The Role of Local Orography on the Development of a Severe Rainfall Event over Western Peninsular Malaysia: A Case Study

Abstract Severe rainfall events are common in western Peninsular Malaysia. They are usually short and intense, and occasionally cause flash floods and landslides. Forecasting these local events is difficult and understanding the mechanisms of the rainfall events is vital for the advancement of tropical weather forecasting. This study investigates the mechanisms responsible for a local heavy rainfall event on 2 May 2012 that caused flash floods and landslides using both observations and simulations with the limited-area high-resolution Met Office Unified Model (MetUM). Results suggest that previous day rainfalls over Peninsular Malaysia and Sumatra Island influenced the development of overnight rainfall over the Strait of Malacca by low-level flow convergence. Afternoon convection over the Titiwangsa Mountains over Peninsular Malaysia then induced rainfall development and the combination of these two events influenced the development of severe convective storm over western Peninsular Malaysia. Additionally, anomalously strong low-level northwesterlies also contributed to this event. Sensitivity studies were carried out to investigate the influence of the local orography on this event. Flattened Peninsular Malaysia orography causes a lack of rainfall over the central part of Peninsular Malaysia and Sumatra Island and produces a weaker overnight rainfall over the Strait of Malacca. By removing Sumatra Island in the final experiment, the western and inland parts of Peninsular Malaysia would receive more rainfall, as this region is more influenced by the westerly wind from the Indian Ocean. These results suggest the importance of the interaction between landmasses, orography, low-level flow, and the diurnal cycle on the development of heavy rainfall events.

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Evolution of physicochemical species concentration in streams based on heavy rainfall event data obtained for high-frequency monitoring

RBRH ◽

10.1590/2318-0331.011616055 ◽

2016 ◽

Vol 21 (4) ◽

pp. 653-665

Author(s):

Rubia Girardi ◽

Adilson Pinheiro ◽

Edson Torres ◽

Vander Kaufmann ◽

Luis Hamilton Pospissil Garbossa

Keyword(s):

Water Quality ◽

Water Level ◽

Atlantic Forest ◽

High Frequency ◽

Heavy Rainfall ◽

Rainfall Event ◽

Heavy Rainfall Event ◽

Rainfall Events ◽

Frequency Monitoring ◽

Heavy Rainfall Events

ABSTRACT Studies carried out over short time intervals assist in understanding the biogeochemical processes occurring relatively fast in natural waters. High frequency monitoring shows a greater variability in the water quality during and immediately after heavy rainfall events. This paper presents an assessment of the surface water quality parameters in the Atlantic Forest biome, caused by heavy rainfall events. The work was developed in two fluviometric sections of the Concordia River watershed, located in the state of Santa Catarina, southern Brazil. The spatial distribution of land use shows the predominance of Atlantic Forest in fluviometric section 1 (FS1) and pasture, forestry, agriculture, and Atlantic Forest in fluviometric section 2 (FS2). In each selected heavy rainfall event, the evolution rainfall height, the water level, and physicochemical parameters of water were analyzed. In all events, the water quality changed due to the heavy rainfall. After the events, an increase in water level and turbidity in both fluviometric sections were detected. In addition, the ammonium ion concentration increased in the river, and the pH value and nitrate concentration decreased. The electrical conductivity presented different behavior in each section. The dissolved oxygen concentration increased in 19 of 27 events. The principal component (PC1) correlated with the turbidity in FS1, and it correlated with level, turbidity, and pH in FS2.

