Atmospheric storm surge modeling methodology along the French (Atlantic and English Channel) coast

Storm surge modeling and forecast are the key issues for coastal risk early warning systems. As a general objective, this study aims at improving high-frequency storm surge variations modeling within the PREVIMER system (www.previmer.org), along the French Atlantic and English Channel coasts. The paper focuses on (1) sea surface drag parameterization and (2) uncertainties induced by the meteorological data quality. The modeling is based on the shallow-water version of the model for applications at regional scale (MARS), with a 2-km spatial resolution. The model computes together tide and surge, allowing properly taking into account tide-surge interactions. To select the most appropriate parameterization for the study area, a sensitivity analysis on sea surface drag parameterizations is done, based on comparisons of modeled storm surges (extracted with a tidal component analysis) with four tidal gauges, during four storm events, and over about 7.5 years, where the observed water level is processed in the same way as the modeling results. The tested drag parameterizations are a constant one, as reported by Moon et al. (J Atmos Sci 61: 2321–2333, 2007), Makin (Bound-Layer Meteorol 115: 169–176, 2005), and Charnock (J Roy Meteor Soc 81: 639–640, 1955). Charnock’s parameterization, either constant with high value (0.022) or relying on a full statistical description of the sea state, enables to improve storm surges forecast with peak errors 10 cm smaller than those computed with the other drag coefficient formulations. The impact of the meteorological forcing quality is evaluated over January 2012 from the comparison between surges modeled with different meteorological data (ARPEGE, ARPEGE High Resolution and AROME) and observations. For event time scale, storm surge computation is highly improved with ARPEGE High Resolution data. For month time scale, statistics of model accuracy are less sensitive to the choice of meteorological forcing. As a conclusion, the Charnock’s parameterization is advised to model storm surges on the French Atlantic and English Channel coasts, whereas the quality requirements regarding meteorological inputs depend on the time scale of interest. Within the PREVIMER system, aiming at forecasting events, ARPEGE High Resolution data are used

Numerical Modelling of a Macrotidal Bay over the Last 9,000 Years: An Interdisciplinary Methodology to Understand the Influence of Sea-Level Variations on Tidal Currents in the Bay of Brest

International audienceEstuaries play a major role in the transfer of sediments from the continents to the shelves and deep ocean basins. Their position at the interface between land and sea promotes them as a key area for the understanding of ocean sediment supply, but yet long-term evolution remains poorly understood. The main reasons of the lack of knowledge about estuaries filling are the lack of hydrodynamic data in the past and the temporal application of numerical models. Oceanographers and geologists have developed numerical models to simulate currents and sedimentation. On one hand, hydro-sediment models allow a good physical representation of estuarine hydrodynamic processes and their impact on sedimentation, but only over time-scale spanning years to decades. On the other hand, stratigraphic diffusive models aim to study the impact of various geological processes on sedimentary basins over millions of years, but they are unable to describe in detail the tidal hydrodynamic processes that govern estuaries. In response to this timescale issue, this study presents a first step attempt to explore the evolution of tidal current distribution in relation with Holocene eustatic variations and seafloor evolution. Here we focus on a macro-tidal estuary, the bay of Brest, where tidal processes dominate, as the estuary is naturally protected from ocean swells. This paper aims to set up a methodology to simulate the (past) tidal currents over a long time period and correlate them with sedimentary data. Major changes in deposit dynamics are first identified from cores and seismic data, and the corresponding paleo-topographies and paleo-sea-levels are rebuilt. A process-based hydrodynamic model (MARS3D) is then used to test the impacts of these paleo-bathymetries on hydrodynamics over a 1-year time span. Four scenarii have been considered, representing four key stages of the Holocene transgression in the Bay of Brest. The simulated barotropic currents distributions were analysed and bottom currents impact on the seafloor compared with sedimentary records to understand past hydrodynamic context and associated sediment spatial distribution over geological time scale. Hydrodynamic simulations and sediments records are linked, in order to propose a reconstruction of the tidal influence on sediments over the last 9000 years. The results show changes of the tidal patterns related to the paleoenvironmental evolution (bathymetry and sea-level variations). Even if a hydro-sediment model would be needed to make a direct correlation between simulated currents and sediment records, this successful application in the Bay of Brest shows that discontinuous modelling can help to understand tidal current evolution and their impact on sediment distribution over long periods

PREVIMER: Improvement of surge, sea level and currents modelling

The pre-operational system PREVIMER provides coastal observations and forecasts along French coasts. It provides, among other variables, currents, sea levels, surges and waves. This paper describes the development and validation of a high temporal (15 minutes) and spatial (250 m) resolution modeling system, based on MARS hydrodynamic model (Lazure and Dumas 2008), along the Atlantic and English Channel coasts. Models benefi t from experiments developed during the PREVIMER project by: (1) taking better into account wind and wave actions (improving surface drag coeffi cient parameterization), (2) taking into account a better meteorological forcing (improving spatial and temporal meteorological resolution). These high resolution models have been integrated in PREVIMER modeling system since 2013

