Linear stability analysis on the onset of viscous fingering due to a non monotonic viscosity profile for immiscible fluids

The onset of viscous fingering in the presence of a non monotonic viscosity profile is investigated theoretically for two immiscible fluids. Classical fluid dynamics predicts that no unstable behavior may be observed when a viscous fluid pushes a less viscous one in a Hele-Shaw cell. Here, we show that the presence of a viscosity gradient at the interface between both fluids destabilize the interface facilitating the spread of the perturbation. The influence of the viscosity gradient on the dispersion relation is analyzed

A method to calculate finite-time Lyapunov exponents for inertial particles in incompressible flows

The present study aims to improve the calculus of finite-time Lyapunov exponents (FTLEs) applied to describe the transport of inertial particles in a fluid flow. To this aim, the deformation tensor is modified to take into account that the stretching rate between particles separated by a certain distance is influenced by the initial velocity of the particles. Thus, the inertial FTLEs (iFTLEs) are defined in terms of the maximum stretching between infinitesimally close trajectories that have different initial velocities. The advantages of this improvement, if compared to the standard method (Shadden et al., 2005), are discussed for the double-gyre flow and the meandering jet flow. The new method allows one to identify the initial velocity that inertial particles must have in order to maximize their dispersion.This work was supported by the Ministerio de Economía y Competitividad under research grant CGL2013-45932-RS

HF Radars for Wave Energy Resource Assessment Offshore NW Spain

Wave energy resource assessment is crucial for the development of the marine renewable industry. High-frequency radars (HF radars) have been demonstrated to be a useful wave measuring tool. Therefore, in this work, we evaluated the accuracy of two CODAR Seasonde HF radars for describing the wave energy resource of two offshore areas in the west Galician coast, Spain (Vilán and Silleiro capes). The resulting wave characterization was used to estimate the electricity production of two wave energy converters. Results were validated against wave data from two buoys and two numerical models (SIMAR, (Marine Simulation) and WaveWatch III). The statistical validation revealed that the radar of Silleiro cape significantly overestimates the wave power, mainly due to a large overestimation of the wave energy period. The effect of the radars’ data loss during low wave energy periods on the mean wave energy is partially compensated with the overestimation of wave height and energy period. The theoretical electrical energy production of the wave energy converters was also affected by these differences. Energy period estimation was found to be highly conditioned to the unimodal interpretation of the wave spectrum, and it is expected that new releases of the radar software will be able to characterize different sea states independentlyThis research was funded by INTERREG V-A Spain–Portugal (POCTEP) project RADAR ON RAIA (0461-RADAR ON RAIA-1-E) co-funded by the European Regional Development Fund (ERDF) (EU)S

Climatology of Lyapunov exponents : the link between atmospheric rivers and large-scale mixing variability

Large-scale tropospheric mixing and Lagrangian transport properties have been analyzed for the long-term period 1979–2014 in terms of the finite-time Lyapunov exponents (FTLEs).Wind field reanalyses from the European Centre for Medium-Range Weather Forecasts were used to calculate the Lagrangian trajectories of large ensembles of particles. Larger values of the interannual and intra-annual mixing variabilities highlight the El Niño Southern Oscillation, the storm track, or the Intertropical Convergence Zone among other largescale structures. The mean baroclinic instability growth rate and the mean atmospheric river occurrence show large correlation values with the FTLE climatology as an indication of their influence on tropospheric mixing in the midlatitudes. As a case study, the role that land-falling atmospheric rivers have on large-scale tropospheric mixing and the precipitation rates observed in Saharan Morocco and the British Isles has been analyzed. The atmospheric river contribution to tropospheric mixing is found to decrease from 15% in Saharan Morocco to less than 5% for the UK and Ireland regions, in agreement with their contribution to precipitation that is 40% larger in the former than in the latter regionThis work was financially supported by Ministerio de Economía y Competitividad and Xunta de Galicia (CGL2013-45932-R, GPC2015/014) and contributions by the COST Action MP1305 and CRETUS Strategic Partnership (AGRUP2015/02). All these programs are co-funded by ERDF (EU)S

