Large-scale Front Research Articles

Time-dependent dispersion coefficients for the evolution of displacement fronts in heterogeneous porous media

We present an approach for quantifying displacement fronts in heterogeneous porous media based on the concept of time-dependent apparent dispersion coefficients. The concept of constant asymptotic macrodispersion generally overestimates the area swept by a displacement front and leads to unrealistic upstream dispersion. We show that the large-scale front spreading can be captured by a one-dimensional advection–dispersion equation that is parameterized by a suitably chosen temporally evolving dispersion coefficient. For purely advective front spreading, we derive an analytical expression based on a predictive continuous time random walk approach, which applies to highly heterogeneous porous media. This analysis elucidates the variability of solute travel times as the key longitudinal spreading mechanism. It shows that the evolution of dispersion can be captured as the sum of exponentials that decay on two dominant time scales. In a particle-based picture, these scales mark the short time at which transported particles start exploring the flow variability and the large time at which the slowest particles start decorrelating their transport velocity. Based on these insights, we propose a heuristic formula that accounts for the impact of local-scale dispersion as an additional decorrelation mechanism. The heuristic expression for the longitudinal dispersion coefficient captures solute spreading for a broad range of Péclet numbers and heterogeneity variances. The proposed approach is tested against direct numerical simulations. It provides a robust and fast method for quantifying the evolution of displacement fronts in heterogeneous porous media with possible applications, for example, in groundwater contamination modelling, underground gas storage, and geothermal energy production.

Assessing atmospheric moisture effects on heavy precipitation during HyMeX IOP16 using GPS nudging and dynamical downscaling

Abstract. Gaining insight into the interaction between atmospheric moisture and convection is determinant for improving the model representation of heavy precipitation, a weather phenomenon that causes casualties and monetary losses in the western Mediterranean region every year. Given the large variability of atmospheric moisture, an accurate representation of its distribution is expected to reduce the errors related to the representation of moist convective processes. In this study, we use a diagnostic approach to assess the sensitivity of convective precipitation and underlying mechanisms during a heavy precipitation event (Hydrological cycle in the Mediterranean eXperiment Intensive Observation Period; HyMeX IOP16) to variations of the atmospheric moisture spatio-temporal distribution. Sensitivity experiments are carried out by nudging a homogenized data set of the Global Positioning System-derived zenith total delay (GPS-ZTD) with sub-hourly temporal resolution (10 min) in 7 and 2.8 km simulations with the COnsortium for Small-scale MOdeling in CLimate Mode (COSMO-CLM) model over the western Mediterranean region. The analysis shows that (a) large atmospheric moisture amounts (integrated water vapour; IWV ∼ 40 mm) precede heavy precipitation in the affected areas. This occurs 12 h prior to initiation over southern France and 4 h over Sardinia, north-eastern Italy and Corsica, which is our main study area. (b) We found that the moisture is swept from the Atlantic by a westerly large-scale front associated with an upper level low on the one hand and evaporated from the Mediterranean Sea and north Africa on the other. The latter moisture transport occurs in the 1 to 4 km layer. (c) COSMO-CLM overestimated the atmospheric humidity over the study region (Corsica), and this was, to a good extent, corrected by the GPS-ZTD nudging. This reduced maximum precipitation (−49 % for 7 km and −16 % for 2.8 km) drastically, considerably improving the precipitation representation in the 7 km simulation. The convection-permitting simulation (2.8 km) without the GPS-ZTD nudging already did a good job in representing the precipitation amount. (d) The two processes that exerted the largest control on precipitation reduction were the decrease of atmospheric instability over Corsica (convective available potential energy; CAPE −35 %) and the drying of the lower free troposphere bringing additional dry air entrainment. In addition, the 7 km simulation showed a weakening of the represented low-pressure system and the associated cyclonic wind circulation. This ultimately reduced the intensity and number of convective updrafts represented over the island. These results highlight the large impact exerted by moisture corrections on precipitating convection and the chain of processes leading to it across scales.

Mesoscale Variability of the Ocean in the Northern Part of the Weddell Sea

Results of a mesoscale survey in the Powell Basin in the northern part of the Weddell Sea are analyzed. The survey included 24 CTD casts (6 × 4) with an interval of six nautical miles. Measurements of water transparency, fluorescence, and plankton catches using the Bongo and Jedy nets were performed. The survey covered a region of an oceanographic front. The front is manifested by sharp horizontal gradients of temperature and chemical properties. The front exists due to the differences in the water properties typical for the entire Weddell Sea and water from the western part of the sea, which is formed in a shallow region covered with ice and icebergs that prevent warming of the upper layer of water by the solar irradiance. The front in the region of the mesoscale survey appears due to the separation of meanders from the large-scale front in the northern part of the Weddell Sea and displacement of the ring with cold water from the bottom elevation to the region of the survey. The differences in the water structures north and south of the front are also pronounced in chemical properties, optical characteristics of the upper layer, and different concentrations and species of phytoplankton, zooplankton, and pelagic fish.

