Thickness Of Mixed Layer Research Articles

Tropical cyclone sensitivity to ocean coupling in the ECMWF coupled model

AbstractWe present an investigation of the performance of the ECMWF coupled atmosphere‐waves‐ocean model for different ocean and atmosphere resolutions on a series of tropical cyclones in the Western Pacific with the aim to better understand the coupled feedback mechanisms in these extreme conditions. For some of the test cases, we only find little impact of coupling the atmosphere to the ocean, while in others, we observe a very large impact. To further understand these differences, we have selected two tropical cyclones (TCs) as case studies: TC Haiyan (with small impact of coupling) and TC Neoguri (with large impact of coupling). The comparison between these two cases suggests that the upper ocean stratification is the key in determining the strength of the coupled feedback. A strong coupled feedback is found whenever the ocean heat content of the upper layer is low while a very weak coupled feedback is found whenever the ocean has a thick warm mixed layer. The oceanographic response to tropical cyclones for the two storms has been compared to sea surface temperature and derived surface currents from drifting buoys and to subsurface observations from Argo and ship launched XBT's. These comparisons show that we are able to realistically reproduce the atmospheric and oceanographic interaction during tropical cyclone conditions which gives us confidence that the coupled modeling system is physically sound.

Observed seasonal and interannual variability of the near-surface thermal structure of the Arabian Sea Warm Pool

The observed seasonal and interannual variability of near-surface thermal structure of the Arabian Sea Warm Pool (ASWP) is examined utilizing a reanalysis data set for the period 1990–2008. During a year, the ASWP progressively builds from February, reaches its peak by May only in the topmost 60m water column. The ASWP Index showed a strong seasonal cycle with distinct interannual signatures. The years with higher (lower) sea surface temperature (SST) and larger (smaller) spatial extent are termed as strong (weak) ASWP years. The differences in the magnitude and spatial extent of thermal structure between the strong and weak ASWP regimes are seen more prominently in the topmost 40m water column. The heat content values with respect to 28°C isotherm (HC28) are relatively higher (lower) during strong (weak) ASWP years. Even the secondary peak in HC28 seen during the preceding November–December showed higher (lower) magnitude during the strong ASWP (weak) years. The influence of the observed variability in the surface wind field, surface net air–sea heat flux, near-surface mixed layer thickness, sea surface height (SSH) anomaly, depth of 20°C isotherm and barrier layer thickness is examined to explain the observed differences in the near-surface thermal structure of the ASWP between strong and weak regimes. The surface wind speed is much weaker in particular during the preceding October and February–March corresponding to the strong ASWP years when compared to those of the weak ASWP years implying its important role. Both stronger winter cooling during weak ASWP years and stronger pre-monsoon heating during strong ASWP years through the surface air–sea heat fluxes contribute to the observed sharp contrast in the magnitudes of both the regimes of the ASWP. The upwelling Rossby wave during the preceding summer monsoon, post-monsoon and winter seasons is stronger corresponding to the weak ASWP regime when compared to the strong ASWP regime resulting in greater cooling of the near-surface layers during the summer monsoon season of the preceding year. On the other hand, the downwelling Rossby wave is stronger during pre-monsoon months during the strong ASWP regime when compared to weak ASWP regime leading to lesser cooling during strong ASWP regime.

Effect of the Number of Injectors on the Mixing Process in a Rapidly Mixed Type Tubular Flame Burner

Three-dimensional simulations are performed to study the non-reactive mixing process in a rapidly mixed type tubular flame burner (RTFB). The current work examines the effect of the number of injectors (N= 2, 4 and 6) on the mixing process by focusing on three criterions (Flow structure, local swirl intensity and mixing layer thickness). The Discrete Phase Model (DPM) is used to track the particle trajectories. Validation of the numerical results is carried out by comparing the predicted particle trajectories, central recirculation zone (CRZ) and tangential velocity results to the experimental data. It is concluded that the model offers a satisfactory prediction of the flow field in a RTFB. Numerical results show that, for the same geometrical swirl number (Sw) and the same Reynolds number (ReT), the increasing of the number of injectors enhances the mixing process by generating a larger reverse flow and reducing the mixing layer thickness. It is also concluded that the local swirl intensity along of the RTFB can be correlated in terms of geometric swirl number and number of injectors.