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Evaluation of the Polarimetric-Radar Quantitative Precipitation Estimates of an Extremely Heavy Rainfall Event and Nine Common Rainfall Events in Guangzhou

Atmosphere ◽

10.3390/atmos9090330 ◽

2018 ◽

Vol 9 (9) ◽

pp. 330 ◽

Cited By ~ 7

Author(s):

Yang Zhang ◽

Liping Liu ◽

Hao Wen ◽

Chong Wu ◽

Yonghua Zhang

Keyword(s):

Heavy Rainfall ◽

Doppler Radar ◽

Heavy Rain ◽

Rainfall Event ◽

Retrieval Algorithm ◽

Heavy Rainfall Event ◽

Polarimetric Radar ◽

Rainfall Events ◽

Factors Affecting ◽

Hourly Rainfall

The development and application of operational polarimetric radar (PR) in China is still in its infancy. In this study, an operational PR quantitative precipitation estimation (QPE) algorithm is suggested based on data for PR hydrometeor classification and local drop size distribution (DSD). Even though this algorithm performs well for conventional rainfall events, in which hourly rainfall accumulations are less than 50 mm, the capability of a PR to estimate extremely heavy rainfall remains unclear. The proposed algorithm is used for nine different types of rainfall events that occurred in Guangzhou, China, in 2016 and for an extremely heavy rainfall event that occurred in Guangzhou on 6 May 2017. It performs well for all data of these nine rainfall events and for light-to-moderate rain (hourly accumulation <50 mm) in this extremely heavy rainfall event. However, it severely underestimated heavy rain (>50 mm) and the extremely heavy rain at stations where total rainfall exceeded 300 mm within 5 h in this extremely heavy rainfall event. To analyze the reasons for underestimation, a rain microphysics retrieval algorithm is presented to retrieve Dm and Nw from the PR measurements. The DSD characteristics and the factors affecting QPE are analyzed based on Dm and Nw. The results indicate that compared with statistical DSD data in Yangjiang (estimators are derived from these data), the average raindrop diameter during this rainfall event occurred on 6 May 2017 was much smaller and the number concentration was higher. The algorithm underestimated the precipitation with small and midsize particles, but overestimated the precipitation with midsize and large particles. Underestimations occurred when Dm and Nw are both very large, and the severe underestimations for heavy rain are mainly due to these particles. It is verified that some of these particles are associated with melting hail. Owing to the big differences in DSD characteristics, R(KDP, ZDR) underestimates most heavy rain. Therefore, R(AH), which is least sensitive to DSD variations, replaces R(KDP, ZDR) to estimate precipitation. This improved algorithm performs well even for extremely heavy rain. These results are important for evaluating S-band Doppler radar polarization updates in China.

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Numerical Study of the Rainfall Event due to the Interaction of Typhoon Babs (1998) and the Northeasterly Monsoon

Monthly Weather Review ◽

10.1175/2009mwr2757.1 ◽

2009 ◽

Vol 137 (7) ◽

pp. 2049-2064 ◽

Cited By ~ 32

Author(s):

Chun-Chieh Wu ◽

Kevin K. W. Cheung ◽

Ya-Yin Lo

Keyword(s):

