The Impact of the Typhoon to East Malaysia on Orographic Effect

Transcription

1 Proceeding of the 2011 IEEE International Conference on Space Science and Communication (IconSpace) July 2011, Penang, Malaysia The Impact of the Typhoon to East Malaysia on Orographic Effect Tan Fuyi, Lim Hwee San and Khiruddin Abdullah School of Physics Universiti Sains Malaysia 11800, Penang, Malaysia Abstract East Malaysia has few mountains ranges which are believed to affect the cloud and rainfall distribution patterns during severe weather especially typhoon season. The study objectives are to investigate and determine the relationship between the typhoon impacts and the topography of East Malaysia. The highest mountain range in East Malaysia was the Crocker Range in Sabah, which divides the state into half. Mountain ranges in the East tend to follow north-south or northeast-southwest paths, rising to over meters houses by Mount Kinabalu, the highest mountain in the country, as well as Mount Trusmadi (2642m) and Mount Tambuyukon (2579m), the second and third highest peak in the country. Typhoon Ketsana was chosen for this study. The hourly rainfall products were obtained from FengYun-2D geostationary satellite. Radiosonde data from Kota Kinabalu station with Skew-T diagram and hodograph were applied in this study. A total of 5 distinct results were found in this cloud and rainfall-orography interaction during the typhoon activity. First, parts of the cloud were blocked by the Crocker Range and caused to slow down of rainfall event for a moment. Second, the heavy rainfall occurred around these mountains range. Third, the variation of cloud and rainfall distribution pattern was related to the mountain shape. Fourth, if the raining cloud was high enough and there were extra outside energy (typhoon system) the rainfall process may be forced to move forward and pass through these mountains range. Besides that, heavy rainfall distribution was more affected by the high mountains close to the shoreline than the interior region of the island. Based on this study, orography effect was important to the weather forecaster since the high mountain range was able to influence the distribution of the cloud and even rainfall patterns during typhoon season. Keywords- East Malaysia; Orographic Effect; Typhoon; Rainfall; Cloud; mountains range I. INTRODUCTION Tropical cyclones (TCs) able to alter the synoptic scale circulation pattern throughout the entire troposphere leading to perturbations in the planetary scale circulation pattern which may have far reaching implications to those regions far away from the TC s center. This has a strong influence on the regional rainfall pattern, even to countries that were not directly on the path of these cyclones. The region covered by western North Pacific Ocean (WNP) and South China Sea (SCS) is the most active cyclogenesis basin with average of 27 cyclones per year and almost half of them reaching typhoon intensities [1]. In this basin, Philippines has been the most frequently hit by TCs and mostly they passed through Philippines Island and continued to travel through South China Sea (SCS) toward west or northwest continental region (Indo China or China). The direct treat of TC was practically nonexistent in Malaysia region but most of the tropical storms which formed over the WNP ocean always moved away from the Sabah shores and continue to move towards northwest direction have trigger the tail end effect of the TCs in term of strong wind and heavy rainfall as well as rough seas to East Malaysia. The effects were felt because East Malaysia was located between SCS and WNP. The effects were depended on the movement, intensity and position of the tropical storms. Typhoon-orography interaction studies have been done by many researchers in term of pressure, intensity and track variation. Topography-Typhoon relationship studies have been done by using a modified moist a geostrophic Q vector and data from Weather Research and Forecasting (WRF) model simulation [2]. By studying the typhoon Haitang of 2005, other researcher figure out the topographic lifting affects the formation of rainfall, whereas the rainfall amounts were forced by the topographic friction effect [2]. In briefly, typhoon rainfall maybe modified and enhanced by the topographic effects. Moreover, technique simulation of Penn State/NCAR Mesoscale Model Version 5 (MM5) was used to investigate dynamics of orographic rainfall associated with the Supertyphoon Bilis (2000) over Central Mountain Range (CMR) of Taiwan [3]. Air was forced up the CMR as the typhoon approached to coastal regions of the Taiwan, heavy rainfall were inevitable. However, results of moisture flux shown a large portion of rainfall over mountain regions was caused by orographically induced vertical motions and convergence. During Bilis event, the sounding data showed that the atmosphere was quite moist and conditionally unstable but in fact there exist a deep layer. Comparison of sounding results of eastern and western side of CMR showed that the air flow patterns were modified by the CMR at western side. By using radiosonde data, Skew-T diagram can indicate level of instability in the atmospheric and able to assess the potential for outbreak of severe weather. Meanwhile, the level of cloud formation and thickness of the cloud can be estimated from this diagram as well. And the wind variation with altitude can be viewed by vertical profile of the atmosphere. During TCs period, lifting condensation level (LCL) and level of free convection (LFC) heights generally /11/$ IEEE 83