210Pb, 137Cs and 7Be in the sediments of coastal lakes on the polish coast: Implications for sedimentary processes

In this study we combined radioisotopes (210Pb, 137Cs and 7Be) and hydrodynamic modeling to investigate sedimentary processes in three coastal lakes on the Polish Baltic coast. The research aimed at establishing the depth of sediment mixing and its effects on sediment geochemistry as well as showing the relationship between lake water salinity and radionuclide distribution in the sediment cores. We established that the intensity of mixing displayed appreciable variability throughout the lakes and the thickness of sediment mixing layer was between <2 and 22 cm. The mixing was primarily due to wind-induced waves. The vertical mixing was shown to shift sulfidation of the sediments towards deeper layers. We found that the distributions of radioisotopes, 137Cs in particular, in the sediment cores from coastal lakes were strongly affected by the early diagenetic processes, which caused diffusive migration of radionuclides. The inventories of 210Pbex and 137Cs in the lakes were positively related to salinity. The high inventories of both isotopes (3.2–10.9 kBq ·m−2 for 210Pbex and 3.0–6.0 kBq·m−2 for 137Cs) in coastal lakes were explained by enhanced sedimentation within estuarine mixing zone and delivery of “additional” 210Pb and 137Cs to the lakes during saltwater ingressions. The results of this study have implications for the paleolimnology, sedimentology and biogeochemistry of coastal lakes.

Constraining foraminiferal calcification depths in the western Pacific warm pool

Highlights • First apparent calcification depth assessment of living foraminifera in the SE WPWP • Deep surface mixed layer causes deep apparent calcification depths. • Deep-dwelling G. hexagonus traces nutrient conditions in equatorial water masses. Abstract Insight into past changes of upper ocean stratification, circulation, and nutrient signatures rely on our knowledge of the apparent calcification depth (ACD) and ecology of planktonic foraminifera, which serve as archives for paleoceanographic relevant geochemical signals. The ACD of different species varies strongly between ocean basins, but also regionally. We constrained foraminiferal ACDs in the Western Pacific Warm Pool (Manihiki Plateau) by comparing stable oxygen and carbon isotopes (δ18Ocalite, δ13Ccalcite) as well as Mg/Ca ratios from living planktonic foraminifera to in-situ physical and chemical water mass properties (temperature, salinity, δ18Oseawater, δ13CDIC). Our analyses point to Globigerinoides ruber as the shallowest dweller, followed by Globigerinoides sacculifer, Neogloboquadrina dutertrei, Pulleniatina obliquiloculata and Globorotaloides hexagonus inhabiting increasing greater depths. These findings are consistent with other ocean basins; however, absolute ACDs differ from other studies. The uppermost mixed-layer species G. ruber and G. sacculifer denote mean calcification depths of ~ 95 m and ~ 120 m, respectively. These Western Pacific ACDs are much deeper than in most other studies and most likely relate to the thick surface mixed layer and the deep chlorophyll maximum in this region. Our results indicate that N. dutertrei appears to be influenced by mixing waters from the Pacific equatorial divergence, while P. obliquiloculata with an ACD of ~ 160 m is more suitable for thermocline reconstructions. ACDs of G. hexagonus reveal a deep calcification depth of ~ 450 m in oxygen-depleted, but nutrient-rich water masses, consistent to other studies. As the δ13C of G. hexagonus is in near-equilibrium with ambient seawater, we suggest this species is suitable for tracing nutrient conditions in equatorial water masses originating in extra-topical regions.

A turbulence characteristic length scale for compressible flows

ABSTRACTThe current RANS models are generally established and calibrated under incompressible condition and these kinds of models could succeed in predicting many features of incompressible flows. However, these models extended to the high-speed, compressible flows are always less accurate. In the paper, a compressible von Kármán length scale is proposed for compressible flows considering the variable densities. It contains no empirical coefficients and is based on phenomenological theory. In the turbulent kinetic equation, the extra unclosed terms induced by non-constant densities are treated as dissipation terms and the equation is closed algebraically via the introduction of the von Kármán length scale. The original and the proposed von Kármán length scale lead to two different kinds of SAS (scale adaption simulation) models, KDO (turbulence kinetic energy dependent only) and CKDO (compressible KDO), respectively. Compressible mixing layer with significant compressibility is studied within standard k–ϵ, k–ω, KDO turbulence models and their compressible versions. The compressibility effects such as the reduced mixing layer thickness, growth rate and turbulence intensity can be reproduced by CKDO. The new length scale can improve the performances of the model in predicting the mixing layer thickness, stream-wise velocity and Reynolds shear stresses when the convective Mach number is 0.8. Besides, the new length scale also leads to accurate computed growth rate when the convective Mach number ranges from 0.1 to 1.0.

