Storm Of Cycle Research Articles

Magnetospheric Time‐History in Storm‐Time Magnetic Flux Dynamics: A Global Simulation Campaign

AbstractThis paper aims to quantify the magnetospheric magnetic flux contents under moderate to intense space weather conditions using global simulations. This study is a companion to Akhavan‐Tafti, Atilaw, et al. (2023, https://doi.org/10.1029/2023JA031832) where magnetic flux evolution is presented for a catalog of storm events, using Heliophysics System Observatory (HSO) observations. For this study, we used the Space Weather Modeling Framework (SWMF) in Geospace configuration to study magnetic flux dynamics for a subset of their storm events (15 events). Simulations reliably resolve the storm‐time magnetic flux Bz and current density |J| asymmetries across the different storm phases. It is revealed that: relative to the quiet period, flux content is enhanced during the storm sudden commencement (SSC) phase in the dayside by ΔBz/Bz, quiet = +17%, and reduced in the nightside magnetosphere (r[RE] < −6 RE) by −15%. At the same time, the cross‐tail current is found to enhance (|J| = 2 nA/m2), which suggests the storm impact in the nightside magnetosphere is much earlier in the storm cycle than previously shown. Concurring with previous studies, a significant depletion of magnetic flux by up to −40%, with day‐night and dawn‐dusk asymmetries, can be seen during the main and recovery phases. This corresponds to the enhanced current density (|J| = 5–8 nA/m2) at ∼6 RE further confirming the role of ring current in driving magnetospheric dynamics during the main and recovery phases. This is in contrast with the SSC phase wherein the Chapman‐Ferraro and cross‐tail currents are the dominant current systems.

Near-field variability of evaposublimation in a montane conifer forest

Methods that combine in-situ measurements, statistical methods, and model simulations with remotely sensed data provide a pathway for improving the robustness of surface flux products. For this research, we acquired eddy-covariance fluxes along a five-tower transect in a snowy montane forest over three consecutive winters to characterize near-field variability of the subcanopy environment. The novel experiment design enabled discriminating near-field evaposublimation sources. Boosted regression trees reveal that the predictive capacity of state variables change with season and storm cycle frequency. High rates of post-storm evaposublimation of canopy-intercepted snow at this site were constrained by short residence time of snow in the canopy due to throughfall and melt. The snow melt-out date for open vs. closed canopy conditions depended on total snowfall accumulation. Compared with low accumulation years, the snow melt-out date under the dense canopy during the high accumulation winter was later than for the open area, as shading became more important later in the season. The field experiments informed an environmental response function that was used to integrate ERA5-Land latent heat flux data at 20-km nominal resolution with USFS Tree Canopy Fraction data at 30-m resolution and showed near-field flux variability that was not resolved in model simulations. Previous evaposublimation results from experiments in alpine and subalpine environments do not directly translate to a montane forest due to differences in process rates.

Statistical relevance of meteorological ambient conditions and cell attributes for nowcasting the life cycle of convective storms

AbstractThe usually short lifetime of convective storms and their rapid development during unstable weather conditions makes forecasting these storms challenging. It is necessary, therefore, to improve the procedures for estimating the storms' expected life cycles, including the storms' lifetime, size, and intensity development. We present an analysis of the life cycles of convective cells in Germany, focusing on the relevance of the prevailing atmospheric conditions. Using data from the radar‐based cell detection and tracking algorithm KONRAD of the German Weather Service, the life cycles of isolated convective storms are analysed for the summer half‐years from 2011 to 2016. In addition, numerous convection‐relevant atmospheric ambient variables (e.g., deep‐layer shear, convective available potential energy, lifted index), which were calculated using high‐resolution COSMO‐EU assimilation analyses (0.0625°), are combined with the life cycles. The statistical analyses of the life cycles reveal that rapid initial area growth supports wider horizontal expansion of a cell in the subsequent development and, indirectly, a longer lifetime. Specifically, the information about the initial horizontal cell area is the most important predictor for the lifetime and expected maximum cell area during the life cycle. However, its predictive skill turns out to be moderate at most, but still considerably higher than the skill of any ambient variable is. Of the latter, measures of midtropospheric mean wind and vertical wind shear are most suitable for distinguishing between convective cells with short lifetime and those with long lifetime. Higher thermal instability is associated with faster initial growth, thus favouring larger and longer living cells. A detailed objective correlation analysis between ambient variables, coupled with analyses discriminating groups of different lifetime and maximum cell area, makes it possible to gain new insights into their statistical connections. The results of this study provide guidance for predictor selection and advancements of nowcasting applications.

