Tatiana Yu. Bocharova

Investigation of fine spatial structure of currents and submesoscale eddies based on satellite radar data and concurrent acoustic measurements

Satellite high-resolution radar data from Envisat ASAR, RADARSAT-2 and TerraSAR-X sensors are used for the detection and investigation of fine structure of currents and submesoscale eddies. The fine structure of sea currents is manifested in radar images as quasiperiodic slick strips of a horizontal scale from tens of meters to several kilometers. The strips are typically elongated along the currents apparently indicating their jet character and intermittence. The focus is on filamentary slick bands involved into small scale vortical processes and allowing estimation of spatial characteristics of submesoscale eddies based on their SAR signatures. Submesoscale or eddies with diameters of less than Rossby radius of deformation could hardly be investigated by traditional oceanographic means due to spontaneity of their appearance, nonstationarity and short lifetimes. Sources of their generation are still not well known. A large archive of satellite data accumulated during 1999-2011 over different parts of the Black, Baltic and Caspian Seas has been analyzed in order to search, systemize and reveal mechanisms of formation and evolution of small scale eddy structures in these areas. Atmospheric and hydrological fronts and current instability have been found to play a significant role in submesoscale eddies generation. Subsatellite measurements using acoustic Doppler profiler (ADCP) have been performed aimed at verification and adjustment of the results obtained based on satellite radar data. The formation and drift of a near-shore submesoscale anticyclonic eddies of 5-7 km in diameter were observed.

River plumes investigation using Sentinel-2A MSI and Landsat-8 OLI data

We present the results of using Sentinel-2A Multispectral Imager Instrument (MSI/S2) and Landsat-8 Operational Land Imager (OLI/L8) data to monitor river plumes in the eastern Black Sea and from the Rhône River in the Mediterranean Sea. The focus is on exploring the possibility to investigate hydrodynamic processes associated with river outflows, in particular internal waves (IWs). Submesoscale IWs having wavelengths less than 50 m and generated by unstable sharp front of a river plume were revealed and their parameters were assessed. A map of surface manifestation of IW trains in the Gulf of Lions was created based on MSI/S2 images. There are different mechanisms of IW generation in river outflow zones, they are determined by a number of parameters including river discharge, bottom topography and presence of tidal currents or inertial period IWs in the shelf zone. A new phenomenon manifested as a chain of quasi circles was discovered. Inertial water motions were suggested as its prime cause, however, this hypothesis is yet to be investigated. An analysis of OLI/L8 and MSI/S2 data enabled us to consider in detail river debouchment streams. For the first time a wave pattern of such stream in the eastern Black Sea was observed in conditions of foehn winds. Usually, foehn winds are distinctly manifested in radar images. A joint analysis of quasi simultaneous ocean color MSI/S2 and Sentinel-1A SAR images demonstrated how water stream wave-like signatures differ from those of foehn winds.

Detecting and tracking small scale eddies in the black sea and the Baltic Sea using high-resolution Radarsat-2 and TerraSAR-X imagery (DTeddie)

High-resolution Synthetic Aperture radar (SAR) images of coastal and off-coastal areas provide a nearly weather independent monitoring of small-scale oceanographic features. In this paper, we describe the advances, which have been achieved within the DTeddie project. Therefore, we present the application of formerly developed algorithms to high-resolution SAR data to estimate the sea surface current at sub-mesoscale. Inside the derived current sea surface current fields, vortical structures, e.g. eddies, will be detected by means of local current anomalies. Another approach uses the alignment of anchored ships as an indicator of the sea surface current. To infer dependent information for each ship, we present an automatic registration method between coastal photographs and the SAR images. Both approaches have proven to yield valuable results with respect to the detection of vortical current flows at sub-mesoscale.

Revealing the influence of various factors on concentration and spatial distribution of suspended matter based on remote sensing data

The paper discusses the results of a research on the influence of various hydrometeorological factors on distribution of suspended matter carried by rivers into the sea. The research is based on remote sensing data received in different bands of electromagnetic spectrum. Suspended matter concentration and integral water turbidity were estimated based on data from MODIS, MERIS, ETM+, TM and OLI sensors. The study was performed for two regions with very different characteristics: the semi-enclosed Gulf of Gdańsk of the Baltic Sea and eastern part of the Black Sea. It is shown that the plume fraction with highest suspended matter concentration of the lowland River Vistula spreads primarily under the impact of wind. Low concentration plume fraction is driven by the longshore current. In case of extraordinary floods, turbid Vistula waters spread in the upper sea layer almost all over the Gulf. The situation in the eastern part of the Black Sea with its narrow shoal and abrupt shelf edge wherein flow highly turbid mountain rivers is quite different. Here, the dominating role is played by runoff. Its intensity determines both plume shape and dimensions. A strong easterly wind can change plume configuration, cause formation of jet-like plumes.

