Severe Rainfall Events Research Articles

Characteristics of Flood Flow with Active Sediment Transport in the Sozu River Flood Hazards at the Severe Rainfall Event in July 2018

This study reveals the characteristics of flood flow with active sediment transport that caused the Sozu River flood disaster in July 2018. The results of field surveys revealed that the basin contains steep mountains and flat areas, facilitating the rapid transport of sediment produced in the mountains to downstream areas during the disaster. Two-dimensional numerical simulations of flood flow with sediment transport are conducted to realize the flood flow characteristics. As a result of the simulation, a huge amount of sediment deposition in the channel near the top of the alluvial fan increases the inundation damage at downstream areas. The results indicate that within two flood peaks during the disaster, the inundation in the second flood peak is more severe than that in the first peak, whereas the discharge of the second peak is much less than that in the first peak, caused by the reduction in the capacity of the original channel owing to the sand deposition during the first and second flood peaks.

Terraced Landscapes and Hydrogeological Risk. Effects of Land Abandonment in Cinque Terre (Italy) during Severe Rainfall Events

During 25 October 2011, an extremely intense rainfall event occurred in Eastern Liguria and Northern Tuscany. Severe damages were registered in the Monterosso and Vernazza basins, located in the famous area of Cinque Terre, which have been affected by hundreds of landslides, mud flows, and erosions. The main feature of the Cinque Terre landscape is the presence of terraced cultivations on steep slopes facing the sea. The area represents a remarkable cultural landscape, is a National Park, and is included in the World Heritage List of the UNESCO. This work aims to analyze the effect of abandoned terraced land on hydrogeological risks and in landslide prevention, by comparing what happened in Cinque Terre to other experiences presented in scientific literature. The cessation of maintenance of dry stone terraces due to the crisis of traditional agriculture was identified as the main cause of failure during the heavy rainfall event. However, we found some contrasting and, in some cases, unexpected results in the literature regarding the effects of the vegetation that develops on abandoned terraces. This paper compares different results in order to better understand what the future of terraced landscapes is and which are the best management strategies for such complex and fragile territories. In particular, where they represent a cultural heritage and a resource for the rural economy.

Biogas Digestate Application Modifies Solute Transport Conditions in Soils and Increases the Release of Phosphorus

Core Ideas Severe rainfall events cause increased phosphorus release from soils. The proportion of DRP increases with increasing rainfall duration. Digestate application causes a 400‐fold increase in pore water P concentrations. The DRP fraction is clearly dominant immediately after fertilization. Digestate application facilitates a flux regime shift with preferential flow pathways. The application of fertilizers to soils may impair the quality of both surface and subsurface waters. Severe rainfall events that follow fertilizer application can increase P release from soils. In this study, suction plates were installed in a loamy soil with the aim of determining the P transport patterns and processes in the soil before and after the application of biogas digestate. Prior to the application of biogas digestate, the pore water concentrations of dissolved reactive P (DRP) were significantly lower than the total P (TP) concentrations, with DRP/TP ratios increasing from 0.03 to 0.89. After fertilizer application, the TP and DRP pore water concentrations increased 400‐fold. The DRP/TP ratio remained almost constant at 0.83. After 220 mm of precipitation, 4.5% of the P mass applied was leached to below the 50‐cm soil depth, indicating the general mobility of P in soils during high‐intensity rainfall. Brilliant Blue dye tracing experiments revealed that organic‐matter‐rich biogas digestate obviously changes the flow and transport patterns in soils from being homogenous to moderately heterogeneous, including preferential flow. Although the P loading across the flux plane had been equilibrated prior to treatment, after the application it was found to be concentrated in preferred transport regions. The emerging conclusion is that the risk of ground and subsequent surface water pollution with P after fertilization with biogas digestate originates not only from the applied P mass but also from the shift in the flow and transport regime caused by the organic‐matter‐rich slurry.