Navigation of Scalar Fronts With Multirobot Clusters in Simulation and Experiment

Environmental fronts are transition regions characterized by a steep gradient in the value of a scalar field. Finding and spatially characterizing these fronts is critical for a variety of scientific applications; however, such work is challenging and usually performed manually, limited to localized characterization, and/or based on assumptions regarding the structure of the front. In this article, we present an automated multirobot control technique to find and spatially characterize general scalar field fronts. This is achieved with a multirobot system that adaptively sizes, orients, and translates based on data sampled by each robot in order to span the width of the front, and to move longitudinally along the front, reversing direction when the end of the front is reached. This span and scan functionality allows the cluster to spatially characterize the front's full extent and to enable comprehensive scientific study of its associated phenomenon. To our knowledge, this is the first comprehensive, adaptive multirobot front-following control strategy proposed for general front structures, and initial verification of its capability is provided through both simulation of behavior on a large-scale front and experimentation with a simple, small-scale indoor multirobot testbed.

Organization of dust storms and synoptic-scale transport of dust by Kelvin waves

Abstract. Based on the large-scale transport of dust driven by the winds parallel to the mountains in the Harmattan, Saudi Arabian, and Bodélé Depression dust storm cases, a detailed study of the generation of Kelvin waves and its possible role in organizing these dust storms and large-scale dust transport was accomplished. For this study, observational and numerical model analyses were done in an in depth manner. For this, MERRA reanalysis data sets; WRF-simulated high-resolution variables; MODIS Aqua and Terra images; EUMETSAT images; NAAPS aerosol modeling plots; and MERRA-2 dust scattering aerosol optical depth (AOD) modeling plots, surface observations, and rawinsonde soundings were analyzed for each of these three case studies. We found that there were meso-β-scale (horizontal length scale of 20–200 km) adjustment processes resulting in Kelvin waves only in the Harmattan and the Bodélé Depression cases. The Kelvin wave preceded a cold pool accompanying the air behind the large-scale cold front instrumental in the major dust storm. We find that this Kelvin wave organized the major dust storm in a narrow zone parallel to the mountains before it expanded upscale (meso-α to synoptic).

Objective Function BF-ETX for RPL Routing Protocol

This paper presents the adaptation and implementation of the objective function in routing protocol RPL (Routing Protocol for Low Power and Lossy Networks) restating in terms of the status of the connection for better performance quality of service offered. In this way, a new routing metric that improves the indicators quality of service was established: network lifetime, packet delivery and energy savings of LLN networks outlined in the framework of the Internet of Things (IoT); all within the functional specifications for these networks of Internet and Engineering Working Group (In English: Internet Engineering Task Force - IETF). It was performed a series of tests and different simulations in Cooja, where BF-ETX generates better results in terms of network latency and power consumption in densely deployed networks and large-scale front MRHOF. This way, it can achieve a better performance of LLNS to large scale in terms of quality of service compared to routing problems.

Особенности структуры поля температуры и крупномасштабных фронтов на поверхности Индоокеанского сектора Южного океана в районах неоднородности рельефа дна

Особенности структуры поля температуры и крупномасштабных фронтов на поверхности Индоокеанского сектора Южного океана в районах неоднородности рельефа дна

NGC 2207/IC 2163: A GRAZING ENCOUNTER WITH LARGE-SCALE SHOCKS

Radio continuum, Spitzer infrared, optical and XMM-Newton X-ray and UVM2 observations are used to study large-scale shock fronts, young star complexes, and the galactic nuclei in the interacting galaxies NGC 2207/IC 2163. There are two types of large-scale shock fronts in this galaxy pair. The shock front along the rim of the ocular oval in IC 2163 has produced vigorous star formation in a dusty environment. In the outer part of the companion side of NGC 2207, a large-scale front attributed to disk or halo scraping is particularly bright in the radio continuum but not in any tracers of recent star formation or in X-rays. This radio continuum front may be mainly in the halo on the back side of NGC 2207 between the two galaxies. Values of the flux density ratio S(8 um)/S(6 cm) of kpc-sized, Spitzer IRAC star-forming clumps in NGC 2207/IC 2163 are compared with those of giant H II regions in M81. We find evidence that in 2001 a radio supernova was present in the core of feature i, a mini-starburst on an outer arm of NGC 2207. X-ray emission is detected from the NGC 2207 nucleus and from nine discrete sources, one of which corresponds to SN 1999ec, and another may be a radio supernova or a background quasar. The X-ray luminosity and X-ray spectrum of the NGC 2207 nucleus suggests it is a highly absorbed, low luminosity AGN.