Turbulent mixing of a passive scalar in grid turbulence

Fractal grids have attracted attention as a new-type of turbulence-generating grid due to their unique characteristics. Recent studies have revealed that such uniqueness appears in the near field of regular grid-generated turbulence. Scalar transport in those flows is also of great interest as it is not yet fully understood. In this study, we investigate the scalar mixing in the near field of regular grid-generated turbulence with various grid configurations. Experiments have been carried out in liquid mixing layers with a Reynolds number of 5000 based on the mesh size of the grid and uniform velocity. Simultaneous measurements of two-component velocities and concentration have been performed by particle image velocimetry and a planar laser-induced fluorescence technique, respectively. The results show that the scaling law using the wake-interaction length scale is applicable for the turbulence intensity in the grid turbulence with different mesh sizes and the same thickness of the grid bar. The turbulence intensity increases as the thickness of the grid bar increases; thus, consequently increasing the scalar diffusion. The streamwise development of the scalar mixing layer thickness collapses onto a single curve by normalization based on the thickness of the grid bar.

CCHE1D 모형을 이용한 저수지 붕괴에 따른 하상변동 해석

본 연구에서는 저수지의 붕괴로 인한 하류부 하천의 하상변동에 대한 해석을 수행하였다. 저수지 붕괴에 따른 1차원 비평형, 비균일 유사의 이송과 하상 변동을 연구를 위해 CCHE1D 모형을 이용하였다. CCHE1D 모형은 비평형 및 비균일 유사해석을 위해 조정거리와 분류된 입자의 입경을 사용하며, 하상 물질의 교환을 위한 혼합층의 개념이 사용된다. CCHE1D 모형을 1996년 저수지 붕괴가 발생한 Ha!Ha!강 유역에 적용하여 저수지 붕괴로 인한 하류부의 비평형, 비균일 유사이송을 해석하고, 계산 결과를 저수지 붕괴전 후에 하류부 실측 하상과 비교하였다. 또한, 조정계수(<TEX>$L_{s,b}$</TEX>), 비평형 계수(<TEX>${\alpha}_s$</TEX>), 혼합층 두께(<TEX>${\delta}_m$</TEX>), 공극률(p')을 포함하는 주요 매개변수에 대한 민감도를 분석하였으며, 대상유역에서는 비평형 계수가 하상변동에 가장 큰 영향을 주는 것으로 나타났다. This study presents the analysis of flood and bed deformation caused by reservoir failure. The CCHE1D is used to simulate 1D non-uniform, non-equilibrium sediment transport and bed deformation. The CCHE1D deals with the adaptation length for non-equilibrium sediment, classified sediment particle for non-uniform sediment and mixing layer for the exchange with the sediment moving with the flow. The model is applied to Ha!Ha! river basin where was experienced reservoir failure in 1996 to analyze non-uniform and non-equilibrium sediment transport. The calculations are compared with morphological bed changes of pre- and post-flood. In addition, model sensitivity to main parameters involving adaptation length (<TEX>$L_{s,b}$</TEX>), non-equilibrium coefficient (<TEX>${\alpha}_s$</TEX>), mixing layer thickness (<TEX>${\delta}_m$</TEX>) and porosity (p') is analyzed. The results indicates that thalweg change is the most sensitive to non-equilibrium coefficient (<TEX>${\alpha}_s$</TEX>) among those parameters in the study area.

An estimation of the effect of a possible wind speed increase on the ocean mixed layer depth at the northern Patagonian continental shelf

A possible deepening of the ocean mixed layer is investigated at two selected points of the Patagonian continental shelf where a significant positive wind speed trend is estimated. Using a 1-dimensional vertical numerical model on a long term simulation (1979–2011) it is found that the mixed layer thickness presents a significant and positive trend of about 10±1.5 cm/yr. The model is forced by atmospheric data from NCEP/NCAR 1 reanalysis and tidal constituents from TPXO 7.2 global model. Several numerical experiments are carried out in order to evaluate the impact of the different atmospheric variables considered in this study (wind components, air temperature, atmospheric pressure, specific humidity and cloud coverage). As a result it is found that a possible increase in the wind speed should be considered as a very significant factor for deepening the ocean mixed layer at the northern Patagonian continental shelf.

