Coastal coverage of ESA’ Sentinel 2 mission

Climatologic and anthropogenic pressures in coastal areas affect the coastal zone at different scales. With the development of new missions in open-access, satellites now represent an attractive solution for a broad public to capture local-scale coastal impacts at large scales. Here, the capability of the Sentinel 2 constellation to cover coastal areas and measure coastal processes –physical and biological. We show that Sentinel 2 enables high-frequency measurements across the globe. Cloud coverage at higher latitudes is overcome by decrease revisit time-intervals. Only around the equator, the longest revisit intervals and high cloud cover probability limits coastal measurements there. Sentinel 2 based methods are capable of estimating Digital Elevation Models for mid- to high-latitude coastal zones and sporadic spots for lower latitudes where 2 orbit swaths overlap. For the majority of the world’s coastal bathymetries can be obtained with the Sentinel 2 imagery surpassing the depth of closure (beyond this offshore limit sediment transport is limited). Only in sheltered areas, wave-based bathymetry inversion is limited but at these areas inversion through colouring (light penetration) prevails. This works shows that Sentinel 2 enables coastal monitoring as never before, large spatial scale with revisits of a few days at most of the world.     

The X-TRACK/ALES multi-mission processing system: New advances in altimetry towards the coast

In the context of the ESA Climate Change Initiative project, a new coastal sea level altimetry product has been developed in order to support advances in coastal sea level variability studies. Measurements from Jason-1,2&3 missions have been retracked with the Adaptive Leading Edge Subwaveform (ALES) Retracker and then ingested in the X-TRACK software with the best possible set of altimetry corrections. These two coastal altimetry processing approaches, previously successfully validated and applied to coastal sea level research, are combined here for the first time in order to derive a 16-year-long (June 2002 to May 2018), high-resolution (20-Hz), along-track sea level dataset in six regions: Northeast Atlantic, Mediterranean Sea, West Africa, North Indian Ocean, Southeast Asia and Australia. The study demonstrates that this new coastal sea level product called X-TRACK/ALES is able to extend the spatial coverage of sea level altimetry data ~3.5 km in the land direction, when compared to the X-TRACK 1-Hz dataset. We also observe a large improvement in coastal sea level data availability from Jason-1 to Jason-3, with data at 3.6 km, 1.9 km and 0.9 km to the coast on average, for Jason-1, Jason-2 and Jason-3, respectively. When combining measurements from Jason-1 to Jason-3, we reach a distance of 1.2–4 km to the coast. When compared to tide gauge data, the accuracy of the new altimetry near-shore sea level estimations also improves. In terms of correlations with a large set of independent tide gauge observations selected in the six regions, we obtain an average value of 0.77. We also show that it is now possible to derive from the X-TRACK/ALES product an estimation of the ocean current variability up to 5 km to the coast. This new altimetry dataset, freely available, will provide a valuable contribution of altimetry in coastal marine research community.     

Lagrangian analysis of satellite-derived currents: Application to the North Western Mediterranean coastal dynamics

Optimal interpolation methods for improving the reconstruction of coastal dynamics from along-track satellite altimetry measurements have been recently developed over the North Western Mediterranean Sea. Maps of satellite-derived geostrophic current anomalies are generated using these methods, and added to different mean circulation fields in order to obtained absolute geostrophic currents. The resulting fields are then compared to standard AVISO products, and their accuracies are assessed with Lagrangian diagnostics. The trajectories of virtual particle clusters are simulated with a Lagrangian code either with new current fields or with the AVISO ones. The simulated trajectories are then compared to 16 in situ drifter trajectories to evaluate the performance of the different velocity fields. The comparisons show that the new current fields lead to better results than the AVISO one, especially over the shallow continental shelf of the Gulf of Lion. However, despite the use of innovative strategies, some altimetry limitations still persist in the coastal domain, where small scale processes remain sub-sampled by conventional altimetry coverage but will benefit from technological development in the near future. Some of the limitations of the Lagrangian diagnostics presently used are also analyzed, but dedicated studies will be required for future further investigations.     