2 quite low, these reflect the very moist and humid low-level conditions [4, 5]. The smaller of the LCL-LFC different will cause to stronger deep convection. By using multiparameters of the sounding data, the development of the storms can be predicted by investigating the unstable environment [6]. In order to well understand the wind behaviors in the air column, hodograph was very useful to determine vertical wind shear and vorticity in the atmosphere. Hodograph has been used to forecast severe thunderstorms [7] and also used to study the orography and topography effects on the wind variations [8, 9]. In addition, hodographs were applied in many TCs studies in order to figure out the behavior of the storms [10, 11]. Thus, hodograph was used as well in this study to investigate the relationship of the tropical storm on the orography effects. II. STUDY AREA East Malaysia consist of the states of Sabah and Sarawak. It is located between SCS and WNP which known as the most active cyclogenesis basin. Normally, TCs will travel away from the Sabah shore but will not make any landfalling into the island. Sometimes the tropical depression will form over SCS and the position was quite near to East Malaysia but it will not landfall to these region as well. However, strong wind and rough sea are always felt by the residents; heavy rains occur occasionally in coastal and mountain regions. This study was more focus on Sabah state. Sabah has few mountains ranges which were believed to affect the cloud and rainfall distribution patterns during severe weather especially typhoon season. The highest mountain range in East Malaysia was the Crocker Range, the backbone of Sabah, which divides the state in half. Mountain ranges in the East tend to follow north-south or northeast-southwest paths, rising to over meters houses by Mount Kinabalu, the highest mountain in the country, as well as Mount Trusmadi (2642m) located in the Trusmadi Range near the town of Tambunan and Mount Tambuyukon (2579m) about 12 km from Mount Kinabalu, the second and third highest peak in the country respectively (Table 1). The topography altitude of East Malaysia can be viewed in different colour code (Fig. 1). Thus, the position of high terrain aera can be clearly determined. III. METHODOLOGY The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data of global digital elevation model v001 (ASTER GDEM) were downloaded from and edited by PCI Geomatics software in order to produce an elevation map over the study area. The main objective of this study was to investigate the relationship of the typhoon with the orographic effects on Sabah region. Mountains information was acquired from and Typhoon Ketsana was one of the popular typhoons in year 2009 because it brought hard impacts to the Southeast Asia (SEA) countries (Fig. 2). Thus, the satellite data for rainfall variation during the typhoon periods was obtained from These satellite images were analyzed and the correlation between rainfall and the mountains region in East Malaysia was investigated. In order to understand the weather conditions in Sabah during Ketsana event, the radiosonde station located at Kota Kinabalu was chosen for this study purpose. The radiosonde data was obtained from Skew-T charts which describe the instability of the atmospheric and including many parameters can be used to determine the level of instability in an air column and assess the potential for an outbreak of severe weather. Since this chart was related to dew point temperature and environment temperature we can understand better the atmospheric condition during typhoon periods. Moreover, hodograph will be applied as well in order to determine the wind behavior. Fig. 1. The Digital Elevation Model of East Malaysia with latitude and longitude were edited with latitude 1 S-9 N and longitude E. 4- point star was represent sounding station at Kota Kinabalu with 5.93 N and E. TABLE I. MOUNTAINS AFFECTED TO THE RAINFALL DISTRIBUTION PATTERNS DURING CYCLOGENESIS OF TYPHOON KETSANA. State Mountain Elevation (m) Latitude (N) Longitude (E) Sabah Kota ~6.08 ~ Kinabalu Trusmadi ~5.55 ~ Tambuyukon ~6.22 ~ IV. RESULTS AND DISCUSSION Atmospheric dynamics were complicated during cyclogenesis since this was a synoptic system. In strong TCs such as typhoon activity has a strong influence on the entire tropospheric circulation and weather of the surrounding regions. Thus, in order to understand the impacts of Ketsana to East Malaysia, Skew-T and hodograph diagrams derived from sounding data (Kota Kinabalu sounding station) were used in this study (Fig. 3). Sounding data acquired at 00Z and 12Z by radiosondes carried aloft by weather balloons containing data on wind speed, temperature and dewpoint 84

