including HAIYAN (2013). Based this, TC RAI had the highest ACE when compared with the composite TCs. TCs such as NELL (1993), WUKONG (2012), and JANGMI (2014) had a negative LFDR, indicating that these TCs did intensify after landfall, while NORRIS (1986), MARGE (1986), and AXEL (1994) had a positive LFDR. A unique finding from the composite TCs is that KAJIKI (2001) had zero LFDR, indicating that this particular TC maintained its intensity before and after landfall. However, what sets TC RAI apart from other TCs is that although it had a higher intensity, its LFDR was only about 0.02. This is in contradiction to the findings from Kaplan and Demaria (1995) wherein they observed that TC's rate of wind speed decay is proportional to wind speed. Yet, this TC was able to maintain its intensity albeit its STY category as exhibited to its near zero LFDR, which indicates that its dissipation rate was very low relative to other composite TCs.

Table 2. Monthly Ocean Heat Content (OHC) Anomaly, Accumulated Cyclone Energy (ACE), and Landfalling Dissipation Rate (LFDR) of the 7 Composite TCs and TC RAI.

Name	Year^a	Monthly OHC Anomaly^b (x±s.d.) kJ cm ^-2	Accumulated Cyclone Energy (ACE)	Landfalling Dissipation Rate (LFDR)
NORRIS	1986	-101±12	11.08	0.46
MARGE	1986	-101±12	12.78	0.54
NELL	1993	-98±19	3.89	-0.15
AXEL	1994	-113±6	9.89	0.42
KAJIKI	2001	79±8	1.35	0
WUKONG	2012	201±14	1.71	-0.08
JANGMI	2014	-15±19	1.02	-0.77
HAIYAN^c	2013	233±19	20.69	0.36
RAI	2021	123±14	18.94	0.02
^aBlue (red) years indicate La Nina (El Nino) years ^bOHC anomaly computed by solving the mean inside the white box (7.5-10.5°N, and 127.5-135.5°E) of Figure 1 ^cDaily OHC anomaly computed to be 115-135 kJ cm^-2 (Lin et al., 2021)

Based on these findings, there are five features unique to TC RAI when compared to landfalling TCs in the Islands of Siargao-Dinagat Islands. Upon landfall, TC RAI is the TC case having: a) MSW of 105 kts and an MSLP of 915 hPa and is the strongest TC to make landfall in Siargao-Dinagat Island; b) a high ACE and a near-zero LFDR indicating a low dissipation rate of the TC during landfall, c) the second fastest translational speed upon landfall (next to NORRIS,1986); d) having a westward track (~ 270°) from LF to LF + 24; and e) having the second largest TC 30 kt radius (next to MARGE, 1986) at 240 km.

3.2.2. Analysis of Environmental Factors

This section covers the environmental factors that contributed to the development, intensification, and maintenance of TC RAI. The ERA5 dataset was used for the analysis of the moisture and vertical wind shear while the GHRSST dataset for SST. This section focuses on the time steps between LF-24 to LF due to the consistent intensification of TC RAI.

SST plays a vital role in the development and intensification of TCs (Kuroda et al., 1998). Although other factors such as weak vertical wind shear, increase in moisture flux, high relative humidity, etc. contribute to TC intensification, TCs moving over waters warmer than 28.5°C (Kuroda et al., 1998) for one to two days often acquire higher intensities eventually. Figure 1a shows that TC RAI passed over waters warmer than 29°C for at least two days before landfall. Another impact in terms of SSTa is that TC RAI developed and intensified during a La Nina season at which temperatures in the ENSO 3.4 region during OND, NDJ, DJF were about − 1.1, -0.9, and − 0.8°C, respectively (Diamond and Schreck, 2022; Null and CCM, 2024). At the time of TC RAI, the SSTa over the Philippine Sea was about + 0.5 to 1.5°C. According to Corporal-Lodangco et al. (2016), during La Nina conditions, TCs in the fourth quarter (OND) have approximately straight-moving tracks. Also, the genesis of these TCs is concentrated to lower latitudes and are formed closer to the Philippines, about west of 160°E (Corporal-Lodangco et al., 2016). This hypothesis is demonstrated in the tracks of TC RAI and the composite TCs shown in Fig. 1b and Fig. 4, respectively.