Restratification of the Surface Mixed Layer with Submesoscale Lateral Density Gradients: Diagnosing the Importance of the Horizontal Dimension

Abstract A depth-cycling towed conductivity–temperature–depth (CTD) and vessel-mounted acoustic Doppler current profiler (ADCP) were used to obtain four-dimensional measurements of the restratification of the surface mixed layer (SML) at a submesoscale lateral density gradient near the subtropical front. With the objective of studying the role of horizontal processes in restratification, the thermohaline and velocity fields were monitored for 33 h by 16 small-scale (≤15 km2) surveys centered on a drogued float. Daytime warming by insolation caused a unidirectional displacement of the initially vertical isopycnals toward increasing density. Across the entire SML (50-m vertical scale), solar insolation accounted for 60% of observed restratification, but over 10-m scales, the percentage decreased with depth from 80% at 25–35 m to ≤25% at 55–65 m. Below 35 m, stratification was enhanced by the vertically sheared horizontal advection of the lateral density gradient due to a near-inertial wave of ∼100-m vertical wavelength that rotated anticyclonically at the inertial frequency. The phase and similar period (25.4 h) of the local inertial period to the diurnal cycle ensured constructive interference with isopycnal displacements due to insolation. Restratification by sheared advection matched that predicted due to vertically sheared inertial oscillations generated during the geostrophic adjustment of a density front, but direct wind forcing may also have generated the wave that was subsequently modified by interaction with mesoscale vorticity associated with a nearby large-scale front. By further including the effects of lateral uncompensated thermohaline inhomogeneity, the authors can account for 100% ± 20% of the observed N 2 during daytime restratification. No detectable restratification due to the slumping of horizontal density gradients under gravity alone was found.

Front propagation in a laminar cellular flow: Shapes, velocities, and least time criterion

We experimentally investigate the propagation of chemical fronts in steady laminar cellular flows at large Péclet numbers and large Damköhler numbers. Fronts are generated in an aqueous solution by an autocatalytic oxydoreduction reaction. They propagate in a channel in which a chain of counter-rotative parallel vortices is induced by electroconvection. We first accurately determine the form, the dynamics and the mean velocity of these fronts in the whole Hele-Shaw regime of the flow. We then address the modeling of the evolution of their mean velocity with the flow amplitude. The structure of the front wakes yields us to reject an effective reaction-diffusion wave as a relevant model for large-scale front propagation. On the other hand, analysis of the role of front heads brings us to introduce a kinematic model at the vortex scale for uncovering the front dynamics. This model addresses the propagation of the front leading point in a chain of vortices whose field is modeled by a two-dimensional solid rotation complemented by a boundary layer. Interestingly, it sensitively relies on the effective trajectory followed by the front leading point. To account for this, a competition is worked out among a one-parameter family of potential trajectories. The actual trajectory is then selected as the fastest one with quite a good agreement with measurements and observations. In particular, the measured effective front velocities are well recovered from the model, including their intrinsic dependence on the boundary layer width. Accordingly, effective front propagation in a laminar steadily stirred medium is thus understood from an optimization principle similar to the Fermat principle of ray propagation in heterogeneous media.

Bright knots along spiral arms in disk galaxies

AbstractMany spiral galaxies show bright knots along their arms on high resolution K-band images. Spectroscopy of such knots suggests that they are very young stellar clusters which formation was triggered by a large-scale front associated to a density wave. We have studied a sample of around 80 disk galaxies (with i < 65°) for which deep K-band maps with a resolution of <1″ are available and present preliminary statistics of such bright knots.

Development of the geometric structure of the thermonuclear-deflagration front in type Ia supernovae

Three-dimensional hydrodynamical simulations of the development of a large-scale instability accompanying deflagration in the degenerate cores of rotating white dwarfs—progenitors of type-Ia supernovae—are presented. The numerical algorithm used is described in detail. An explicit, conservative, Godunov-type TVD difference scheme was employed for the computations. Large-scale convective processes are important as the deflagration front propagates. The supernova explosion is strongly nonspherically symmetric; a large-scale front structure emerges and propagates most rapidly along the rotational axis. The arrival of fresh thermonuclear fuel to the central region of the core can result in flares and the destruction of the core.