3 profile through each level of the atmosphere until the balloon bursts. Skew-T charts display valuable measurements in graphic presentation of data. Furthermore, in order to better and easy to understand what the wind is doing in the atmosphere a graph is plotted to display the change of wind speed and direction with height (vertical wind shear) in a simple diagram so called as hodograph. The stability of the atmosphere and the weather status or conditions can be determined by analyzing the parameters in both Skew-T and hodograph diagrams. From the observation of Feng Yun 2D geostationary satellite images (rainfall product), strong wind and heavy rain invaded East Malaysia three times during the period of 05:15 to 16:15 UTC at 25 Sep 2009 (Fig. 4) and 20:15 UTC 25 Sep 2009 to 11:15 UTC 26 Sep 2009 (Fig. 5). Whereas the third invaded period (12:15 to 23:15 UTC on 26 Sep 2009) was not included in this study since the TCs on orographic effects was not significant. Hence, sounding data for 00Z and 12Z measured from 25 to 26 Sep in year 2009 at Kota Kinabalu, Sabah was acquired to investigate the atmospheric conditions. Sounding data (atmospheric behavior) are used to enhance and describe the satellite data (precipitation) to help us understand the impacts of the Ketsana to East Malaysia. exist in this air column. Thus, we expected the thunderstorm was coming due to the tail effect of the Ketsana which located over middle part of Philippines region. Fig. 2. The track and stage development of Typhoon Ketsana.*(JWTC) Before the tail effect reaching East Malaysia on September 25, 2009 at 00Z, sounding data at Kota Kinabalu station (refer to Fig. 3) showed that Lifted index (Li) was 3.88 so we can expect the thunderstorm was approaching since the greater the negative number of Li, the better the chance for thunderstorms and the greater threat for severe weather. In addition, Convective Available Potential Energy (CAPE) was 1786Jkg -1 indicated that the atmosphere was unstable at this moment; convective inhibition (CIN) from Skew-T showed value as many as -45.1Jkg -1 was represented that convection is developed and able to overcome for storm initiation. Besides that, the vertical wind shear veered with height from 1000mb to around 450mb. Even though, the vertical wind shear was not strong, due to the spinning updraft deep convection was believed to Fig. 3. Skew-T diagrams above show the instability of the atmosphere; hodographs display the change of the wind speed and direction with height (vertical wind shear). In order to view the variation of the atmosphere during Ketsana development, 4 charts were plotted at 00Z and 12Z for 25 to 26 Sep in year 2009 at Kota Kinabalu, Sabah. These figures were obtained from the University of Wyoming Web site. Based on the Fig. 4, Ketsana over Philippines strongly influenced the atmospheric circulation, thus the weather of East Malaysia was changed as well. From the Feng Yun 2D geostationary satellite data, the rainfall covered most of the 85

4 part of East Malaysia from UTC on 25 Sep Rainfall formed rapidly in East Malaysia and heavy rainfall occurred at northern part of Sabah region. During the formation of raining cloud (05:15 to 09:15UTC), rainfall distinctly distributed to the west side of the East Malaysia. This phenomenal was believed caused by the orographic effects. The light brown and yellow colour in Fig. 1 indicates the altitude of the East Malaysia s topography with m and m respectively. The lifted condensation level (LCL) from sounding data is the pressure level a parcel of air reaches saturation or once the relative humidity first reaches 100% in the parcel showed that the lifted air is condensed to be cloud base. LCL was around 926mb (~744m) at 00Z and 932mb (~685m) at 12Z on 25 Sep 2009 (Fig. 3). Thus, the high mountains in East Malaysia (refer to Fig. 1) have disturbed and blocked the movement of the raining cloud at 00Z and 12Z on 25 Sep 2009 since the cloud base at the moment was low enough. The 07:15UTC image shows the most significant effect of rainfall patterns affected by the high terrain (Fig. 4). Fig. 4. The rainfall (mm) images from UTC at 25 Sep 2009 were acquired from FY-2D geostationary satellite. The rainfall distribution patterns were varying with time. The significant synoptic variation to the atmosphere due to development of Ketsana was clearly shown in this period. The storm position was located at Philippines but the storm tail effect was reach to East Malaysia. The rain event in following hours also blocked by the mountain range until the continuous atmospheric circulation was strong enough to bring the raining cloud forward. So from these images we can notice that the raining structure moved forward by slow motion. Meanwhile, heavy rainfall developed over North Sabah and then the raining distribution area was getting larger and moving southward (Fig. 4). The possibility of the heavy rain over the northern of Sabah region were believed caused by i) Sabah is surrounded by open sea which may cause to moist in extreme condition when TC nearby Sabah region, and this phenomenal is proved by sounding data by using Skew-T diagram from 12Z on 25 Sep 2009 (Fig. 3) to show that dew point temperature and environment temperature line was close to each other; ii) a lot of water vapor brought along by the tail effect of Ketsana and the occurrence the vertical 86