Table 2 shows the monthly ocean heat content (OHC), which was averaged over the white box in Fig. 1, of the composite TCs, TC RAI, and HAIYAN (2013). The white box covers the area where TC RAI intensified from TS to STY TC within 36 hours Table 2 shows a positive OHC anomaly during TC RAI in this area of about 123 ± 14 kJ cm^− 2. It is also worth noting that HAIYAN (2013) had the highest monthly OHC anomaly at 233 ± 19 kJ cm^− 2, while AXEL (1994) had the lowest OHC anomaly at -113 ± 6kJ cm^− 2. Compared to other STYs, the daily OHC anomaly for HAIYAN (2013) and HAGIBIS (2019) were estimated to be 115–135 kJ cm^− 2 and 140–160 kJ cm^− 2, respectively (Lin et al., 2021). TC RAI had SST and OHC conditions that favored its intensification to STY in the WNP (Lin et al., 2021).

Vertical wind shear is a major contributor to TC generation, weakening, and maintenance of vertical structure (Wong and Chan, 2004). From LF-24 until LF-6, the TC moved through areas of high vertical wind shear (10–20 m s^− 1) within the 1° radius of the TC as shown in Fig. 6. Zehr (1992) and Flatau et al. (1994) noted that TCs can usually withstand a vertical wind shear of 9 m s^− 1–12.5 m s^− 1. Above this threshold, TC development is already impeded. This suggests that TC RAI was able to intensity despite moving in an area of high vertical wind shear.

Figure 7 shows the vertically integrated water vapor (VIWV) from 1000 to 700 hPa (Zomeren and Delden, 2007) during TC RAI’s approach in the Philippine Sea. It can be seen that the TC passed through an area of rich moisture content with about 60 to 80 kgm^− 2 of water vapor from LF-24 to LF-6. High environmental moisture, specifically in the rear quadrants of the TC, provides a favorable condition for TC intensification (Wu et al., 2015) as shown in Fig. 7d.

High specific humidity is observed along the eastern Philippine coast as the TC approaches at LF-24 to LF + 12 (Fig. 8). This enabled ample intensification time for the TC through moisture transport, as described earlier. Moreover, moisture from the northeast of the Philippines was also drawn towards the central Philippines as depicted at LF-24 and LF-12. This allowed the TC to consistently gain moisture as it approached the Philippines, which resulted in greater precipitation (Chien and Kuo, 2011) and higher energy. In terms of moisture flux, a positive and strong convergence can be seen along the TC center (Fig. 9). A consistent moisture supply from the central Philippine region may also explain why the TC maintained its intensity during and after its initial landfall.

3.3. Analysis relative to the composite TCs

Similar to the methodology of Chien and Kuo (2011), it is essential to examine the difference of the environmental factors between TC RAI and the composite TCs. The meteorological fields for the seven TCs were retrieved from the ERA5 dataset and the average compared to those of TC RAI.

A key finding in the composite analysis is that TC RAI was able to intensify from LF-24 to LF-6 in a high wind shear environment. Recall that Zehr (1992) has noted that a vertical wind shear of > 12.5 m s^− 1 impedes the development of a TC. Yet Figure S4 shows the difference of the vertical wind shear of TC RAI and the composites. Based on visual inspection, TC RAI had a consistently higher vertical wind shear as it approached the Philippines, but this environmental condition did not impede the intensification of the TC. In particular, the difference of the vertical wind shear between TC RAI and the composites is greater than 10 m s^− 1 within its core. This merits further investigation since this contradicts the findings of Zehr (1992) and Flatau et al. (1994) given TC RAI’s increased intensity amid this high wind shear condition.