Statistical analysis on the ionospheric response over South American mid- and near high-latitudes during 70 intense geomagnetic storms occurred in the period of two decades

The first-time statistical response of the positive and negative ionospheric storms phases using Vertical Total Electron Content (VTEC) measurements during 70 geomagnetic storms at near high- and mid-latitudes regions in the Antarctic and Argentine/Chilean sectors in the Southern hemisphere are investigated. The study covers the years between 1999 and 2018 of solar cycles 23 and 24, using the Dst ≤ −100 nT as a criterion for all 70 storms selected. Significant features of solar cycle, seasonal and local time of ionospheric storms are showed. Our results indicate that the occurrence of geomagnetic storms follows a pattern of solar activity dependence, and also indicate a predominance of positive and positive-negative phases during autumn, winter, and spring at mid-latitudes and winter at near high-latitudes. Negative and negative-positive phases occur during all seasons at near high- and mid-latitudes. In addition, positive phases occur more frequently during the daytime while the negative phases occur predominantly in nighttime. There is also a predominance of positive and positive-negative phases simultaneously at near high- and mid-latitudes in the Antarctic and Argentine/Chilean sectors. The percentages of occurrence of positive and positive-negative phases are of 50% and 19%, respectively, at mid-latitude and 60% and 22%, respectively, at near high-latitudes. Negative and negative-positive phases are below 9% at both latitudes.

Role of Thermodynamics and Dynamics in the Diurnal Cycle, Propagation, and Progression of Convective Storms in the Eastern Flank of the Indian Monsoon Trough

Abstract This study investigates the diurnal cycle, propagation, and progression of convective storms (CSs) on the eastern edge of India’s monsoon trough (MT) using 9 years of S-band radar measurements with satellite and reanalysis datasets. CSs initiate over ocean during midnight–early morning hours and propagate onshore in succeeding hours. CSs exhibit two semidiurnal peaks, one during afternoon hours over inland areas and another during midnight–early morning hours in oceanic/coastal locations. The deep and intense afternoon peak over inland regions is attributed to land surface heating and associated destabilization. The weak and shallower but organized midnight–morning peak and propagation of CSs toward the coast are attributed to the nocturnal land breeze and its interaction with prevailing onshore flow. The observed lead–lag of a few hours in the diurnal cycle of different cumulus modes correspond to the transition of congestus into deep and then, often, into overshooting modes. Moisture budget analysis showed atmospheric regulation of this transition through thermodynamic (congestus moistening) and dynamic processes (vertical advection). Theoretical time scales were invoked to estimate the relative role of vertical advective versus congestus moistening for promoting the afternoon transition from congestus to deeper modes. Comparing the time scales for congestus moistening (18–46 h) and dynamics (3 h) with the actual transition time scales (2–4 h) reveal that congestus moistening is too slow to explain the observed lead–lag in CS modes. Though both thermodynamic and dynamic processes moisten the midlevel prior to deep/overshooting convection, vertical advection is the dominant dynamic process for the observed congestus–deep–overshooting transition. Significance Statement Tropical rainfall is usually linked with convection in the morning and afternoon hours. We look at the basic physical processes that lead to those convective activities peaks. The afternoon peak is linked to maximum heating, resulting in an unstable environment, whereas the morning peak is linked to the interaction of large-scale monsoon flow with a land breeze. Furthermore, daily solar heating visually shows a shallow-to-deep progression of convection. The moist midlevel environment was shown to precede such convective development in a day. The large-scale monsoon flow is a dominant cause of this moistening. The monsoon dynamic flow takes roughly 2–3 h to sufficiently moist shallow storms into deep storms, whereas the local thermodynamic moistening process takes about 18–46 h.