Spatio-temporal variability of the wet component of the troposphere – Application to satellite altimetry

Due to the presence of water vapour and cloud liquid water in the atmosphere, the wet component of the troposphere is responsible for a delay in the propagation of the altimeter signals, the Wet Path Delay (WPD). The high space–time variability of the water vapour distribution makes the modelling of WPD difficult, its effect still being one of the main error sources in satellite altimetry applications, e.g. in the estimation of Mean Sea Level (MSL). The understanding and the quantification of the WPD variability on various spatial and temporal scales are the main purposes of this study, in view to improve the MSL error budget. The dominant timescales of WPD variability and its correlation with Sea Level Anomaly (SLA) are examined. In these analyses, the atmospheric reanalysis ERA-Interim model from the European Centre for Medium-Range Weather Forecasts (ECMWF) is used to derive a global dataset of daily grids of WPD, spanning a 28-year period from January 1988 to December 2015. The Seasonal-Trend decomposition procedure based on Loess (STL) is used to extract precise WPD annual and interannual signals. Linear trends have been derived from the interannual time series and the contribution of each STL component was mapped globally, allowing the understanding of the WPD variability in spatial terms. The correlation between SLA and WPD is mapped and decomposed into seasons using monthly mean grids, for a period of 21-years, from January 1993 to December 2013.Aiming at inspecting the sensitivity of the results to the used data set, the WPD temporal analysis is extended to the data set provided by the Special Sensor Microwave Imager (SSM/I) and SSM/I Sounder (SSM/IS) Sensors. The WPD from SSM/I(S) is compared against those from the ERA-Interim and from the National Centers for Environmental Prediction (NCEP).Results show that climate phenomena, especially the El Niño Southern Oscillation (ENSO) are the cause for this high variability, since they affect the water vapour and temperature. The observed trends from ERA-Interim, computed globally and over ocean regions only, allow concluding that WPD is increasing with time by approximately 0.1?mm per year, and the maximum trends are observed for the Pacific North and Indian Oceans. High correlation between WPD and SLA is found over the western tropical Pacific.The comparison between WPD from SSM/I(S) and from ERA-Interim and NCEP, allows concluding that the trends computed using only the SSM/I(S) measurement points are substantially larger.     

ASAR imaging for coastal upwelling in the Baltic Sea

Analysis of Envisat Advanced Synthetic Aperture Radar (ASAR) and Aqua/Terra Moderate Imaging Spectrometer (MODIS) infrared (IR) imagery of coastal upwelling in the southeastern Baltic Sea is presented. It is found that upwelling features are well distinct in the SAR images, and the leading imaging mechanism appears to be the change of the marine atmospheric boundary layer (MABL) stratification over the sea surface temperature (SST) front. This finding is supported by model calculations of the MABL transformation supplemented with the SAR backscatter calculations based on the CMOD4 model. In addition an empirical dependence of the SAR contrasts over the upwelling region on the wind speed and the SST drop is suggested. Finally, surface slicks accumulated in the sea surface current convergence zones generate additional distinct features in SAR imagery. This effect is interpreted within the framework of the coastal current circulation model based on analysis of the SST snapshot.     

Semi-automatic determination of the Azores Current axis using satellite altimetry: Application to the study of the current variability during 1995–2006

Satellite altimetry has been widely used to study the variability of the ocean currents such as the Azores Current (AzC) in the North Atlantic. Most analyses are performed over the region that encloses the current, thus being somehow affected by other oceanographic signals, e.g., eddies. In this study, a new approach for extracting the axis of a zonal current solely based on satellite altimetry is presented. This is a semi-automatic procedure that searches for the maximum values of the gradient of absolute dynamic topography (ADT), using the geostrophic velocity as auxiliary information. The advantage of this approach is to allow the analyses to be performed over a buffer centered on the current axis instead of using a wider region. It is here applied to the AzC for the period June 1995–October 2006.     

Satellite-observed biological variability in the equatorial Pacific during the 2009–2011 ENSO cycle

The event of 2009–2011 El Niño Southern Oscillation (ENSO) provides an opportunity to gain insight into the biological variability of the equatorial Pacific Ocean for an entire ENSO cycle with satellite and in situ observations. Even though El Niño and La Niña in general led to respectively weakened and enhanced chlorophyll-a concentration and net primary production (NPP) along the equatorial Pacific Ocean during the 2009–2011 ENSO cycle, biological responses were highly disparate along the equator and attributed to different driving mechanisms. In the eastern equatorial Pacific east of 150°E, the El Niño-La Niña biological change was in general small except for the transition period even though sea surface temperature (SST) showed over ∼5 °C drop from El Niño to La Niña. In the central-eastern (170°W–140°W) equatorial Pacific, moderate change of biological activity is attributed to the changes of thermocline driven by the eastward propagating equatorial Kelvin waves and changes of zonal currents and undercurrents. Highest biological response in this ENSO cycle was located in the central (170°E–170°W) and central-western (150°E–170°E) equatorial Pacific with quadruple chlorophyll-a concentration and over ∼400 mg C m⁻² d⁻¹ increase of NPP from El Niño in 2009 to La Niña in 2010. However, spatial pattern of ENSO biological variability as represented with NPP is not exactly the same as chlorophyll-a variability. Wind-driving mixing of nutrients and eastward advection of the oligotrophic warm pool waters are attributed to this significant biological variability in this region.     