At LF-24, there was a higher RH at 700 hPa east of the Philippines, and north of the TC for the case of TC RAI (Fig. 10a). In contrast, the atmosphere over this region is dry for the case of the composite TCs (Fig. 10b) due to weakened southerly wind, which carries moisture rich air. Seeing the difference of TC RAI from the composite in Fig. 10c, a positive RH anomaly of about 10 to 30% exists to the north of the TC. Examining the difference at subsequent time steps in Fig. 11, we can observe that the wind circulation around TC RAI is consistently greater than the composite TCs as manifested by the wind convergence to the center of TC RAI in all the difference plots from Figs. 10 to 13. Furthermore, Fig. 11 shows a consistently moist environment over central Philippines from LF to LF + 24. The wind vectors from this humid environment also points towards the TC circulation, which is conducive to the sustenance of TC energy, as well as a consistent moisture supply. To reiterate, high environmental moisture provides a favorable condition for TC intensification (Wu et al., 2015) and the abovementioned SH and RH of TC RAI are consistently high compared to the composite TC environment. This is in connection with the earlier discussion on Table 2, which showed TC RAI having the highest ACE and LFDR of about 0.02 (near zero), indicating that the TC had comparable intensities before and after landfall. In contrast, the composite TCs show a varied LFDR, three of the cases having a positive LFDR, indicating effective dissipation. The higher moisture content in central Philippines may also manifest itself in greater accumulated rainfall as shown in Figure S1, where there was at least 100 mm of accumulated rainfall along the path of TC RAI.

The influence of the western North Pacific Subtropical High (WNPSH) on the TC trajectory can also be seen in Fig. 12a. At LF-24, it can be observed that the TC is directly below the WNPSH, with its extension shown in the 576 gpm contour line in the 15–20°N band. Another signal for the remarkable WNPSH is its relationship to the low to mid-level easterly winds. These easterlies are located on the southern flank of the WNPSH. In the case of TC RAI, easterly winds were stronger than those in the composite. Hence, the aforementioned fields may have provided a stronger steering flow, driving TC RAI towards the lower latitudes and with a westward track throughout its traversal in the Philippines. This has been observed by Hung et al. (2020), where an intrusion of the WNPSH delays the recurvature of the TCs in Taiwan. This study has pointed out that extension of this system acts like a barrier to block the typhoons from recurving northwards. As such, TCs are now likely to be driven by the easterly flow, following the westward route (Hung et al., 2020). Similar to the composite and in the case of RAI, there is a lack of a westerly flow from the equatorial Southeast Asia as shown in Figs. 11c and d as the TC makes landfall in the Philippines. The lack of a strong and significant westerly flow from SCS allows the TC to delay its recurve and continue its straight and westward trajectory. This may likely explain the westward bearing of the TC until it reached the SCS, where it recurved northwards. Though the extension of the WNPSH is also manifested on the composite TCs as shown in the 576 gpm contour line, the higher low-level RH of TC RAI makes it unique compared to the composite TC.

Figure 13 shows that TC RAI’s vorticity is greater than the composite TCs. Moreover, the high humidity east of the Philippines contributed to the moisture supply of the TC, consistent with Figs. 10a and 12a. This increased low-level moisture is apparent in all the difference plots shown in Figs. 10c, 12c, and 13c, which show that TC RAI traverses an area of higher than usual RH. This positive RH anomaly across the low levels was also observed in the case study made by Chien and Kuo (2011) for MORAKOT (2009). Cross-examining Fig. 5a, this may likely explain the maintenance of TC intensity greater than 80 kts, throughout its traversal in central Philippines.

This study characterized TC RAI, which devastated northeastern Mindanao and central Philippines on December 16, 2021. In summary, the important findings are:

TC RAI made landfall in southern Philippines with an MSW of 105 kts and MSLP of 915 hPa having a 30 kt radius of 240 km. This TC had a high ACE, a near-zero LFDR, the second fastest speed among landfalling STYs in the Philippines from 1979–2020, and a westward track of 270° during landfall;

Environmental conditions were conducive to TC intensification before landfall, which included high SST, positive OHC anomaly, high SH, strong convergence, and high water vapor content. Despite the high vertical wind shear around its core, the TC was able to maintain its structure and intensify; and

Extension of the WNPSH with a strong easterly wind at the southern flank allowed for a westward track for the TC. A plentiful moisture supply from the Philippine Sea was drawn into the TC during approach and landfall, which resulted in heavy rainfall and energy sustenance.

