Favorable Conditions for the Development of a Heavy Rainfall Event over Oahu during the 2006 Wet Period

Abstract During the 2006 wet period, as eastward-moving transient disturbances passed through a semipermanent low pressure system west of Hawaii, southerly winds east of the low strengthened bringing in higher than usual amounts of moisture from the deep tropics to Hawaii. All five heavy rainfall episodes during the wet period occurred during a southerly wind regime. Favorable conditions for the development of the Kahala Mall flood case on 31 March 2006 are examined. A high low-level θe axis across Hawaii indicated the existence of convective instability over Hawaii. A transient midlatitude trough extending southward merged with the semipermanent subtropical trough. The tropopause folding associated with the deepening subtropical trough contributed to the spinup of the Kona low. The advection of high-PV air in the upper troposphere enhanced upward motion downstream over Hawaii. The Weather Research and Forecasting Model (WRF) simulation shows that latent heat release contributed to an eastward shift of the moisture tongue and enhanced moisture convergence at low levels. The horizontal distributions of instability indices, especially the K index, from WRF modeling can provide useful forecast guidance for the development of heavy rainfall. On 31 March, heavy rainfall occurred on the lee side of the Ko’olau Mountain Range with maximum rainfall at the summit as a convective line followed by an intense storm moved inland along the south shore and continued to advance northward through the range. As the convective cells moved across the mountain range, radar echoes intensified with deeper echo tops and higher vertically integrated liquid water content.

Download Full-text

Mesoscale processes for super heavy rainfall of Typhoon Morakot (2009) over Southern Taiwan

Atmospheric Chemistry and Physics ◽

10.5194/acp-11-345-2011 ◽

2011 ◽

Vol 11 (1) ◽

pp. 345-361 ◽

Cited By ~ 20

Author(s):

C.-Y. Lin ◽

H.-m. Hsu ◽

Y.-F. Sheng ◽

C.-H. Kuo ◽

Y.-A. Liou

Keyword(s):

Heavy Rainfall ◽

Large Scale ◽

Specific Humidity ◽

Mountain Range ◽

Typhoon Morakot ◽

Data Set ◽

The North ◽

Wave Trains ◽

Convective Cells ◽

Mesoscale Processes

Abstract. Within 100 h, a record-breaking rainfall, 2855 mm, was brought to Taiwan by typhoon Morakot in August 2009 resulting in devastating landslides and casualties. Analyses and simulations show that under favorable large-scale situations, this unprecedented precipitation was caused first by the convergence of the southerly component of the pre-existing strong southwesterly monsoonal flow and the northerly component of the typhoon circulation. Then the westerly component of southwesterly flow pushed the highly moist air (mean specific humidity >16 g/kg between 950 and 700 hPa from NCEP GFS data set) eastward against the Central Mountain Range, and forced it to lift in the preferred area. From the fine-scale numerical simulation, not only did the convergence itself provide the source of the heavy rainfall when it interacted with the topography, but also convective cells existed within the typhoon's main rainband. The convective cells were in the form of small rainbands perpendicular to the main one, and propagated as wave trains downwind. As the main rainband moved northward and reached the southern CMR, convective cells inside the narrow convergence zone to the south and those to the north as wave trains, both rained heavily as they were lifted by the west-facing mountain slopes. Those mesoscale processes were responsible for the unprecedented heavy rainfall total that accompanied this typhoon.

Download Full-text

Analysis and Simulations of a Heavy Rainfall Event Associated with the Passage of a Shallow Front over Northern Taiwan on 2 June 2017

Monthly Weather Review ◽

10.1175/mwr-d-21-0113.1 ◽

2021 ◽

Keyword(s):

Heavy Rainfall ◽

Heavy Rainfall Event ◽

Mountain Range ◽

Cold Air ◽

Taipei Basin ◽

Resolution Model ◽

Northern Side ◽

High Resolution Model ◽

River Valleys ◽

Northern Taiwan

Abstract From 0200 to 1000 LST 2 June 2017, the shallow, East-West oriented Mei-Yu front (< 1 km) cannot move over the Yang-Ming Mountains (with peaks ∼ 1120 m) when it first arrives. The postfrontal cold air at the surface is deflected by the Yang-Ming Mountains and moves through the Keelung River and Tamsui River valleys into the Taipei Basin. The shallow northerly winds are anchored along the northern side of the Yang-Ming Mountains for 8 hours. In addition, the southwesterly barrier jet with maximum winds in the 900–950-hPa layer brings in abundant moisture and converges with the northwesterly flow in the southwestern flank of the Mei-Yu frontal cyclone. Therefore, torrential rain (> 600 mm) occurs over the northern side of the Yang-Ming Mountains. From 1100 to 1200 LST, with the gradual deepening of the postfrontal cold air, the front finally passes over the Yang-Ming Mountains and arrives at the Taipei Basin, which results in an E-W oriented rainband with the rainfall maxima over the northwestern coast and Taipei Basin. From 1300 to 1400 LST, the frontal rainband continues to move southward with rainfall over the northwestern slopes of the Snow Mountains. In the prefrontal southwesterly flow, the orographic lifting of the moisture-laden low-level winds results in heavy rainfall on the southwestern slopes of the Snow Mountains and the Central Mountain Range. With the terrain of the Yang-Ming Mountains removed in the high-resolution model, the Mei-Yu front moves quickly southward without a rainfall maximum over the northern tip of Taiwan.