Joint assimilation of Aquarius‐derived sea surface salinity and AVHRR‐derived sea surface temperature in an ocean general circulation model using SEEK filter: Implication for mixed layer depth and barrier layer thickness

AbstractSea surface salinity (SSS) from Aquarius mission and sea surface temperature (SST) from Advanced Very High Resolution Radiometer (AVHRR) for the years 2012–2014 are assimilated into the global Massachusetts Institute of Technology General Circulation Model (MITGCM). Investigation of the impact of assimilation of these two data sets on simulated mixed layer depth (MLD) and barrier layer thickness (BLT) forms the core of our study. The method of assimilation is the Singular Evolutive Extended Kalman (SEEK) filter. Several assimilation runs are performed. Single‐parameter assimilation, as well as joint assimilation, is conducted. To begin with, the model simulated SST and SSS are compared with independent Argo observations of these two parameters. Use of latitudinally varying error variances, which is a novel feature of our study, gives rise to the significant improvement in the simulation of SSS and SST. The best result occurs when joint assimilation is performed. Afterward, simulated MLD and BLT are compared with the same parameters derived from Argo observations forming an independent validation data set. Comparisons are performed both in temporal and spatial domains. Significant positive impact of assimilation is found in all the cases studied, and joint assimilation is found to outperform single‐parameter assimilation in each of the cases considered. It is found that simulations of MLD and BLT improve up to 24% and 29%, respectively, when a joint assimilation of SSS and SST is carried out.

Parameters of the upper mixed layer and thermocline layer and chlorophyll- &lt;i&gt;a &lt;/i&gt;in the western deep basin of the Bering Sea in summer and fall of 2002-2013

Spatial distribution and seasonal and interannual variability are determined for parameters of the upper mixed layer and seasonal thermocline in the western deep basin of the Bering Sea in summer and fall of 2002-2013 and compared with chlorophyll- a concentration at the sea surface. The mixed layer is formed by solar heating of the sea surface and wind mixing. Its thickness doesn’t exceed 10-15 m in summer but increases up to 25-40 m in fall. The thermocline becomes deeper from summer to fall, too, in particular its lower bound that sinks from 37 to 48 m, on average; however, its thickness decreases from 20-25 to 16-22 m and temperature and density gradients increase from 0.23-0.25 to 0.37-0.45 °C/m and from 0.032 to 0.062 units/m, respectively, that means that the thermocline becomes steadier. Tendency to cooling was observed for both layers in the last decade, the process was more visible in autumn, with a shift at the upper bound of thermocline (from the mean temperature of 9.8 °C in 2002-2006 to the mean temperature of 8.4 °C in 2007-2013) and gradual decreasing at its lower bound (from 3.3 °C in 2003 to 2.1 °C in 2013), and not so obvious in summer. Year-to-year fluctuations of salinity in summer/autumn were within the limits 32.77-33.03/32.60-32.96 psu for the upper bound of the thermocline and 32.98-33.14/33.06-33.13 psu for its lower bound and had slight negative tendency. In both seasons, the mixed layer depth has significant negative correlation with the temperature at the upper bound of the thermocline (R 2 = 0.6) and the temperature at the lower bound of the thermocline correlates significantly with salinity at its both bounds (R 2 > 0.4). Spring bloom of chlorophyll- a has no relationship neither with temperature in the mixed layer in April and May, nor with the rate of warming between these months. Strong fall bloom of chlorophyll- a develops if the fall cooling begins in conditions of high temperature at the sea surface (> 6 °C) that negatively depends on the mixed layer thickness, so slow cooling and delay of the fall mixing are favorable for chlorophyll- a blooming in autumn.

