ANALYSIS OF IONOSPHERIC TEC ANOMALIES DURING SUPER TYPHOONS MEGI AND MERANTI

In this paper, we detected and analyzed ionospheric anomalies during the 2016 super typhoons Megi and Meranti using the sliding interquartile range method applied to the ionospheric total electron content (TEC) grid data from the Global Ionospheric Model (GIM) provided by the Center for Orbit Determination in Europe (CODE). We found that ionospheric TEC anomalies occurred 2 days before the origination of super typhoon Megi. The anomalies occurred from 02:00 to 12:00 local time over a period of 10 h, with a maximum outlier of 12 TEC units over the typhoon center. Anomalies occurred in the ionosphere over both the area near the typhoon center and the corresponding equatorial magnetic conjugate region. The anomalies in both areas showed approximately the same trends. It is very likely that the disturbances from the ionospheric anomalies over the typhoon center were one of the precursors for the typhoon event. Similar ionospheric anomalies also occurred in the ionosphere 2 days before the formation and development of super typhoon Meranti...

Analysis of Combining SAR and Optical Optimal Parameters to Classify Typhoon-Invasion Lodged Rice: A Case Study Using the Random Forest Method

Lodging, a commonly occurring rice crop disaster, seriously reduces rice quality and production. Monitoring rice lodging after a typhoon event is essential for evaluating yield loss and formulating suitable remedial policies. The availability of Sentinel-1 and Sentinel-2 open-access remote sensing data provides large-scale information with a short revisit time to be freely accessed. Data from these sources have been previously shown to identify lodged crops. In this study, therefore, Sentinel-1 and Sentinel-2 data after a typhoon event were combined to enable monitoring of lodging rice to be quickly undertaken. In this context, the sensitivity of synthetic aperture radar (SAR) features (SF) and spectral indices (SI) extracted from Sentinel-1 and Sentinel-2 to lodged rice were analyzed, and a model was constructed for selecting optimal sensitive parameters for lodging rice (OSPL). OSPL has high sensitivity to lodged rice and strong ability to distinguish lodged rice from healthy rice. After screening, Band 11 (SWIR-1) and Band 12 (SWIR-2) were identified as optimal spectral indices (OSI), and VV, VV + VH and Shannon Entropy were optimal SAR features (OSF). Three classification results of lodging rice were acquired using the Random Forest classification (RFC) method based on OSI, OSF and integrated OSI–OSF stack images, respectively. Results indicate that an overall level of accuracy of 91.29% was achieved with the combination of SAR and optical optimal parameters. The result was 2.91% and 6.05% better than solely using optical or SAR processes, respectively.

The Effect of Storm-Induced Precipitation on Flooding in Macau City

The extent and depth of coastal flooding caused by tropical cyclone can be underestimated without considering the contribution by storm-induced precipitation. In this study, the dynamic flooding processes in Macau are simulated accounting for the combined effect of storm surge and precipitation during Typhoon Mangkhut in 2018. The surface wind and atmospheric pressure during this typhoon event are generated by a parametric vortex model and are used to drive a surge–tide–wave model for producing storm-induced water levels. The precipitation rate from ERA5 reanalysis data is utilized in this modelling suite as an additional source of water mass. The numerical model is first thoroughly validated by comparing the numerical results against the measured water levels and significant wave heights at tidal gauge and wave buoy stations. By switching on/off the rainfall function in the mass conservation and momentum equations, we compare and analyze the differences of inundation depths and inundation extent induced by precipitation. The differences in the maximum inundation depths vary spatially and the increments owing to precipitation are less than 50 mm in most flooded regions, the associated contribution is less than 10% in areas with moderate and severe flooding. The effect of precipitation as additional rainfall-induced forces in the momentum equations plays a more important role than that in the mass conservation. The inundation depths induced by tide and surge are approximately two times of that induced by precipitation. Moreover, the compound effects by ocean waves and precipitation tend to decrease the increment of precipitation-induced inundation depths by precipitation alone. Finally, we also assess the performance of the overland flow model Itzï on Macau Peninsula for the same typhoon event. The present study helps identifying vulnerable areas subject to heavy precipitation in Macau and gaining more understanding of flooding mechanisms by different physical drivers.