Ultrasensitive detection of p24 in plasma samples from people with primary and chronic HIV-1 infection

HIV-1 Gag p24 has long been identified as an informative biomarker of HIV replication, disease progression and therapeutic efficacy, but the lower sensitivity of immunoassays in comparison to molecular tests and the interference with antibodies in chronic HIV infection limits its application for clinical monitoring. The development of ultrasensitive protein detection technologies may help overcoming these limitations. Here we evaluated whether immune-complex dissociation combined with ultrasensitive digital ELISA Simoa technology could be used to quantify p24 in plasma samples from people with HIV-1 infection. We found that, among different immune-complex dissociation methods, only acid-mediated dissociation was compatible with ultrasensitive p24 quantification by digital ELISA, strongly enhancing p24 detection at different stages of HIV-1 infection. We show that ultrasensitive p24 levels correlated positively with plasma HIV-RNA and HIV-DNA and negatively with CD4+ T cells in the samples from people with primary and chronic HIV-1 infection. In addition, p24 levels also correlated with plasma D-dimers and IFNα levels. P24 levels sharply decreased to undetectable levels after initiation of combined antiretroviral treatment (cART). However, we identified a group of people who, 48 weeks after cART initiation, had detectable p24 levels despite most having undetectable viral loads. These people had different virologic and immunologic baseline characteristics when compared with people who had undetectable p24 after cART. These results demonstrate that ultrasensitive p24 analysis provides an efficient and robust mean to monitor p24 antigen in plasma samples from people with HIV-1 infection, including during antiretroviral treatment, and may provide complementary information to other commonly used biomarkers.ImportanceThe introduction of combined antiretroviral treatment has transformed HIV-1 infection in a manageable condition. In this context, there is a need for additional biomarkers to monitor HIV-1 residual disease or the outcome of new interventions, such as in the case of HIV cure strategies. The p24 antigen has a long half-life outside viral particles and it is therefore a very promising marker to monitor episodes of viral replication or transient activation of the viral reservoir. However, the formation of immune-complexes with anti-p24 antibodies makes its quantification difficult beyond acute HIV-1 infection. We show here that, upon immune-complex dissociation, new technologies allow the ultrasensitive p24 quantification in plasma samples throughout HIV-1 infection, at levels close to that of viral RNA and DNA determinations. Our results further indicate that ultrasensitive p24 quantification may have added value when used in combination with other classic clinical biomarkers

Infragravity waves: from driving mechanisms to impacts

Infragravity (hereafter IG) waves are surface ocean waves with frequencies below those of wind-generated “short waves” (typically below 0.04 Hz). Here we focus on the most common type of IG waves, those induced by the presence of groups in incident short waves. Three related mechanisms explain their generation: (1) the development, shoaling and release of waves bound to the short-wave group envelopes (2) the modulation by these envelopes of the location where short waves break, and (3) the merging of bores (breaking wave front, resembling to a hydraulic jump) inside the surfzone. When reaching shallow water (O(1–10 m)), IG waves can transfer part of their energy back to higher frequencies, a process which is highly dependent on beach slope. On gently sloping beaches, IG waves can dissipate a substantial amount of energy through depth-limited breaking. When the bottom is very rough, such as in coral reef environments, a substantial amount of energy can be dissipated through bottom friction. IG wave energy that is not dissipated is reflected seaward, predominantly for the lowest IG frequencies and on steep bottom slopes. This reflection of the lowest IG frequencies can result in the development of standing (also known as stationary) waves. Reflected IG waves can be refractively trapped so that quasi-periodic along-shore patterns, also referred to as edge waves, can develop. IG waves have a large range of implications in the hydro-sedimentary dynamics of coastal zones. For example, they can modulate current velocities in rip channels and strongly influence cross-shore and longshore mixing. On sandy beaches, IG waves can strongly impact the water table and associated groundwater flows. On gently sloping beaches and especially under storm conditions, IG waves can dominate cross-shore sediment transport, generally promoting offshore transport inside the surfzone. Under storm conditions, IG waves can also induce overwash and eventually promote dune erosion and barrier breaching. In tidal inlets, IG waves can propagate into the back-barrier lagoon during the flood phase and induce large modulations of currents and sediment transport. Their effect appears to be smaller during the ebb phase, due to blocking by countercurrents, particularly in shallow systems. On coral and rocky reefs, IG waves can dominate over short-waves and control the hydro-sedimentary dynamics over the reef flat and in the lagoon. In harbors and semi-enclosed basins, free IG waves can be amplified by resonance and induce large seiches (resonant oscillations). Lastly, free IG waves that are generated in the nearshore can cross oceans and they can also explain the development of the Earth's “hum” (background free oscillations of the solid earth)

Article 8 MSFD Assessment Guidance

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