Passive scalar mixing in Mc

We investigate passive scalar mixing in spatially developing supersonic shear layer flows formed by two planar streams by means of large eddy simulation. After validation of the numerical procedures employing a high-order hybrid WENO/compact scheme by comparing the results of simulation with the results obtained from a well characterized experimental case, the effects of the convective Mach number, Mc, flow velocity ratio, r = U1/U2, and fluid density ratio, s = ρ1/ρ2, between Stream 1 and Stream 2 on passive scalar mixing are considered by examining the variation of mixing layer thickness and the mixing efficiency represented by the transport of the passive scalar. Mc is specified from 0.2 to 0.8. The evolution of large-scale coherent structures is well reproduced, with vortices undergoing rolling up, pairing, merging, and breaking up. The mixing layer thickness and mixing efficiency both decrease as Mc increases. As s increases, the mixing layer thickness increases, and the mixing efficiency of an entrained fluid decreases. As r increases, the mixing layer thickness decreases while the mixing efficiency increases. While the present results are not applicable to the examination of micro-mixing properties, which require assessment by experimental methods or high computing cost direct numerical simulation, they are useful for evaluating the effects of different flow parameters on macro-performance, which is equally important for scramjet combustor design and evaluation in engineering.

Investigation on flow and mixing characteristics of supersonic mixing layer induced by forced vibration of cantilever

The mixing process has been an important issue for the design of supersonic combustion ramjet engine, and the mixing efficiency plays a crucial role in the improvement of the combustion efficiency. In the present study, nanoparticle-based planar laser scattering (NPLS), particle image velocimetry (PIV) and large eddy simulation (LES) are employed to investigate the flow and mixing characteristics of supersonic mixing layer under different forced vibration conditions. The indexes of fractal dimension, mixing layer thickness, momentum thickness and scalar mixing level are applied to describe the mixing process. Results show that different from the development and evolution of supersonic mixing layer without vibration, the flow under forced vibration is more likely to present the characteristics of three-dimensionality. The laminar flow region of mixing layer under forced vibration is greatly shortened and the scales of rolled up Kelvin–Helmholtz vortices become larger, which promote the mixing process remarkably. The fractal dimension distribution reveals that comparing with the flow without vibration, the turbulent fluctuation of supersonic mixing layer under forced vibration is more intense. Besides, the distribution of mixing layer thickness, momentum thickness and scalar mixing level are strongly influenced by forced vibration. Especially, when the forcing frequency is 4000Hz, the mixing layer thickness and momentum thickness are 0.0391m and 0.0222m at the far field of 0.16m, 83% and 131% higher than that without vibration at the same position, respectively.

Seasonal variability in the thermohaline structure of the Western Pacific Warm Pool

Using the 28A degrees C isotherm to define the Western Pacific Warm Pool (WPWP), this study analyzes the seasonal variability of the WPWP thermohaline structure on the basis of the monthly-averaged sea temperature and salinity data from 1950 to 2011, and the dynamic and thermodynamic mechanisms based on the monthly-averaged wind, precipitation, net heat fluxes and current velocity data. A Delta T=-0.4A degrees C is more suitable than other temperature criterion for determining the mixed layer (ML) and barrier layer (BL) over the WPWP using monthly-averaged temperature and salinity data. The WPWP has a particular thermohaline structure and can be vertically divided into three layers, i.e., the ML, BL, and deep layer (DL). The BL thickness (BLT) is the thickest, while the ML thickness (MLT) is the thinnest. The MLT has a similar seasonal variation to the DL thickness (DLT) and BLT. They are all thicker in spring and fall but thinner in summer. The temperatures of the ML and BL are both higher in spring and autumn but lower in winter and summer with an annual amplitude of 0.15A degrees C, while the temperature of the DL is higher in May and lower in August. The averaged salinities at these three layers are all higher in March but lower in September, with annual ranges of 0.41-0.45. Zonal currents, i.e., the South Equatorial Current (SEC) and North Equatorial Counter Current (NECC), and winds may be the main dynamic factors driving the seasonal variability in the WPWP thermohaline structure, while precipitation and net heat fluxes are both important thermodynamic factors. Higher (lower) winds cause both the MLT and BLT to thicken (thin), a stronger (weaker) NECC induces MLT, BLT, and DLT to thin (thicken), and a stronger (weaker) SEC causes both the MLT and BLT to thicken (thin) and the DLT to thin (thicken). An increase (decrease) in the net heat fluxes causes the MLT and BLT to thicken (thin) but the DLT to thin (thicken), while a stronger (weaker) precipitation favors thinner (thicker) MLT but thicker (thinner) BLT and DLT. In addition, a stronger (weaker) NECC and SEC cause the temperature of the three layers to decrease (increase), while the seasonal variability in salinity at the ML, BL, and DL might be controlled by the subtropical cell (STC).