5 wind shear at Kota Kinabalu area (00Z and 12Z from SkewT and hodograph respectively); iii) the updraft wind may enhance the formation of cloud ; iv) the impact was more distinct over mountains range region since the mountains structure might force the water vapor up to the top of mountains and condense in finally to form cloud. Even though the cloud base might be blocked by the high terrains and slowed down, then the cloud will pass over because there were caused by outer forces such as TCs activity. However, the rain event was weakening due to lack of water vapor supplied in inland regions. Fig. 5. The rainfall (mm) images from 20:15 UTC 25 Sep 2009 to 11:15 UTC 26 Sep 2009 were acquired from FY-2D geostationary satellite. At time moment, the storm landfall into Philippines and passed through it in finally then only continue travel to South China Sea toward westward. This is the second times of the cyclone tail effect to East Malaysia. The second hit by the Ketsana tail effects to East Malaysia were obvious than the first round. From Fig. 5, the outer circulation of the Ketsana shows that the rainfall distribution was forced to travel toward East Malaysia and was separated from the storm system especially at period of 20:15 23:15 UTC on Sep 25. Rainfall event came along 87

6 with this circulation forces to East Malaysia shoreline. At time 00:15 UTC the LCL over Kota Kinabalu was about 958mb (~442m) so the cloud base at this time moment was pretty low which the clouds motion were believed to be highly disturbed by the mountain ranges over the Sabah Island. The level of free convection (LFC) is the level at which a lifted parcel of air will become equal in temperature to that of the environmental temperature. LFC was only 924mb (~730m) at 00:15 UTC so this very small different between the LCL and LFC indicated the deep convection existed in this column [4, 5]. In addition, the very low LFC in Sabah indicate thunderstorms were very easily initiated and maintained over this region. This statement was supported by other researcher who stated that if LFC heights were lower than about 3000m or more than 700mb, thunderstorm was easily trigger in this condition [12]. Furthermore, the low level troposphere content very high relative humidity value; the CAPE value with 3258Jkg- 1 has shown the atmosphere was very instable and the large negative value for Li as well as large positive value for Sweat Index (SI) further explained that the severe weather was came. This entire symptom was due to the tail effect of the Ketsana. On the other hands, the variation of the rainfall distribution was dependent on the structure of the topography in East Malaysia especially in Sabah region. During 20:15 UTC (Fig. 5), rainfall cover the northern part of the Sabah and then at 21:15 to 23:15 UTC the cloud base encountered the Crocker Range in north-south and northeast-southwest paths and houses by Mount Kinabalu (4095.2m). Meanwhile, Mount Trusmadi (2642m) and Mount Tambuyukon (2579m) also influence the movement of the cloud and indirectly change to rainfall distribution patterns. The rainfall patterns were tending to follow this mountains structure in northeast-southwest path (20:15 23:15 UTC Sep 25) so the rain event was absent in the middle or inland of Sabah since raining cloud has been blocked by the Crocker Range. Furthermore, the rainfall distributions over Sabah from 04:15 to 06:15 UTC Sep 26 also almost had the same distribution patterns with 20:15 23:15 UTC Sep 25. However, the strong Ketsana circulation forced the cloud to move toward northeast direction and tend to travel according Ketsana circulation pattern (21:15 UTC Sep 25 to 06:15UTC Sep 26). Hence the raining cloud was forced to travel through or crawl over the mountain range. The raining cloud was clearly stuck by the high terrain region during 07:15 UTC Sep 26 and the rainfall distribution patterns are base on the mountain structure. The following hours 08:15 to 11:15 UTC raining activity had weaken due to lack of water supplying over inland region. After the end of the raining event, the sounding data at 12Z on Sep 26 (Fig. 3) shows that cloud base was hang at level 874mb (~1226m) and the CAPE value was only 92.65Jkg as well as Li has a positive value with 0.28 so these information indicated the thunderstorm was over. -1 V. CONCLUSION From this study, typhoon activities were believed to affect East Malaysia in term of cloud formation and rainfall distribution patterns with conditionally (such as the typhoon track or location, intensity and its diameter). The impacts of the TCs to East Malaysia are more significant when it is associated with orographic effects. The sounding data with multi-parameters can assist the weather forecasters to investigate the atmospheric conditions and then the other techniques such as satellite and radar can be utilized to determine the latest weather. ACKNOWLEDGEMENT The authors gratefully acknowledge the financial support from the RU grant, account number: 1001/PFIZIK/ and USM fellowship used to carry out this project. The authors appreciate the help from NASA for providing free ASTER GDEM data. Thanks are also extended to the National Satellite Meteorological Center (NSMC) from China Meteorological Administration (CMA) for providing the free satellite data used in this study. REFERENCES [1] C. J. N. Greg J. Holland, "Global Guide to Tropical Cyclone Forecasting (Global Overview)," [2] C. Yue, "Quantitative analysis of torrential rainfall associated with typhoon landfall: A case study of typhoon Haitang (2005)," Progress in Natural Science, vol. 19, pp , [3] Y.-L. L. Nicholas C. Witcraft, and Ying-Hwa Kuo, "Dynamics of Orographic Rain Associated with the Passage of a Tropical Cyclone over a Mesoscale Mountain," TAO, vol. 16, pp , [4] J. M. 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