Two polarization radar imagery of sea surface: the dependence on atmospheric stability

It is shown that the two polarizations of microwave radar signal at low grazing angles sense different scatterers: horizontally polarized signal scatters predominantly from steep waves, while vertically polarized one scatters primarily from free Bragg waves. Ocean radar images being sensitive to surface wind fluctuations may be treated as an indicator of atmospheric boundary layer conditions. Radar images of surface imprints of atmospheric patterns near a coastal mountain ridge are considered. In this case the stratification of atmospheric boundary layer near sea surface plays the major role in the formation of lee structures downwind the ridge.

Long-term monitoring of sea ice conditions in the Kerch Strait by remote sensing data

The results of multi-year satellite monitoring of ice conditions in the Kerch Strait connecting the Black and Azov Seas are discussed. The issue gained importance in view of the ongoing construction of the Crimean Bridge across the strait. Our monitoring has been based on the whole variety of available satellite data including visible and radar data over the past 17 years. Every year the Azov Sea becomes fully or partially covered by ice during the cold season. In severe winters, ice often is carried to the Kerch Strait and even the Black Sea. An analysis of ice drift hydrometeorological conditions is presented. The ice conditions of 2017 are under special consideration. Everyday satellite monitoring of the Kerch Strait, including the construction area of the Crimean Bridge, revealed ice formation and drift features on the way from the Azov Sea through the Kerch Strait as well as ice interaction with the piers of the main and technological bridges under construction. It was found that, even under strong northeast winds, ice can pass neither through the piers, nor via the widest shipway. At present, it is hard to discern the impacts of the two bridges on floating ice, nevertheless when the construction is over and the technological bridge is gone, by all appearances the main bridge will strongly affect ice conditions in the Kerch Strait. This perspective calls for continuous satellite monitoring of the area that is enabled by cutting-edge systems and technologies.

Sea surface imprints of wind field patterns in the Black Sea coastal zone as viewed by satellite SAR

The main goal of this study is to derive information on parameters characterizing airflows over the sea surface, particularly downwind natural obstacles from the analysis of SAR imagery. A series of ERS-2 SAR images of the sea surface recorded over north-eastern part of the Black Sea near the Caucasus coast is considered. This set of radar data has been collected in various seasons during 4 years. Related meteorological data, weather and cloudiness maps are available. A distinctive feature of the region is the mountain ridge stretched along shore and having a complicated structure with multiple canyons opening to the sea. Depending on the airflow characteristics radar images of sea surface can bear imprints associated with convection, internal waves in atmosphere, air jets, mushroom-like flows, lee waves, vortices, etc. Radar image patterns are investigated in relation to airflow speed and direction, boundary layer stratification and local topography.

SAR observations of dynamic processes in the Bering Strait

The analysis of ERS-1,2 SAR data of the Bering Strait, obtained in summer 1995, 1998 and 2000 has allowed to observe many important oceanic processes. They include meandering fronts and vortex activity (well pronounced in the eastern part of the Strait), strong jet current at the Prince Wales Cape, intrusions of the cold Chukchi Sea water (extensions of the Siberian Coastal Current) in the western part of the Strait. These images demonstrate some interesting diversions from the northward current typical for the Bering Strait. The flow direction in the central part of the Bering Strait can be revealed from prominent wake features. Also, generation of internal waves on fronts can be detected in some images. Especially impressive is the large variety of surface manifestations of vortex streets north of the Fairway Rock due to high variability of the incident flow. The asymmetry of the Rock causes a special form of the Karman vortex street consisting of a row of single vortices. Specific arc-like features resembling internal wave manifestations can be observed in ERS-SAR images, obtained in the Bering Strait and surrounding waters. Two possible reasons of such features - meander of low salinity water or/and interaction of tidal currents with the bottom relief are discussed.