Heavy Rainfall ◽

Cold Front ◽

The Philippines ◽

Rainfall Event ◽

Heavy Rainfall Event ◽

Rainfall Distribution ◽

Low Level ◽

Sensitivity Experiments ◽

Control Simulation ◽

Monsoon Strength

A heavy rainfall event in the Taiwan area associated with the interaction between Typhoon Babs (1998) and the East Asia winter monsoon is studied. Typhoon Babs is a case in point demonstrating the often-observed phenomenon that heavy rainfall can be induced in the eastern and/or northeastern region of Taiwan. Such heavy rainfall was caused by the joint convergent flow associated with the outer circulation of typhoons and the strengthening northeasterly monsoon in late typhoon season, even though Babs remained distant from Taiwan when it moved through the island of Luzon in the Philippines and stayed over the South China Sea. This heavy rainfall event is simulated in this study using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) with three nested domains and a highest horizontal resolution of 6.67 km. The control experiments with Kain–Fritsch cumulus parameterization perform well in terms of both simulated track and intensity. The 20-km resolution simulation reproduces the correct rainfall distribution during the three days studied, and the fine domain with 6.67-km resolution further improves the maximum simulated rainfall to very close to the observations. A series of sensitivity experiments that include model physics, terrain effect, typhoon vortex structure, and monsoon strength is performed, aiming at investigating the predictability of this typhoon–monsoon–terrain system when some of its components are perturbed. The rainfall event is analyzed based on two rainfall modes of different dominant mechanisms: monsoon mode during 0000 UTC 24–25 October and topographic mode during 0000 UTC 25–26 October. Removal of the Taiwan terrain in one of the sensitivity experiments results in completely different rainfall distribution due to the lack of the convection by orographic lifting, and the terrain is also found to play a key role in changing the low-level convergence pattern between the typhoon circulation and monsoonal northeasterlies. When the radius of the bogus vortex is reduced, the cold front to the north migrates southward in a faster pace than in the control simulation, and rain rate at the front also increases such that total accumulated rainfall at northern Taiwan is comparable with that in the control simulation but with shifted maximum position. In the extreme case in which no bogus vortex is implanted at all, rainfall is mainly associated with evolution of the cold front (pure frontal mode). In addition, a technique is developed to modify the monsoon strength over China. It is found that low-level (1000–700 hPa) reduction in monsoon strength weakens interaction with the typhoon, and rain distribution remains the same as in the control simulation. However, the simulated typhoon track is considerably sensitive to the deep-layer (1000–300 hPa) monsoon strength.

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Improvement of Heavy Rainfall Simulated with SST Adjustment Associated with Mesoscale Convective Complexes Related to Severe Flash Flood in Luwu, Sulawesi, Indonesia

Atmosphere ◽

10.3390/atmos12111445 ◽

2021 ◽

Vol 12 (11) ◽

pp. 1445

Author(s):

Erma Yulihastin ◽

Danang Eko Nuryanto ◽

Trismidianto ◽

Robi Muharsyah

Keyword(s):

Heavy Rainfall ◽

Flash Flood ◽

Wrf Model ◽

Flash Floods ◽

Heavy Rainfall Event ◽

Limited Area ◽

Pressure System ◽

The North ◽

Mesoscale Convective ◽

Persistent Heavy Rainfall

Flash flooding is an important issue as it has a devastating impact over a short time and in a limited area. However, predicting flash floods is challenging because they are connected to convection systems that rapidly evolve and require a high-resolution forecasting system. In addition, modeling a case study of a mesoscale convective complex (MCC) is the key to improving our understanding of the heavy rainfall systems that trigger flash floods. In this study, we aim at improving modeling skills to simulate a heavy rainfall system related to flash-flood-producing MCCs. We simulated a heavy rainfall event related to a severe flash flood in Luwu, Sulawesi, Indonesia, on 13 July 2020. This flood was preceded by persistent heavy rainfall from 11 to 13 July 2020. In this case, we investigated the role of sea surface temperature (SST) in producing the persistent heavy rainfall over the region. Therefore, we explore the physical and dynamic processes that caused the heavy rainfall using a convection-permitting model with 1 km resolution and an experiment comparing the situation with and without updated SST. The results show that the heavy rainfall was modulated by the development of a pair of MCCs during the night. The pair of MCCs was triggered by a meso-low-pressure system with an anti-cyclonic circulation anomaly over the Makassar Strait and was maintained by the warm front passing between the sea and land over central Sulawesi. This front was characterized by moist–warm and cold–dry low-level air, which may have helped extend the lifetime of the MCCs. The north-westward propagation of the MCCs was due to the interaction between predominantly a south-easterly monsoon and SST anomalies. This study suggested that the long-lived (>10 h) MCCs (>80,000 km2 cloud shield) and persistent precipitation are reproduced well in the updated SST scenario in the WRF model. This relatively simple technique in the running model provides a new strategy for improving flash flood forecasting by better predicting rainfall as an input in the hydrological model. Our findings also indicated a long-lived MCC maintained by back-building mechanisms from night to morning inland as an exceptional MCC, which does not correspond to a previous study.

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