Analyzing vertical dust distribution and associated meteorological characteristics over Acidalia Planitia during a regional and global dust event

This study analyzed the vertical distribution of dust and associated atmospheric structural changes over Acidalia Planitia during one regional dust event (RDE) (MY 32, Ls = 220°) and one global dust event (GDE) (MY 28, Ls = 260°), using Mars Climate Sounder observations and Mars Weather Research and Forecasting (MarsWRF) model simulations. Multilinear Regression Coefficient (MLRC) analysis suggests that dustiness at 25–35 km and ~ 40–50 km altitudes contributed significantly to the column integrated opacity during the RDE and GDE, respectively. Both dust events reduced water ice opacity at ~40 km altitude. The atmosphere subsequently warmed ~10 K during the RDE and ~ 30 K during the GDE because of dust radiative heating. An inversion layer formed below ~20 km altitude during RDE due to the combined effect of reduced surface temperature and the downwelling radiation from suspended dust. However, the GDE's much larger opacity at higher altitudes helped form a similar inversion layer at 40 km. This atmospheric warming with the inversion layer below could be associated with heating/cooling layers centered around 35–50 and 20–35 km heights, influencing the variability of water ice within them. The MarsWRF simulations showed downwelling over Acidalia Planitia at ~35–50 km altitude, which supports the presence of the heating layer due to the suspended atmospheric dust during the GDE. However, findings from the heating rate analysis indicated a dominance of dust radiative heating compared to adiabatic heating due to compression on the atmospheric warming during the dust storm occurrences. The simulated boundary layer height and surface radiation flux suggest weaker vertical mixing from the surface and a surface energy budget dominated by downward radiation from the suspended dust, which helped form heating/cooling layers and drove variability in water ice clouds. The Empirical Orthogonal Function analysis (carried out using MCS observations) suggests that the seasonal cycle of the southern hemispheric dust storms, northern hemispheric active storm track, and the cap-edge storms possibly influenced the seasonality observed in the heating/cooling layer clouds.

Relation between Dst* and interplanetary parameters during single-step geomagnetic storms

In this work, the relation between Dst* (solar wind pressure corrected Dst) and solar wind parameters (north–south component of the interplanetary magnetic field Bz and east–west component of the electric field Ey) in the main phase of the single-step geomagnetic storms was analyzed on their peak (Dst*p and Bzp/Eyp) and difference values (ΔDst*p and ΔBz/ΔEy). From 1995 to 2019, 133 storms with peak Dst* ≤ −50 nT were selected. It was found that the correlations between Dst*p and Bzp/Eyp (r = 0.60 and −0.69) were higher than those between Dst*p and ΔBz/ΔEy (r = −0.49 and −0.45, respectively). The correlation between the variations of ΔDst* and ΔBz/ΔEy were intermediate between those ranges (r = 0.52 and 0.45, respectively). Multiple linear correlation of Dst*p with Bzp/ΔBz and Eyp/ΔEy explains 38 and 48% of the variance of Dst*p, respectively, with a larger effect of peak values of solar wind parameters. There were twice more single-step storms in cycle 23 than in cycle 24, but the average values of peaks and variations did not show significant differences. In terms of Eyp criteria for moderate (−100 nT < Dst*p ≤ −50 nT) and intense (Dst*p ≤ 100 nT) geomagnetic storms, >93% of the moderate storms had Eyp > 3 mV·m−1, while 96% of intense storms had Eyp > 5 mV·m−1. On ΔEy , 87% of the moderate storms had ΔEy ≤ 10 mV·m−1, while 72% of the intense storms had ΔEy > 10 mV·m−1. Thus it may be concluded that Bzp and Eyp were important for Dst*p of the single-step storms and that Eyp and ΔEy values could be interplanetary criteria for the moderate and intense storms.