Uso del modelo roms (Sistema de Modelado Oceánico Regional) para mejorar la predicción de la temperatura de la superficie del agua de mar del modelo mercator

Global models are generally capable of reproducing the observed trends in the globally averaged sea surface temperature (SST). However, the global models do not perform as well on regional scales. Here, we present an ocean forecast system based on the Regional Ocean Modelling System (ROMS), the boundary conditions come from the MERCATOR ocean system for the North Atlantic (1/6° horizontal resolution). The system covers the region of the northwestern Iberian Peninsula with a horizontal resolution of 1/36°, forced with the Weather Research and Forecasting Model (WRF) and the Soil Water Assessment Tool (SWAT). The ocean model results from the regional ocean model are validated using real-time SST and observations from the MeteoGalicia, INTECMAR and Puertos Del Estado real-time observational networks. The validation results reveal that over a one-year period the mean absolute error of the SST is less than 1°C, and several sources of measured data reveal that the errors decrease near the coast. This improvement is related to the inclusion of local forcing not present in the boundary condition model.Los modelos globales son capaces de reproducir con suficiente fiabilidad las tendencias observadas a gran escala en la SST (temperatura la superficie del agua del mar, por sus siglas en inglés). Sin embargo estos modelos no muestran tanta precisión en escalas locales. En este trabajo se presenta un sistema de predicción oceánico basado en el modelo regional oceánico ROMS anidado al modelo oceánico MERCATOR (resolución 1/6°) para el Atlántico Norte. El sistema se extiende a lo largo del noroeste de la Península Ibérica con una resolución horizontal de 1/36° y forzado con los modelos WRF para las condiciones atmosféricas y el modelo SWAT para los caudales de los ríos. El sistema de predicción se validó utilizando SST de OSTIA y observaciones provenientes de la redes de medición en tiempo real de Puertos del Estado, Intecmar y Meteogalicia. Los resultados muestran que para el periodo de un año estudiado el error medio absoluto de la SST es menor de 1°C, y las comparaciones contra observaciones puntuales muestran que este error es incluso menor cerca de costa. Los autores estiman que este comportamiento se debe a la inclusión de diferentes forzamientos locales no presentes en el modelo de condiciones de contorno

Tagging moisture sources with Lagrangian and inertial tracers: application to intense atmospheric river events

Two Lagrangian tracer tools are evaluated for studies on atmospheric moisture sources and pathways. In these methods, a moisture volume is assigned to each particle, which is then advected by the wind flow. Usual Lagrangian methods consider this volume to remain constant and the particle to follow flow path lines exactly. In a different approach, the initial moisture volume can be considered to depend on time as it is advected by the flow due to thermodynamic processes. In this case, the tracer volume drag must be taken into account. Equations have been implemented and moisture convection was taken into account for both Lagrangian and inertial models. We apply these methods to evaluate the intense atmospheric rivers that devastated (i) the Pacific Northwest region of the US and (ii) the western Iberian Peninsula with flooding rains and intense winds in early November 2006 and 20 May 1994, respectively. We note that the usual Lagrangian method underestimates moisture availability in the continent, while active tracers achieve more realistic resultsERA-Interim data were supported by the ECMWF. This work was financially supported by Ministerio de Economía, Industria y Competitividad (CGL2017-89859-Rand CGL2013-45932-R), with contributions by the COST Action MP1305 and CRETUS strategic partnership (AGRUP2015/02). All these programs are co-funded by the ERDF (EU)S

Atmospheric Temperature Effect in secondary cosmic rays observed with a two square meter ground-based detector