A Laboratory Model of Thermocline Depth and Exchange Fluxes across Circumpolar Fronts*

Abstract A laboratory experiment has been constructed to investigate the possibility that the equilibrium depth of a circumpolar front is set by a balance between the rate at which potential energy is created by mechanical and buoyancy forcing and the rate at which it is released by eddies. In a rotating cylindrical tank, the combined action of mechanical and buoyancy forcing builds a stratification, creating a large-scale front. At equilibrium, the depth of penetration and strength of the current are then determined by the balance between eddy transport and sources and sinks associated with imposed patterns of Ekman pumping and buoyancy fluxes. It is found that the depth of penetration and transport of the front scale like [(fwe)/g′]L and weL2, respectively, where we is the Ekman pumping, g′ is the reduced gravity across the front, f is the Coriolis parameter, and L is the width scale of the front. Last, the implications of this study for understanding those processes that set the stratification and transport of the Antarctic Circumpolar Current (ACC) are discussed. If the laboratory results scale up to the ACC, they suggest a maximum thermocline depth of approximately h = 2 km, a zonal current velocity of 4.6 cm s−1, and a transport T = 150 Sv (1 Sv ≡ 106 m3 s−1), not dissimilar to what is observed.

Development of Optical Mesosphere Thermosphere Imagers (OMTI)

The Optical Mesosphere Thermosphere Imagers (OMTI) have been developed to investigate the dynamics of the upper atmosphere through nocturnal airglow emissions. The OMTI consist of an imaging Fabry-Perot interferometer, three all-sky cooled-CCD cameras, three tilting photometers, and a Spectral Airglow Temperature Imager (SATI) with two container houses to install them in. These instruments measure wind, temperature and 2-dimensional airglow patterns in the upper atmosphere at multi-wavelengths of OI (557.7 nm and 630.0 nm), OH (6–2) bands, O2 (0, 1) bands, and Na (589.3 nm), simultaneously. Examples of the data are shown for the cameras, the photometers, and the SATI based on the airglow observation at a mid-latitude station in Japan. Good correlation of the photometer and SATI observations is obtained. A comparison is shown for small- and large-scale wave structures in airglow images at four wavelengths around the mesopause region using four cooled-CCD cameras. We found an event during which large-scale bands, small-scale row-like structures, and large-scale front passage occur simultaneously.

Upper mixed layer temperature anomalies at the North Atlantic storm-track zone

Abstract. Synoptic sea surface temperature anomalies (SSTAs) were determined as a result of separation of time scales smaller than 183 days. The SSTAs were investigated using daily data of ocean weather station "C" (52.75°N; 35.5°W) from 1 January 1976 to 31 December 1980 (1827 days). There were 47 positive and 50 negative significant SSTAs (lifetime longer than 3 days, absolute value greater than 0.10 °C) with four main intervals of the lifetime repetitions: 1. 4–7 days (45% of all cases), 2. 9–13 days (20–25%), 3. 14–18 days (10–15%), and 4. 21–30 days (10–15%) and with a magnitude 1.5–2.0 °C. An upper layer balance model based on equations for temperature, salinity, mechanical energy (with advanced parametrization), state (density), and drift currents was used to simulate SSTA. The original method of modelling taking into account the mean observed temperature profiles proved to be very stable. The model SSTAs are in a good agreement with the observed amplitudes and phases of synoptic SSTAs during all 5 years. Surface heat flux anomalies are the main source of SSTAs. The influence of anomalous drift heat advection is about 30–50% of the SSTA, and the influence of salinity anomalies is about 10–25% and less. The influence of a large-scale ocean front was isolated only once in February-April 1978 during all 5 years. Synoptic SSTAs develop just in the upper half of the homogeneous layer at each winter. We suggest that there are two main causes of such active sublayer formation: 1. surface heat flux in the warm sectors of cyclones and 2. predominant heat transport by ocean currents from the south. All frequency functions of the ocean temperature synoptic response to heat and momentum surface fluxes are of integral character (red noise), though there is strong resonance with 20-days period of wind-driven horizontal heat advection with mixed layer temperature; there are some other peculiarities on the time scales from 5.5 to 13 days. Observed and modelled frequency functions seem to be in good agreement.