Comparison of assimilating all‐sky and clear‐sky infrared radiances from Himawari‐8 in a mesoscale system

This study examines the advantages of infrared all‐sky radiance (ASR) assimilation over traditional clear‐sky radiance (CSR) assimilation using a mesoscale LETKF data assimilation system. To effectively assimilate ASR data from the Himawari‐8 geostationary satellite, a cloud‐dependent quality‐control procedure and an observation error model were developed. A single humidity band to be assimilated and thinning distance were determined based on observation error statistics. The operational pre‐processing and parameter settings, such as observation errors and an adaptive bias correction for CSR, were incorporated into the LETKF data assimilation system.A comparison of the impacts of assimilating ASRs and CSRs was accomplished using single‐cycle and 10‐day cycle assimilation experiments. Study results revealed that ASR assimilation provided a higher degree of improvement in the first‐guess fit for conventional observations and satellite retrievals with respect to temperature, moisture and wind. Furthermore, ASR assimilation displayed a more stable improvement in the prediction of a severe rainfall event because it has more universal data coverage than CSR. Adaptive bias correction schemes with two different sets of predictors for ASRs were tested and revealed the difficulty in extracting additional information for the assimilation of no‐bias corrected ASR due to complicated bias factors. This was in contrast to the CSR assimilation, where bias correction had a positive impact.

Utilization of a C-band Polarimetric Radar for Severe Rainfall Event Analysis in Complex Terrain over Eastern China

Polarimetric radar measurements and products perform as the cornerstones of modern severe weather warning and nowcast systems. Two radar quantitative precipitation estimation (QPE) frameworks, one based on a radar-gauge feedback mechanism and the other based on standard rain drop size distribution (DSD)-derived rainfall retrieval relationships, are both evaluated and investigated through an extreme severe convective rainfall event that occurred on 23 June 2015 in the mountainous region over eastern China, using the first routinely operational C-band polarimetric radar in China. Complex rainstorm characteristics, as indicated by polarimetric radar observables, are also presented to account for the severe rainfall field center located in the gap between gauge stations. Our results show that (i) the improvements of the gauge-feedback-derived radar QPE estimator can be attributed to the attenuation correction technique and dynamically adjusted Z–R relationships, but it greatly relies on the gauge measurement accuracy. (ii) A DSD-derived radar QPE estimator based on the specific differential phase (KDP) performs best among all rainfall estimators, and the interaction between the mesocyclone and the windward slope of the mountainous terrain can account for its apparent overestimation. (iii) The rainstorm is mainly dominated by small-sized and moderate-sized raindrops, with the mean volume diameter being less than 2 mm, but its KDP column (KDP > 3°·km−1) has a liquid water content that is higher than 2.4815 g·m−3, and a high raindrop concentration (Nw) with log10(Nw) exceeding 5.1 mm−1m−3. In addition, small hailstones falling and melting are also found in this event, which further aggregates Nw upon the severe rainfall center in the gap between gauge stations.

Strategies to Maintain Natural Biocontrol of Soil-Borne Crop Diseases During Severe Drought and Rainfall Events.

In many parts of the world, agricultural ecosystems are increasingly exposed to severe drought, and rainfall events due to climate changes. This coincides with a higher vulnerability of crops to soil-borne diseases, which is mostly ascribed to decreased resistance to pathogen attacks. However, loss of the natural capacity of soil microbes to suppress soil-borne plant pathogens may also contribute to increased disease outbreaks. In this perspectives paper, we will discuss the effect of extreme weather events on pathogen-antagonist interactions during drought and rainfall events and upon recovery. We will focus on diseases caused by root-infecting fungi and oomycetes. In addition, we will explore factors that affect restoration of the balance between pathogens and other soil microbes. Finally, we will indicate potential future avenues to improve the resistance and/or recovery of natural biocontrol during, and after water stresses. As such, our perspective paper will highlight a knowledge gap that needs to be bridged to adapt agricultural ecosystems to changing climate scenarios.

MODELING SEWERAGE OVERFLOW IN AN URBAN RESIDENTIAL AREA USING STORM WATER MANAGEMENT MODEL

Storm Water Management Model (SWMM) is used in this study for simulating rainfall driven flow in a sewer system that can cause overflow in sewer networks. The simulation is carried out in a residential area located in the Karbala city of Iraq where storm water network is not fully developed. The study reveals that severe rainfall events cause sewer overflow and surface inundation in the area. A number of measures have been proposed including capacity improvement of sewer system, reduction of discharge peak and reduction of exfiltration as a countermeasure to solve the inundation problem. It is expected that the study will help in operational management of the sewerage system and mitigate the flooding problem.