A New View on Risk of Typhoon Occurrence in the Western North Pacific

To study high impact tropical cyclone (TC) is of crucial importance due to its extraordinary destruction potential that leads to major losses in many coastal areas in the Western North Pacific (WNP). Nevertheless, because of the rarity of high-impact TCs, it is difficult to construct a robust risk assessment based on the historical best track records. This paper aims to address this issue by introducing a computationally simple and efficient approach, using data from the THORPEX Interactive Grand Global Ensemble (TIGGE) archive with the application of impact-oriented tracking algorithm, to build a physically consistent high impact typhoon event set with non-realised TC events – data equivalent to more than 10 000 years of TC events. The temporal and spatial characteristics of the new event set is consistent to the historical TC climatology in the WNP. It is shown that this TC event set contains ~ 100 and ~ 77 times more Very Severe Typhoons and Violent Typhoons than the historical records, respectively. Furthermore, this approach can be used to improve the return period estimation of TC-associated extreme wind. Consequently, a robust extreme TC hazard risk assessment, reflective of the current long-term climate variability phase, can be achieved using this approach.

Analysis of flooding potential with different return periods-A case study of Dianbao River in Kaohsiung City, Taiwan

<p>　　In Taiwan, when the rainy season comes, the extreme rainfall and typhoon events cause floods and economic losses in the middle and lower reaches, which impacts on the safety of people's lives. In this study, we took Dianbao River in Kaohsiung City as an example and simulated the rainfall-runoff in the upstream water catchment area based on the HEC-HMS model and used its results as the flow input condition of the FLO-2D model. The two models were validated by the Kongrey typhoon event in 2013 and the Megi typhoon event in 2016. In terms of upstream watershed, the analysis results of the HEC-HMS rainfall-runoff errors for the Kongrey typhoon and the Megi typhoon were as follows: percent errors of peak discharge (EQ<sub>P</sub>) were 0.6% and 4.6%, respectively; errors of time to peak (ET<sub>P</sub>) were 0 hour and 2 hours, respectively; coefficients of efficiency (CE) were 0.89 and 0.91, respectively. In the Dianbao River, the FLO-2D model error analysis results of Kongrey typhoon and Megi typhoon events were as follows: percent errors of peak water level (EW<sub>P</sub>) were 13.51% and 4.71%, respectively; errors of time to peak (ET<sub>P</sub>) were 1 hour and 0 hour, respectively; coefficients of efficiency (CE) were 0.69 and 0.79, respectively. The simulation and validation of the two typhoon-inundated areas were reasonable and then the model was applied to explore the flood potential of the Dianbao River during different flood return periods.</p><p><strong>Keywords</strong><strong>：</strong><strong>HEC-HMS</strong><strong>、</strong><strong>FLO-2D</strong><strong>、</strong><strong>rainfall-runoff</strong><strong>、</strong><strong>error analysis</strong><strong>、</strong><strong>flooding potential</strong></p>

Application of Multiple-Temporal Satellite Imagery with UAV Technology to evaluate Post-typhoon Event Remediation Efficiency in Shihmen Reservoir Watershed

<p>This study utilized multiple-temporal satellite imagery with UAV and IoT technology to evaluate and monitor the post-typhoon event remediation effectiveness of soil and water conservation of Shihmen Reservoir Watershed from 2015 to 2018.</p><p>A combination of the historical event-based landslide inventory and a collection and recent satellite imagery with coverage of the area pre- and post-typhoon MANGKHUT in 2018 were applied to evaluate landslide process, evolution and sediment environment change. In addition, two UAV operations were completed and captured over 160km2 in the 5 sub-watersheds to validate the remediation effectiveness and environmental change.</p><p>The results show that the landslide area within Shihmen Reservoir is less than that of the 1994 typhoon Aere and has no increased tendency. Effective conservation and remediation work can effectively reduce the sediment discharge of meteorological events and decrease the turbidity of the water at the storage point. In addition, the vegetation coverage rate of the entire Shihmen Reservoir watershed is close to 90%. Except for the occasional localized deforestation, the vegetation coverage has gradually stabilized.</p><p>Keywords: Shihmen reservoir, Remediation Efficiency, UAV and IoT Technology</p>