Enhanced Wind-Driven Downwelling Flow in Warm Oceanic Eddy Features during the Intensification of Tropical Cyclone Isaac (2012): Observations and Theory

AbstractTropical cyclones (TCs) typically produce intense oceanic upwelling underneath the storm’s center and weaker and broader downwelling outside upwelled regions. However, several cases of predominantly downwelling responses over warm, anticyclonic mesoscale oceanic features were recently reported, where the ensuing upper-ocean warming prevented significant cooling of the sea surface, and TCs rapidly attained and maintained major status. Elucidating downwelling responses is critical to better understanding TC intensification over warm mesoscale oceanic features. Airborne ocean profilers deployed over the Gulf of Mexico’s eddy features during the intensification of tropical storm Isaac into a hurricane measured isothermal downwelling of up to 60 m over a 12-h interval (5 m h−1) or twice the upwelling strength underneath the storm’s center. This displacement occurred over a warm-core eddy that extended underneath Isaac’s left side, where the ensuing upper-ocean warming was ~8 kW m−2; sea surface temperatures &gt;28°C prevailed during Isaac’s intensification. Rather than with just Ekman pumping WE, these observed upwelling–downwelling responses were consistent with a vertical velocity Ws = WE − Rogδ(Uh + UOML); Ws is the TC-driven pumping velocity, derived from the dominant vorticity balance that considers geostrophic flow strength (measured by the eddy Rossby number Rog = ζg/f), geostrophic vorticity ζg, Coriolis frequency f, aspect ratio δ = h/Rmax, oceanic mixed layer thickness h, storm’s radius of maximum winds Rmax, total surface stresses from storm motion Uh, and oceanic mixed layer Ekman drift UOML. These results underscore the need for initializing coupled numerical models with realistic ocean states to correctly resolve the three-dimensional upwelling–downwelling responses and improve TC intensity forecasting.

Double pulse Laser Induced Breakdown Spectroscopy measurements on ITER-like samples

Laser Induced Breakdown Spectroscopy (LIBS) is an attractive analytical technique for the in situ analysis of the chemical composition of the components inside the vacuum vessel of ITER and for estimating the tritium content in the machine. To improve its sensitivity a dual pulse (DP) configuration, with two laser pulses delayed in the range of nanosecond to several microseconds can be used.In this work DP measurements carried out jointly by ENEA and IPPLM associations in the frame of EFDA task WP13-IPH-A01-P3-01 on samples resembling ITER in vessel components after material re-deposition (still including carbon) are presented. A DP Nd:YAG laser, operating at 1064nm, with delay between pulses ranging from 20ns to 80ms and pulse duration of 8–12ns was used to interact with tungsten sample coated with 3μm thick mixed layer of C/Al/W (Al was used instead of Be).An enhancement of the spectroscopic emission of the elements was observed compared to the standard LIBS measurements. A Calibration Free method was applied to the data for getting the concentration of each element. Results are in good agreement with the concentrations found by the post-mortem energy dispersive X-ray (EDX) analysis.

A priori evaluation of subgrid-scale combustion models for diesel engine applications

Abstract Large Eddy Simulation (LES) is being increasingly used for turbulent reacting flows in diesel engines. The accuracy of LES depends strongly on the accuracy of the subgrid-scale models, which are employed. In this study, two-dimensional DNS of autoigniting turbulent mixing layers are performed at temperature and pressure conditions similar to those observed in diesel engines. The DNS results are used to evaluate the performance of two common subgrid-scale combustion models that are employed in diesel combustion modeling – the perfectly stirred reactor (PSR) model and the unsteady flamelet progress variable (UFPV) model. It is shown that the effect of turbulence on the reaction zone is not felt during the early ignition stage, and in this stage, the PSR model performance is superior to that of the UFPV model. For a laminar mixing layer, which evolves under the influence of turbulence into a turbulent mixing layer, the PSR model performance deteriorates drastically with increasing time, and the UFPV model is shown to be a more suitable subgrid-scale model. The choice of the subgrid-scale combustion model is also shown to strongly depend on the filter size, i.e. the LES grid size. For filter sizes smaller than the mixing layer thickness, the PSR model performs reasonably well whereas the UFPV model outperforms the PSR model for larger filter sizes.