Evaluation of Microphysics and Cumulus Schemes of WRF for Forecasting of Heavy Monsoon Rainfall over the Southeastern Hilly Region of Bangladesh

Accurate forecasting of heavy rainfall is crucial for the improvement of flood warning to prevent loss of life and property damage due to flash-flood-related landslides in the hilly region of Bangladesh. Forecasting heavy rainfall events is challenging where microphysics and cumulus parameterization schemes of Weather Research and Forecast (WRF) model play an important role. In this study, a comparison was made between observed and simulated rainfall using 19 different combinations of microphysics and cumulus schemes available in WRF over Bangladesh. Two severe rainfall events during 11th June 2007 and 24–27th June 2012, over the eastern hilly region of Bangladesh, were selected for performance evaluation using a number of indicators. A combination of the Stony Brook University microphysics scheme with Tiedtke cumulus scheme is found as the most suitable scheme for reproducing those events. Another combination of the single-moment 6-class microphysics scheme with New Grell 3D cumulus schemes also showed reasonable performance in forecasting heavy rainfall over this region. The sensitivity analysis confirms that cumulus schemes play a greater role than microphysics schemes for reproducing the heavy rainfall events using WRF.

Biodiversity bottleneck: seedling establishment under changing climatic conditions at the boreal–temperate ecotone

Climate change is increasing global temperatures, severe rainfall events, and the occurrence and severity of drought. Changes in global climate may have negative consequences for particular plant species and for biodiversity overall. In the short term, altered temperature and precipitation regimes may have the most severe effects on plant species near their range limits and in the earliest stages of plant development. To address these issues, we assessed seedling emergence, early survival, and growth of 18 boreal, temperate, and exotic woody species at the boreal–temperate forest ecotone in central Minnesota. We experimentally warmed forest plots to mimic projected warming by the end of the twenty-first century (+ 1.7 °C and + 3.4 °C). We also experimentally removed summer rainfall (~ 42% reduction) to simulate drought conditions in this region. We found that emergence and survival of boreal and exotic species was lower in experimentally warmed plots. This was exacerbated by drought. Temperate species emergence and survival was largely unaffected by climate manipulations (on average). Conversely, temperate seedling growth was greater in warmer conditions, but only when paired with drought. We found that overall seedling species richness was reduced by warming, mostly due to lower boreal and exotic species emergence and survival (conifers were also strongly negatively affected across species-range groups). If temperate seedling emergence and survival does not compensate for loss of boreal species, these forests may experience loss of biodiversity (and associated ecosystem functions) in the future.

Analysis of synoptic patterns in relationship with severe rainfall events in the Ebre Observatory (Catalonia)

The study has obtained a classification of the synoptic patterns associated with a selection of extreme rain episodes registered in the Ebre Observatory between 1905 and 2003, showing a return period of not less than 10 years for any duration from 5 min to 24 h. These episodes had been previously classified in four rainfall intensity groups attending to their meteorological time scale. The synoptic patterns related to every group have been obtained applying a multivariable analysis to three atmospheric levels: sea-level pressure, temperature, and geopotential at 500 hPa. Usually, the synoptic patterns associated with intense rain in southern Catalonia are featured by low-pressure systems advecting warm and wet air from the Mediterranean Sea at the low levels of the troposphere. The configuration in the middle levels of the troposphere is dominated by negative anomalies of geopotential, indicating the presence of a low or a cold front, and temperature anomalies, promoting the destabilization of the atmosphere. These configurations promote the occurrence of severe convective events due to the difference of temperature between the low and medium levels of troposphere and the contribution of humidity in the lowest levels of the atmosphere.

Effective approaches to extending medium-term forecasting of persistent severe precipitation in regional models

A set of methods designed to improve (i.e. extend) the medium-term forecasting of persistent severe rainfall (PSR) events in China using the regional Weather Research and Forecasting model are summ...

A mass-wasting dominated Quaternary mountain range, the Coastal Range in eastern Taiwan

Fluvial bedrock incision, which creates topographic relief and controls hillslope development, has been considered the key medium linking denudation and tectonic uplift of unglaciated mountains. This article, however, shows a different scenario from the Coastal Range in eastern Taiwan. This range, with the steepness inherited from pre-orogenic volcanoes, has been subject to mass wasting even before its emergence above sea level no earlier than Middle Pleistocene. Numerous terraced alluvial fans/fan deltas record the ancient mass movements of the range, including rock avalanches. Multiple radiocarbon dates <16 ka cal BP reveal the recurrence intervals of these movements of over several thousand years. The largest event is dated ∼15 ka cal BP, and the two second largest, 9–8 ka cal BP. These mass movements were sourced from ridges with minimum heights of 350–400 m, have sequences not clearly related to the known climate-change events, and are believed to have been triggered mainly by severe rainfall events, large earthquakes, or their combinations. The resulting fluctuation of sediment yield has episodically changed river behavior, forming river terraces in catchments >1 km2. Alluvial terraces are typically exhibited close to the source ridges of mass movements, and strath terraces along the downstream parts of rivers. Both were created when enormous sediment supply had exceeded or matched the prevailing river transport capacity. This process, along with the protection by giant boulders from mass movement, disturbed the long-term incision trend of rivers in response to tectonic uplift. As a result, the observed Holocene bedrock incision at most sites has not kept pace with the tectonic uplift. The spatial contrast in mass-wasting histories further accounts for the great diversity of the terrace sequences, even in areas with similar tectonic and base-level conditions.