New Insights into the South American Low-Level Jet from RELAMPAGO Observations

AbstractThe Remote sensing of Electrification, Lightning, And Mesoscale/microscale Processes with Adaptive Ground Observations (RELAMPAGO) campaign produced unparalleled observations of the South American low-level jet (SALLJ) in central Argentina with high temporal observations located in the path of the jet and upstream of rapidly growing convection. The vertical and temporal structure of the jet is characterized using 3-hourly soundings launched at two fixed sites near the Sierras de Córdoba (SDC), along with high-resolution reanalysis data. Objective SALLJ identification criteria are applied to each sounding to determine the presence, timing, and vertical characteristics of the jet. The observations largely confirm prior results showing that SALLJs most frequently come from the north, occur overnight, and peak in the low levels, though SALLJs notably peaked higher near the end of longer-duration events during RELAMPAGO. This study categorizes SALLJs into shorter-duration events with jet cores peaking overnight in the low levels and longer 5–6-day events with elevated jets near the end of the period that lack a clear diurnal cycle. Evidence of both boundary layer processes and large-scale forcing were observed during shorter-duration events, whereas synoptic forcing dominated the longer 5–6-day events. The highest amounts of moisture and larger convective coverage east of the SDC occurred near the end of the 5–6-day SALLJ events.Significance StatementThe South American low-level jet (SALLJ) is an area of enhanced northerly winds that likely contributes to long-lived, widespread thunderstorms in Southeastern South America (SESA). This study uses observations from a recent SESA field project to improve understanding of the variability of the SALLJ and the underlying processes. We related jet occurrence to upper-level environmental patterns and differences in the progression speed of those patterns to varying durations of the jet. Longer-duration jets were more elevated, transported moisture southward from the Amazon, and coincided with the most widespread storms. These findings enable future research to study the role of the SALLJ in the life cycle of storms in detail, leading to improved storm prediction in SESA.

Mechanisms of Short-Duration Heavy Rainfall in the Western Pacific Subtropical High Area: An Analysis of Two Rainstorms of 2018 in Hunan Province, China

ABSTRACT This study examines two short-duration heavy rainfall cases of 2018 in Hunan province, China, using observations and the NCEP FNL analyses. Short-duration heavy rainfall occurring in the western Pacific subtropical high (WPSH) area is one of the major natural hazards during spring and summer over the middle reaches of the Yangtze River basin. Rainstorms in the two cases selected showed high echo-top height but low storm centroid elevation, resembling the “low-echo centroid” structure. Weak vertical shear was observed in the mid-lower troposphere, thereby leading to a short life cycle of storms. Before the heavy rainfall events, in addition to substantial changes in convective available potential energy (CAPE) and convective inhibition (CIN), both cases were characterized by a lack of troughs and jet streams in the middle and upper troposphere over the middle reaches of the Yangtze River basin. A proper configuration of the WPSH extending to the middle reaches of the Yangtze River basin along with low-level synoptic systems (e.g. a surface convergence line or the cold air invading) favoured moisture supply and lifting mechanisms for convection and the rainstorms. During the hot afternoon hours, the thermals forming at the surface further enhanced convection and the rainstorms.

BEACH PROFILE EVOLUTION IN URBAN AND PERI-URBAN AREAS IN THE VILLA GESELL DISTRICT, BUENOS AIRES, ARGENTINA

The coastal villages with the greatest touristic affluence of the province of Buenos Aires, Argentina, are located on the Eastern Sand Barrier. The characteristics of the beaches of Villa Gesell, one of the most visited districts, have been studied before but there is no recent data available. The present work represents an update of the state of the beaches of this district, comparing data collected between 1994 and 2004 with sporadic measurements taken up to 2010. In 2021, two new seasonal surveys were carried out with the completion of four beach profiles distributed throughout the district and covering urban and peri-urban areas. It was observed that beaches in urban areas, where coastal dunes have been deteriorated or eliminated as a result of urban development and some anthropic practices, presented smaller beach widths and lower slopes. Beaches in peri-urban areas, with less building occupation on the waterfront, had greater beach widths and slopes, as well as larger volumes and sediment balances. Seasonal winter/summer cycles, episodic storm cycles and anthropogenic action have been identified as modifying processes of these beaches. Coastal urbanization and practices linked to sun and sea tourism affect coastal dynamics. Thus, sustained monitoring over time is a necessary tool to evaluate possible changes and preserve the beaches that represent a fundamental tourist resource for these localities.