A high resolution 2 m$^2$ tracking detector, based on timing Resistive Plate Chamber (tRPC) cells, has been installed at the Faculty of Physics of the University of Santiago de Compostela (Spain) in order to improve our understanding of the cosmic rays arriving at the Earth's surface. Following a short commisioning of the detector, a study of the atmospheric temperature effect of the secondary cosmic ray component was carried out. A method based on Principal Component Analysis (PCA) has been implemented in order to obtain the distribution of temperature coefficients, $W_T(h)$, using as input the measured rate of nearly vertical cosmic ray tracks, showing good agreement with the theoretical expectation. The method succesfully removes the correlation present between the different atmospheric layers, that would be dominant otherwise. We briefly describe the initial calibration and pressure correction procedures, essential to isolate the temperature effect. Overall, the measured cosmic ray rate displays the expected anticorrelation with the effective atmospheric temperature, through the coefficient $\alpha_T= -0.279 \pm 0.051$ \%/K. Rates follow the seasonal variations, and unusual short-term events are clearly identified too.Comment: 18 pages, 10 figures, Submitted to Earth and Space Scienc

Influence of finite-time Lyapunov exponents on winter precipitation over the Iberian Peninsula

Seasonal forecasts have improved during the last decades, mostly due to an increase in understanding of the coupled ocean–atmosphere dynamics, and the development of models able to predict the atmosphere variability. Correlations between different teleconnection patterns and severe weather in different parts of the world are constantly evolving and changing. This paper evaluates the connection between winter precipitation over the Iberian Peninsula and the large-scale tropospheric mixing over the eastern Atlantic Ocean. Finite-time Lyapunov exponents (FTLEs) have been calculated from 1979 to 2008 to evaluate this mixing. Our study suggests that significant negative correlations exist between summer FTLE anomalies and winter precipitation over Portugal and Spain. To understand the mechanisms behind this correlation, summer anomalies of the FTLE have also been correlated with other climatic variables such as the sea surface temperature (SST), the sea level pressure (SLP) or the geopotential. The East Atlantic (EA) teleconnection index correlates with the summer FTLE anomalies, confirming their role as a seasonal predictor for winter precipitation over the Iberian PeninsulaThis work was financially supported by the Ministerio de Economía y Competitividad and Xunta de Galicia (CGL2013-45932-R, GPC2015/014), and contributions by COST Action MP1305 and the CRETUS Strategic Partnership (AGRUP2015/02). All these programs are co-funded by the ERDF (EU). The support of Office of Naval Research grant no. N00014-16-1-2492 is also acknowledgedS

Quality Assessment and Practical Interpretation of the Wave Parameters Estimated by HF Radars in NW Spain

High-frequency (HF) radars are efficient tools for measuring vast areas and gathering ocean parameters in real-time. However, the accuracy of their wave estimates is under analysis. This paper presents a new methodology for analyzing and validating the wave data estimated by two CODAR SeaSonde radars located on the Galician coast (NW Spain). Approximately one and a half years of wave data (January, 2014–April, 2015) were obtained for ten range cells employing two different sampling times used by the radar software. The resulting data were screened by an updated method, and their abundance and quality were described for each radar range cell and different wave regime; the latter were defined using the spectral significant wave height (Hm0) and mean wave direction (Dm) estimated by two buoys and three SIMAR points (SImulación MARina in Spanish, from the wave reanalysis model by Puertos del Estado (PdE)). The correlation between the results and the particularities of the different sea states (broadband or bimodal), the wind and the operation of the devices are discussed. Most HF radar wave parameters’ errors occur for waves from the NNE and higher than 6 m. The best agreement between the Vilán radar and the Vilano-Sisargas buoy wave data was obtained for the dominant wave regime (from the northwest) and the southwest wave regime. However, relevant contradictions regarding wave direction were detected. The possibilities of reducing the wave parameters’ processing time by one hour and increasing the numbers of range cells of the radars have been validatedThis research was funded by Interreg Atlantic Area project MyCOAST (EAPA 285/2016) and INTERREG V-A Spain-Portugal (POCTEP) project RADAR_ON_RAIA co-funded by the European Regional Development Fund (ERDF) (EU). V.P.-M. and A.B. acknowledge financial support by CRETUS strategic partnership (ED431E2018/01), co-funded by the ERDF (EU) http://www.usc.es/cretus/S