A Combined Approach to Improving the Regional Model Forecasts for the Rainy Season in China

A combined forecasting methodology, into which the spectral nudging, lateral boundary filtering, and update initial conditions methods are incorporated, was employed in the regional Weather Research and Forecasting (WRF) Model. The intent was to investigate the potential for improving the prediction capability for the rainy season in China via using as many merits of the global model having better predictability as it does for the large-scale circulation and of the regional model as it does for the small-scale features. The combined methodology was found to be successful in improving the prediction of the regional atmospheric circulation and precipitation. It performed best for the larger magnitude precipitation, the relative humidity above 800 hPa, and wind fields below 300 hPa. Furthermore, the larger the magnitude and the longer the lead time, the more obvious is the improvement in terms of the accumulated rainfall of persistent severe rainfall events.

Geomorphic effects caused by heavy rainfall in southern Calabria (Italy) on 30 October–1 November 2015

ABSTRACTA severe rainfall event occurred in southern Calabria between 29 October and 2 November 2015, causing two deaths and serious damage to transport infrastructure. Widespread slope erosion and thousands of shallow landslides were triggered on the slopes, combined with flooding and transport of debris along the streams. Rains recorded by the regional gauge network and the national radar monitoring system were analysed by means of Kriging techniques. Ground effects were surveyed in the field, and mapped using post-event air photos taken along the coastal sector. Shallow landslides, soil erosion (including sheet, rill, and gully erosion), flooding, lateral erosion, and debris deposition along streambeds, overflow on lateral slopes, and fan deposition at the mouths of the streams were mapped at 1:60,000 scale. Isohyets of the rainfall event – in terms of cumulative maxima over 24 h – are also shown on the Main Map. In Annex A, a list of notices of processes and/or damage, arranged by municipalities, with coordinates, types of ground effect, and synthetic descriptions, is also provided.

Spatio-temporal changes in rate of soil loss and erosion vulnerability of selected region in the tropical forests of Borneo during last three decades

An attempt has been made to analyze the spatial-temporal characteristics of soil erosion vulnerability and soil loss from the forested region in the north-eastern Borneo, Sarawak, Malaysia during the last three decades (1991–2015) using the revised universal soil loss equation (RUSLE) and geographical information systems (GIS). The components of RUSLE such as rainfall erosivity (R), soil erodibility (K), slope-length and steepness (LS), cover management (C) and conservation practice (P) factors were grouped into two categories by keeping one set as temporally changing and others as static. Among them the R and C factors are calculated for the years 1991, 2001 and 2015 whereas the K and LS factors are considered for the single time frame. Because of the forested nature of the study area, the P factor is kept constant for the whole analysis. The R factor and C factor is shown changes in values and its distribution over the years, which reflected in the final soil loss and erosion vulnerability map as a change in the rate of erosion and spatial domain. The analysis of three time slices has shown that the maximum value of the soil loss per unit area i.e. at erosion hotspots, is relatively similar throughout at around 1636 to 1744 t/ha/y. This is the result of maximum values of R factor and C factor i.e. high rainfall erosivity combined with lack of vegetation cover in those hotspots, which are generally steeply sloping terrain. The reclassification of annual soil loss map into erosion vulnerability zones indicated a major increase in the spatial spread of erosion vulnerability from the year 1991 to 2015 with a significant increase in the high and critical erosion areas from 2.3% (1991) to 31.5% (2015). In 1991, over 84% of the study area was under low erosion vulnerability class but by the year 2015 only 12% was under low erosion vulnerability class. Moreover, a dynamic nature in the erosion pattern was found from the year 1991 to 2015 with more linear areas of land associated with higher rate of soil loss and enhanced erosion vulnerability. The linearity in the spatial pattern is correlated with the development of logging roads and logging activities which has been confirmed by the extraction of exposed areas from satellite images of different years of analysis. The findings of the present study has quantified the changes in vegetation cover from dense, thick tropical forest to open mixed type landscapes which provide less protection against erosion and soil loss during the severe rainfall events which are characteristic of this tropical region.