ОСОБЕННОСТИ ДЕФОРМАЦИЙ БЕРЕГОВОГО ПЕСЧАНОГО ПРОФИЛЯ ЗА ПОЛНЫЙ ШТОРМОВОЙ ЦИКЛ

Deformations of the coastal accumulative profile during a full storm cycle can be related both to the special aspects of individual storms and to seasonal cycles of wave activity. There is a concept of a “winter” and a “summer” type of coastal profile. The difference between these types of profiles is the isolation of the outer underwater berm in winter due to the transfer of sand material to depth and an increase of slopes in the surf zone in summer due to the transfer of sand material to the shore. In this case, the post-storm relief is determined by the character of the wave intensity reduction phase, i.e., the storm attenuation phase. The attenuation of the storm can be prolonged and accompanied by local peaks in wave heights, which will decrease gradually. In this case, the storm has a high potential for beach recovery and the underwater berm may shift toward the shore. In another case, storm attenuation can occur relatively quickly – the original profile is not recovered and at the end of the storm, the profile shape reflects the erosion in the peak phase. Relevant is the determination of the special features of the resulting deformations at the end of the storm cycle, in particular at the end of the storm attenuation phase. In this paper, the special features of storm deformations of the coastal sand profile, which were observed during field experiment on the Bulgarian coast and during monitoring observations on the Baltic Spit, are considered. In one case (the Bulgarian coast) it is a transition from the typical “summer” profile to the “winter” profile. In another case (Baltic Spit), a shift of the outer underwater berm toward the shore was revealed, which can be considered as a transition to the “summer” profile. It is revealed that the features of the storm attenuation stage in the two cases under consideration have specific differences. It is shown that transformations of the coastal profile, characterized as seasonal, can occur during the full cycle of a single storm event.

Physical processes controlling the diurnal cycle of convective storms in the Western Ghats

Diurnal variation of convective storms (CSs) during monsoon season and associated physical mechanisms are significantly important for accurate forecast of short-time and extreme precipitation. The diurnal cycle of CSs is investigated using ground-based X-band radar, Tropical Rainfall Measuring Mission Precipitation Radar, and reanalysis data during the summer monsoon (June–September of 2014) over complex mountain terrain of Western Ghats, India. Diurnally, CSs show a bimodal distribution in the coastal areas, but this bimodality became weak along the upslope regions and on the mountain top. The first occurrence mode of CSs is in the afternoon–evening hours, while the second peak is in the early-morning hours. The diurnal cycle’s intensity varies with location, such that it reaches maximum in the afternoon–evening hours and early morning on the mountain top and coastal areas, respectively. Two possible mechanisms are proposed for the observed diurnal variation in CSs (a) the radiative cooling effect and (b) the surface wind convergence induced by the interaction between land-sea breeze, local topography and large-scale monsoon winds. It is also observed that the CSs developed on the mountain top during afternoon–evening hours are deeper than those along the coast. The higher moisture in the lower- and mid-troposphere, higher instability and strong upward motion facilitate deeper CSs during afternoon–evening hours.

Organic carbon and nitrogen dynamics during a peatland storm event: How dissolved combined amino acids reveal the spatial and temporal separation of organic molecules

The export of fixed carbon from peatlands is likely important in driving freshwater ecosystem productivity in head water streams. Here we studied the stream composition from three peatlands, and the sub-catchment containing these peatlands, in response to a natural storm event. We investigated the dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) mobilised by this event as well as the dissolved combined amino acids (DCAAs; proteins, peptides and free AAs) component of the DOC/DON. The storm pulse was characterised by a strong DOC/DON/DCAA pulse reflecting the mobilisation of materials from the peat profile. Through the storm cycle a transient increase in the C:N ratio of the DOC was observed (ranging over 20 – 50 across the four sites), likely reflecting the mobilisation of less decomposed material from higher in the peat soil profile. By comparing the DCAA and DON dynamics we show that a DCAA-rich component (fast-DCAA) is mobilised earlier in the storm cycle, likely reflecting a higher mobility in the peat soil. Similarly, comparison of the DCAA and DOC dynamics suggests that at least part of the DOC is mobilised more rapidly than fast-DCAA (fast-DOC). While we may not expect the composition of a storm pulse to be uniform through the cycle, this study shows that within the DOC and DON pools there is spatial and temporal separation of molecular groups. Proteins (in principal) represent both highly bioavailable N-sources as well as potential biological trigger molecules, so our work shows that these compounds are more dominant in the early part (rising limb) of the storm pulse. This understanding of the changing composition of DOC through a storm peak also provides greater insight into the concentration-discharge (C-Q) response of peatlands; our work shows that the activation of water paths higher in the peat profile at high flows leads to the mobilisation of different pools of DOC such that the molecular characteristics of DOC change through the storm cycle.