The results further showed that the wind speed during LF of TC RAI was about 105 kts, which is the second highest MSW among TCs that made landfall in the Philippines. Comparing its MSLP among other 160 TCs that made landfall in the Philippines, TC RAI is in the 96th percentile. In terms of speed, the TC made landfall at about 16.7 kts, which is the second fastest when compared to other TCs in the STY category. A composite analysis of seven TCs with similar track and landfalling location in December shows that TC RAI had a more westward direction, a consistently larger TC radius after landfall, greater speed from LF to LF + 12, and greater intensity from LF-12 to LF + 24.

Analysis of the environmental conditions shows that the TC traversed through SSTs warmer than 29°C, and that the SSTa around the Philippine Sea during the TC event was about + 0.5 to 1.5°C, during a La Nina Season. The OHC shows a positive anomaly of about 123 ± 14 kJ cm^− 2 along the path of TC RAI during its intensification. Vertical wind shear near the TC core was estimated to be about 10–20 ms^− 1, and there exists a rich moisture content along the path of the TC. This resulted in greater moisture convergence upon landfall.

Comparison with the composite of seven TCs that made landfall in the area, with nearly similar track and month, reveals that the low-level RH during TC RAI was anomalously high at about 10 to 30% along the central and Eastern Philippines. This allowed the TC to draw moisture and maintain its strength. Moreover, the difference plots of the environmental condition during TC RAI and the composite show wind convergence at the center of TC RAI’s location throughout its traversal, indicating that the TC had stronger winds compared to the composites. The westward trajectory may also be attributed to the extension of the western WNPSH located along 20°N, which allowed TC RAI to maintain a straight and westward movement and delayed its recurving northward direction until it reached west of the Philippines. Moreover, robustness analysis using the JRA55 data set indicates the same trends (i.e. higher RH, extension of the WNPSH, higher SH, and higher moisture transport).

This study gave an initial overview of the characteristics of TC RAI in terms of speed, trajectory, and direction, as well as the environmental factors behind the event. These findings are schematically shown in Fig. 14. Further research such as idealized numerical simulations may provide a greater understanding of the environmental factors (i.e., SST, humidity, steering, etc.) that contributed to the intensification of the TC. A comparison of observed ground-based data from TC RAI (i.e., station data, doppler radar) with satellite and reanalysis data will help strengthen the conclusion about TC RAI’s unique features and those of future STYs that are bound to happen in the future.

Acknowledgements

Part of this study was supported by Grant-in-Aid for Scientific Research (No. 23H00030; PI Jun Matsumoto,20H01386; PI Yoshiyuki Kajikawa of Kobe University, and 22H04938; PI Kei Yoshimura of the University of Tokyo). L.M.P. Olaguera, C.E. Petilla, F.A.T. Cruz, and J.R.T. Villarin were supported by the Manila Observatory's project: High-Definition Clean Energy, Climate, and Weather Forecasts for the Philippines. Clint Eldrick R. Petilla is thankful for the scholarship provided by the Department of Science and Technology-Science Education Institute (DOST-SEI) and the Office of Admission and Aid of the Ateneo de Manila University.

The authors declare that they have no competing interests.

Competing Interests

The authors declare that they have no competing interests.

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The Unique Features of Typhoon Rai (2021): An observational study

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Abstract

Figures

1. Introduction

2. Data and Methodology

3. Results and Discussion

3.1. Observations of TC RAI

3.2. Unique Features of TC RAI

3.2.1. Translational Speed, Intensity, and Direction of Movement

3.2.2. Analysis of Environmental Factors

3.3. Analysis relative to the composite TCs

4. Summary and Conclusions

Declarations

References

Supplementary Files

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