Atoll Groundwater Movement and Its Response to Climatic and Sea-Level Fluctuations

Groundwater resources of low-lying atoll islands are threatened due to short-term and long-term changes in rainfall, wave climate, and sea level. A better understanding of how these forcings affect the limited groundwater resources was explored on Roi-Namur in the Republic of the Marshall Islands. As part of a 16-month study, a rarely recorded island-overwash event occurred and the island’s aquifer’s response was measured. The findings suggest that small-scale overwash events cause an increase in salinity of the freshwater lens that returns to pre-overwash conditions within one month. The overwash event is addressed in the context of climate-related local sea-level change, which suggests that overwash events and associated degradations in freshwater resources are likely to increase in severity in the future due to projected rises in sea level. Other forcings, such as severe rainfall events, were shown to have caused a sudden freshening of the aquifer, with salinity levels retuning to pre-rainfall levels within three months. Tidal forcing of the freshwater lens was observed in electrical resistivity profiles, high-resolution conductivity, groundwater-level well measurements and through submarine groundwater discharge calculations. Depth-specific geochemical pore water measurements further assessed and confirmed the distinct boundaries between fresh and saline water masses in the aquifer. The identification of the freshwater lens’ saline boundaries is essential for a quantitative evaluation of the aquifers freshwater resources and help understand how these resources may be impacted by climate change and anthropogenic activities.

An Attempt to Improve the Forecasting of Persistent Severe Rainfall Using the Spectral Nudging and Update Cycle Methods

Abstract Using the interior spectral nudging and update cycle (SN+UIC) methods in the regional Weather Research and Forecasting (WRF) Model, the numerical predictions of four persistent severe rainfall (PSR) events during the preflood season in south China were investigated, based on the fact that the global model has an advantage in predicting the large-scale atmospheric variation and the regional model is better in terms of simulating small-scale changes. The simulation results clearly indicated that the SN+UIC improved the prediction of the PSR events’ daily precipitation for moderate, heavy, and torrential rains (10–100 mm day−1). It also improved the simulative forecasts of the two categories of rain with accumulated precipitation above 50 and 100 mm at lead times of 5–11 days. Moreover, the longer the forecast lead time is, the larger the decrease in the Brier score. Additionally, the SN+UIC method decreased the root-mean-square error for accumulated rainfall (6.2%) and relative humidity (5.67%).

Preliminary Test of a Data Assimilation System with a Regional High-Resolution Atmosphere–Ocean Coupled Model Based on an Ensemble Kalman Filter

AbstractAn ensemble Kalman filter (EnKF) that uses a regional mesoscale atmosphere–ocean coupled model was preliminarily examined to provide realistic sea surface temperature (SST) estimates and to represent the uncertainties of SST in ensemble data assimilation strategies. The system was evaluated through data assimilation cycle experiments over a one-month period from July to August 2014, during which time a tropical cyclone (TC) as well as severe rainfall events occurred. The results showed that the data assimilation cycle with the coupled model reproduced SST distributions realistically even without assimilating SST and sea surface salinity observations, and atmospheric variables provided to ocean models can, therefore, control oceanic variables physically to some extent. The forecast error covariance calculated in the EnKF with the coupled model showed dependency on oceanic vertical mixing for near-surface atmospheric variables due to the difference of variability between the atmosphere and the ocean as well as the influence of SST variations on the atmospheric boundary layer. The EnKF with the coupled model reproduced the intensity change of Typhoon Halong (2014) during the mature phase more realistically than with an uncoupled atmosphere model, although there remained a degradation of the SST estimate, particularly around the Kuroshio region. This suggests that an atmosphere–ocean coupled data assimilation system should be developed that is able to physically control both atmospheric and oceanic variables.

Improving the regional model forecasting of persistent severe rainfall over the Yangtze River Valley using the spectral nudging and update cycle methods: a case study

AbstractChina's worst flooding since 1998 occurred over the Yangtze River Valley from 30 June to 6 July 2016. This study investigated the event using a new method – the spectral nudging and update cycle (SN + UIC) – in the regional Weather Research and Forecasting model, fuller use of small‐scale features by using multi‐scale blending. The SN + UIC was found to be successful in improving the prediction of this persistent severe rainfall event; and the larger the magnitude and longer the lead time, the more obvious the improvement. It was also found that the use of this new method decreased the root‐mean‐square error for related meteorological variables.

On the Dynamics of the Large-Scale Circulation during the Persistent Severe Rainfall Events in Southern China

On the Dynamics of the Large-Scale Circulation during the Persistent Severe Rainfall Events in Southern China