Diagnosing the Influence of a Receding Snow Boundary on Simulated Midlatitude Cyclones Using Piecewise Potential Vorticity Inversion

AbstractPrevious research has found a relationship between the equatorward extent of snow cover and low-level baroclinicity, suggesting a link between the development and trajectory of midlatitude cyclones and the extent of preexisting snow cover. Midlatitude cyclones are more frequent 50–350 km south of the snow boundary, coincident with weak maxima in the environmental Eady growth rate. The snow line is projected to recede poleward with increasing greenhouse gas emissions, possibly affecting the development and track of midlatitude cyclones during Northern Hemisphere winter. Detailed examination of the physical implications of a modified snow boundary on the life cycle of individual storms has, to date, not been undertaken. This study investigates the impact of a receding snow boundary on two cyclogenesis events using Weather Research and Forecasting Model simulations initialized with observed and projected future changes to snow extent as a surface boundary condition. Potential vorticity diagnosis of the modified cyclone simulations isolates how changes in surface temperature, static stability, and relative vorticity arising from the altered boundary affect the developing cyclone. We find that the surface warm anomaly associated with snow removal lowered heights near the center of the two cyclones investigated, strengthening their cyclonic circulation. However, the direct effect of snow removal is mitigated by the stability response and an indirect relative vorticity response to snow removal. Because of these opposing effects, it is suggested that the immediate effect of receding snow cover on midlatitude cyclones is likely minimal and depends on the stage of the cyclone life cycle.

The Evolution of Sediment Sources Over a Sequence of Postfire Sediment‐Laden Flows Revealed Through Repeat High‐Resolution Change Detection

AbstractPostfire debris flows are particularly complex to study because they do not form discrete initiation locations and commonly involve multiple simultaneously operating erosional processes. Although recent work has begun to elucidate a more mechanistic understanding of postfire debris flows, there is still a paucity of detailed sediment budgets characterizing these events. In this study, we seek to understand how postfire sediment sources and erosional processes change over multiple storm cycles. To do this, we performed repeat high‐resolution change detection in a headwater catchment burned by the 2018 Holy Fire in the Santa Ana Mountains, California, USA. This included terrestrial laser scanning in a zero‐order catchment (0.95 ha) and unmanned aerial vehicle structure from motion of a headwater channel network (up to 6.5 ha). During the initial storm events that produced runoff‐generated debris flows, we found that the evacuation of dry ravel and prefire colluvium accounted for half of the eroded material. These initial flows also acted to clear out much of the material stored within downstream headwater channel networks. In subsequent storm events of equal or greater rainfall intensity, total erosion from the study site was subdued, and the relative importance of shallow hillslope erosion from interrill and rill erosion was increased, as has been noted in similar studies in the region. Overall, this suggests that channel sediment supplies may be more rapidly depleted than hillslope sources, which may drive a trend of decreasing sediment fluxes over time from burned headwater catchments subject to repeated runoff events.

Dynamics of Barred Shore Profile on the Temporal Scale of a Storm Cycle

A model is developed to simulate the morphodynamics of a barred shore profile on the scale of a storm cycle, comprising the phases of high and low wave activity. In the CROSS-PB model, the main mechanisms governing sediment transport in wave shoaling and surf zones are taken into consideration, in particular, skewness and asymmetry of the wave orbital velocities, undertow, and infragravity oscillations associated with wave groups. By comparing calculations with observed data, it is concluded that the model correctly reproduces seaward displacement of bars during a storm and shoreward under moderate or weak wave conditions. The first process is shown to be provided by balance of transport mechanisms when the peak offshore transport is located at the front slope of bar. The second process is conditioned by dominant shoreward transport and its sharp decay over the bar crest. Testing of the model yields estimates for the calibration coefficients, and it is concluded that shoreward sediment flux is mainly controlled by wave asymmetry.