Download Full-text

The Okhotsk-Japan Circulation Pattern and the Heavy Rainfall in Beijing in 2012 Summer

Advances in Meteorology ◽

10.1155/2015/483970 ◽

2015 ◽

Vol 2015 ◽

pp. 1-10 ◽

Cited By ~ 1

Author(s):

Yafei Wang ◽

Jianzhao Qin ◽

Lijuan Zhu

Keyword(s):

Heavy Rainfall ◽

Large Scale ◽

Circulation Pattern ◽

Northern China ◽

Reanalysis Data ◽

Heavy Rainfall Event ◽

Baikal Lake ◽

Ural Mountains ◽

Pressure System ◽

Blocking High

Using station precipitation and reanalysis data, we examined the evolution of the large-scale circulations associated with the heavy rainfall event that occurred around July 21, 2012 (721 heavy rainfall). This study focuses on a role that the large-scale circulations named “the Okhotsk-Japan (OKJ) circulation pattern” played in causing the heavy rainfall case. We found that the 721 heavy rainfall occurred under a background of the OKJ circulation that persisted for about 10 days. However, the pattern was different from the normal OKJ circulation, for this circulation pattern accompanied a blocking high between the Ural Mountains and the Baikal Lake. This difference resulted from the seasonal change of the basic flow. The related Rossby wave propagated eastward during the persisting period of the dominated OKJ pattern. This caused the development of a low-pressure system around the Baikal Lake and the weakening of a ridge around the Okhotsk Sea. The slow evolution of the OKJ circulation created a favorable environment for the moisture transport to northern China, assisting in the generation of the 721 heavy rainfall.

Download Full-text

Comparison between 3D-Var and 4D-Var data assimilation methods for the simulation of a heavy rainfall case in central Italy

Advances in Science and Research ◽

10.5194/asr-14-271-2017 ◽

2017 ◽

Vol 14 ◽

pp. 271-278 ◽

Cited By ~ 7

Author(s):

Vincenzo Mazzarella ◽

Ida Maiello ◽

Vincenzo Capozzi ◽

Giorgio Budillon ◽

Rossella Ferretti

Keyword(s):

Data Assimilation ◽

Heavy Rainfall ◽

Hydrological Cycle ◽

Central Italy ◽

Probability Of Detection ◽

Precipitation Forecast ◽

Heavy Rainfall Event ◽

Tyrrhenian Sea ◽

Pressure System ◽

The Impact

Abstract. This work aims to provide a comparison between three dimensional and four dimensional variational data assimilation methods (3D-Var and 4D-Var) for a heavy rainfall case in central Italy. To evaluate the impact of the assimilation of reflectivity and radial velocity acquired from Monte Midia Doppler radar into the Weather Research Forecasting (WRF) model, the quantitative precipitation forecast (QPF) is used.The two methods are compared for a heavy rainfall event that occurred in central Italy on 14 September 2012 during the first Special Observation Period (SOP1) of the HyMeX (HYdrological cycle in Mediterranean EXperiment) campaign. This event, characterized by a deep low pressure system over the Tyrrhenian Sea, produced flash floods over the Marche and Abruzzo regions, where rainfall maxima reached more than 150 mm 24 h−1.To identify the best QPF, nine experiments are performed using 3D-Var and 4D-Var data assimilation techniques. All simulations are compared in terms of rainfall forecast and precipitation measured by the gauges through three statistical indicators: probability of detection (POD), critical success index (CSI) and false alarm ratio (FAR). The assimilation of conventional observations with 4D-Var method improves the QPF compared to 3D-Var. In addition, the use of radar measurements in 4D-Var simulations enhances the performances of statistical scores for higher rainfall thresholds.

Download Full-text

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.

Download Full-text

Statistical Validation of the Predicted Amount and Start Time of Heavy Rainfall in 2015 Based on the VIL Nowcast Method

Journal of Disaster Research ◽

10.20965/jdr.2019.p0248 ◽

2019 ◽

Vol 14 (2) ◽

pp. 248-259 ◽

Cited By ~ 2

Author(s):

Koyuru Iwanami ◽

◽

Kohin Hirano ◽

Shingo Shimizu

Keyword(s):

Experimental Data ◽

Liquid Water ◽

Heavy Rainfall ◽

Heavy Rain ◽

Liquid Water Content ◽

Rainfall Amount ◽

Start Time ◽

Statistical Validation ◽

Evaluation Period ◽

Convective Cells

We statistically evaluated the rainfall amount predicted by VIL Nowcast, which is designed using vertically integrated liquid water content (VIL) through experimental data obtained over five months from June to October of 2015. The accuracy of predictions for the start time of heavy rain, which are vital for issuing warnings concerning localized heavy rain, was also reviewed. We revealed that VIL Nowcast could predict the rainfall amount more accurately than conventional methods up to the first 20 min of the evaluation period (30 min in total) with superior accuracy for the start time of severe rain from isolated convective cells in the first 10 min.

Download Full-text