The impact of fluctuating light on the dinoflagellate Prorocentrum micans depends on NO3− and CO2 availability

Increasing atmospheric pCO2 and its dissolution into oceans leads to ocean acidification and warming, which reduces the thickness of upper mixing layer (UML) and upward nutrient supply from deeper layers. These events may alter the nutritional conditions and the light regime to which primary producers are exposed in the UML. In order to better understand the physiology behind the responses to the concomitant climate changes factors, we examined the impact of light fluctuation on the dinoflagellate Prorocentrum micans grown at low (1μmolL−1) or high (800μmolL−1) [NO3−] and at high (1000μatm) or low (390μatm, ambient) pCO2. The light regimes to which the algal cells were subjected were (1) constant light at a photon flux density (PFD) of either 100 (C100) or 500 (C500) μmolm−2s−1 or (2) fluctuating light between 100 or 500μmolphotonsm−2s−1 with a frequency of either 15 (F15) or 60 (F60) min. Under continuous light, the initial portion of the light phase required the concomitant presence of high CO2 and NO3− concentrations for maximum growth. After exposure to light for 3h, high CO2 exerted a negative effect on growth and effective quantum yield of photosystem II (F′v/F′m). Fluctuating light ameliorated growth in the first period of illumination. In the second 3h of treatment, higher frequency (F15) of fluctuations afforded high growth rates, whereas the F60 treatment had detrimental consequences, especially when NO3− concentration was lower. F′v/F′m responded differently from growth to fluctuating light: the fluorescence yield was always lower than at continuous light at 100μmolm−2s−1, and always higher at 500μmolm−2s−1. Our data show that the impact of atmospheric pCO2 increase on primary production of dinoflagellate depends on the availability of nitrate and the irradiance (intensity and the frequency of irradiance fluctuations) to which the cells are exposed. The impact of global change on oceanic primary producers would therefore be different in waters with different chemical and physical (mixing) properties.

How diffusivity, thermocline and incident light intensity modulate the dynamics of deep chlorophyll maximum in Tyrrhenian Sea.

During the last few years theoretical works have shed new light and proposed new hypotheses on the mechanisms which regulate the spatio-temporal behaviour of phytoplankton communities in marine pelagic ecosystems. Despite this, relevant physical and biological issues, such as effects of the time-dependent mixing in the upper layer, competition between groups, and dynamics of non-stationary deep chlorophyll maxima, are still open questions. In this work, we analyze the spatio-temporal behaviour of five phytoplankton populations in a real marine ecosystem by using a one-dimensional reaction-diffusion-taxis model. The study is performed, taking into account the seasonal variations of environmental variables, such as light intensity, thickness of upper mixed layer and profiles of vertical turbulent diffusivity, obtained starting from experimental findings. Theoretical distributions of phytoplankton cell concentration was converted in chlorophyll concentration, and compared with the experimental profiles measured in a site of the Tyrrhenian Sea at four different times (seasons) of the year, during four different oceanographic cruises. As a result we find a good agreement between theoretical and experimental distributions of chlorophyll concentration. In particular, theoretical results reveal that the seasonal changes of environmental variables play a key role in the phytoplankton distribution and determine the properties of the deep chlorophyll maximum. This study could be extended to other marine ecosystems to predict future changes in the phytoplankton biomass due to global warming, in view of devising strategies to prevent the decline of the primary production and the consequent decrease of fish species.

Graphitic Schottky Contacts to Si formed by Energetic Deposition

ABSTRACTCarbon films deposited by filtered cathodic vacuum arc have been used to form high quality Schottky diodes on p-Si. Energetic deposition with an applied substrate bias of -1 kV and with a substrate temperature of 100 °C has produced carbon diodes with rectification ratios of ∼ 3 × 106, saturation currents of ∼0.02 nA and ideality factors close to unity (n = 1.05). Simulations were used to estimate the effective work function and the thickness of an interfacial mixed (C/SiO2) layer from the current/voltage characteristics of the diodes.