Effect of repeated rise and fall of water level on seepage-induced deformation and related stability analysis of Princeville levee

The Princeville levee, and flooding associated with Hurricanes Floyd and Matthew, is used as a case study in which the analyses are focused on the effect of repeated rise and fall of water levels (representing severe storm cycles) on the stability of the levee and the risk of failure. The analyses included strain-based and strength reduction approaches and are conducted using the finite element program Plaxis 2D. The limit equilibrium stability software “Slope/W” was also used for comparative study. The strain-based limit state approach considers the uncertainty of soil properties and is used to characterize the levee performance under repeated storm loading in terms of damage levels (or limit states). The strain-based analyses results show a progressive development of plastic shear strain zone within the levee as the number of storm cycles is increased. The accumulation of such shear strain leads to increasing the probability of exceeding a given performance limit state. As more flooding cycles are introduced, the shear strain values increase by a factor of 3.5 from cycle 1 to 6, and therefore reflect the increasing level of failure risk. In parallel, the deterministic stability factor of safety obtained from limit equilibrium method remains unchanged and slightly changes for strength reduction method with an increased number of rises and falls of the water level. The consideration of “rapid” drawdown conventionally used in limit equilibrium stability analyses (where no consideration for time is included), instead of more realistic rate based on drawdown hydrograph leads to conservative estimate of factor of safety. The analyses results demonstrate the increase in risk with repeated hydraulic loading.

Illuminating seafloor faults and ocean dynamics with dark fiber distributed acoustic sensing.

Distributed fiber-optic sensing technology coupled to existing subsea cables (dark fiber) allows observation of ocean and solid earth phenomena. We used an optical fiber from the cable supporting the Monterey Accelerated Research System during a 4-day maintenance period with a distributed acoustic sensing (DAS) instrument operating onshore, creating a ~10,000-component, 20-kilometer-long seismic array. Recordings of a minor earthquake wavefield identified multiple submarine fault zones. Ambient noise was dominated by shoaling ocean surface waves but also contained observations of in situ secondary microseism generation, post-low-tide bores, storm-induced sediment transport, infragravity waves, and breaking internal waves. DAS amplitudes in the microseism band tracked sea-state dynamics during a storm cycle in the northern Pacific. These observations highlight this method's potential for marine geophysics.

DEFORMATION OF COASTAL PROFILE DURING DIFFERENT STORM PHASES

The aim of the work is to describe the mechanisms controlling the sediment transportation at different stages of the storm cycle including three main phases, namely development, stabilization and attenuation. The resulting morphological effect of the considered storm depends presumably not only on its strength, but also on the relative duration of the destructive and recovery phases. Two full-scale experiments conducted on the Bulgarian coast in 2016 and 2018 provided materials on the shore profile dynamics under conditions of storm waves. It proved that the profile deformations usually considered to be seasonal may happen in the course of a single extreme event of once a year frequency. Field experiment data analysis proved the concept that long mild waves move sediments onshore (cause accumulative profile) and short steep waves create erosional profile with sandbar. In studied case waves steeper than 0.02 create sandbars and milder than 0.02 – transport sandbars onshore. It was observed that in some cases accumulative phase corresponds to decrease of the peak period while mean period stays unchanged, which indicates narrowing of the wave spectrum.

Modeling a Shore Profile Formed by Storm Cycle Impact

A model is developed for predicting the sand shore profile formed by storm event impact. The model takes into account the cumulative effect of elementary processes responsible for changes in the nearshore relief and relies on both the law of conservation of mass and the adopted geometrical scheme for the coastal profile. The latter is approximated by analytical curves, the properties of which are related to the acting wave parameters. The morphological consequences of a given storm are determined by the concept of effective wave height taking into account the temporal structure of the storm cycle and, in particular, the relative duration of the storm decay phase. The model is verified by published observation data obtained for the US Atlantic and Pacific coasts.