Second order experimental statistics of rain attenuation at Ka band in a tropical location

Some second order rain attenuation statistics such as fade duration and fade slope are investigated on the basis of experimental measurements of received signals using the GSAT-14 satellite beacon signal at 20.2 GHz for three years (2014–2016) over the tropical location Ahmedabad (23.02 ⁰E, 72.51⁰N), India with an Elevation angle of 63⁰. Existing models of fade duration are compared with experimental data in this study and exponent of power law model of fade duration at Ka band is further explored. A new model for fade duration for Ka band for tropical locations is proposed where the constant of exponent of attenuation in the power law is found to be 0.143 instead of 0.055 used in ITU-R. Other relevant parameters for implementation of fade mitigation technique to prevent the link outage like cumulative distribution of signal fade rate, maximum and minimum fade rise and fade fall are also studied. Fade slope asymmetry over tropical region is also investigated. Keeping in view of exploiting the commercial launch of Ka band in Indian region there is an urgent need for validation of the existing models of fade slope (specially looking into fade symmetry) and fade duration. It will help the SATCOM (Satellite Communication) link designer to improve closed loop fade mitigation technique to minimize the possible link failure/link outage over the tropical region.     

Investigation of Satellite Trace (ST) and Multi-reflected Echo (MRE) ionogram signatures and its possible correlation to nighttime spread F development from Cyprus over the solar mini-max (2009–2016)

Multi-reflected echoes (MREs) and satellite traces (STs) are referred in literature as ionogram signatures of Travelling Ionospheric Disturbances (TIDs) which is a phenomenon that apparently drives spread F development mainly at nighttime mid-latitude ionosphere. A long-term statistical study has been undertaken to investigate the morphological aspect of these signatures over the lower midlatitude European station of Nicosia, Cyprus (35.19°N, 33.38°E geographic; magnetic dip. 29.38°N) by inspecting all ionograms recorded by the DPS-4D digisonde in the interval 2009–2016. The results underline the systematic manifestation of these TID signatures over Cyprus with a possible (although not quite clear) solar activity dependence and a distinctive seasonal and diurnal occurrence rate with a seasonal peak of STs during summer and of MREs during January to April. Based on the experimental results of the present study, the seasonal occurrence rate of MREs and STs is found to increase by 75% and 56% during high solar activity periods. Satellite traces are well known ionogram signatures of TIDs and mostly correlated to the nighttime spread F formation. The occurrence of mid-latitude spread Fs over European longitude sector normally increases during summer. The occurrences of TIDs are also prominent at this interval of the year over nighttime mid-latitude ionosphere. The presence of MREs as an ionogram signature of TIDs over mid-latitude ionosphere is unique in nature.     

Rio de la Plata estuarine system: Relationship between river flow and frontal variability

This paper deals with the application of SeaWIFS images to characterize spatial and temporal variability of fronts in the Rio de la Plata estuarine system over the period 2000–2003. We aim to depict the relationship between river outflow and variability of fronts’ loci on monthly to ENSO-related timescales and the influence of the winds along Rio de la Plata (axial winds) on the abrupt changes in frontal dynamics over synoptic timescales. During the studied period both La Niña (July 1999–June 2000) and El Niño (April 2002–May 2003) events induced significant displacements of fronts. Three distinct fronts were analyzed between river, estuarine, coastal and marine waters of the Rio de la Plata: Main Turbidity Front, Main Marine Front, and Secondary Marine Front. Their number, location and separation seem to be mainly related to river outflow and second, to fresh (>8 m/s) axial winds. During low discharge periods (i.e. summer time and/or La Niña events) these winds induce abrupt changes in the location of fronts (100–200 km) and greater separation between them over synoptic timescale, whereas during high river discharge or ENSO years some of the variability of fronts location is explained by the river’s outflow fluctuations, especially by the high variability of the River Uruguay discharge.