DETECTION AND DISCRIMINATION OF SEA SURFACE FILMS IN THE COASTAL ZONE OF NORTHEASTERN USING SAR DATA

M. Mityagina., O. Lavrova, T. Bocharova

Space Research Institute, Russian Academy of Sciences, 117997, , , Profsoyuznaya str. 84/32, Telephone:7 495-333-5078; Fax: 7 495-333-1056 E-mail: [email protected]

ABSTRACT coastal zone, from to the Kerch Strait is of vital necessity. Results of radar monitoring of the sea surface in the Russian sector of the Black Sea coastal zone are Satellite observations of the region have been performed presented. The attention was focused on the detection of over the past 5 years. A series of experiments were oil pollution as well as biogenic and anthropogenic carried out with the aim to investigate the possibility of surfactant films. The basic data were high resolution monitoring coastal zone processes by means of satellite radar images obtained by synthetic aperture radars (SAR) radar instruments [1, 2, 3]. on board Envisat and ERS-2 satellites of the European The developed methods and techniques [4] were applied Space Agency. Spatial and temporal variations of in a pilot project of operational monitoring of the sea different film type manifestations in SAR images were surface state and pollution conducted in 2006. The region analyzed. The effect of coastal zone water circulation of interest is shown in Fig. 1. patterns and river outflow intensity on the dynamics of biogenic films was determined. Areas of the most intense oil pollution were outlined.

1. INTRODUCTION The Black Sea is the most isolated sea from the World Ocean. It is connected to the Mediterranean only through the narrow and winding Bosphorus Strait, a 35-km natural channel, as little as 40 m deep and 700 m wide in places. The ratio of the Black Sea catchment-area and its surface area exceeds 6, compared to a 0.4 average throughout the world. The northeastern coastal zone of the Black Sea is a unique environmental complex and the only Russia’s recreation area on the warm sea. Figure 1. Region of interest. Meanwhile, the region is affected by severe anthropogenic pressure because of large ports situated there, intense agriculture and ever growing residential 2. TASKS OF MONITORING construction. Every year about 110 thousand tons of oil The tasks of the satellite monitoring of the sea state and products are estimated to be discharged into the sea. A sea surface pollution of are closely interrelated because dramatic situation has built up in the vicinity of the pollutants become part of marine environment and biggest Russian port of Novorossiisk – the transportation evolve according to its intrinsic mechanisms. point for an annual export of 2 million tons of oil. A considerable contribition into pollution of the coastal The following tasks were performed in the course of zone is provided by the decrepit local sewage systems. monitoring: The main risk regions are those of the cities of • operational mapping of water state and film pollution Novorossiisk, , and Sochi and (anthropogenic and biogenic) parameters; anchorage sites where ships often discharge wastewaters. Therefore, a comprehensive system of operational • analysis of meteorological conditions and its impact monitoring of the entire Russian sector of the Black Sea on weathering, drift and spread of pollutants;

______Proc. ‘Envisat Symposium 2007’, Montreux, Switzerland 23–27 April 2007 (ESA SP-636, July 2007)

• defining local circulation patterns and their impact on shows that biogenic films are accumulated on the sea weathering, drift and spread of pollutants; surface mostly along the periphery of large phytoplankton masses. • deducing typical local film pollution evolution modes. 3.2. Biogenic films as indicators of water Sea surface images obtained by satellite synthetic dynamics aperture radars (Envisat ASAR and ERS-2 SAR) constitute the data basis. The circulation in the Black Sea is characterized by a strong basin-wide current along the shore in cyclonic Interpretation of SAR images of the sea surface largely direction - the Rim Current. It embraces the entire sea depends on the availability of information on processes along its periphery and is characterized by high in the near-surface layers of the atmosphere and water. hydrodynamic instability. The combination of the Rim These processes include variations of near-surface wind, Current and near-shore anticyclonic eddies plays an convection in the atmosphere, intensity of precipitation, extremely important role in the hydrodynamics and wave height and propagation direction, growth of ecology of the coastal zone [7]. cyanobacteria in the photic layer (sea bloom), change in chlorophyll-a concentration on the sea surface. Beside Biogenic films are very sensitive to surface currents and, SAR data interpretation, this information is also required as a rule, reproduce the shape of local circulation pattern. for analyzing mesoscale sea water dynamics. Water dynamics forces biogenic films to accumulate along the flow lines of surface currents and in this way The basic data (Envisat ASAR, ERS-2 SAR) is therefore emphasize vortex structures, fronts, ship wakes, etc. complemented by other satellite information on the state Since the films influence the backscattering of of the sea surface and sea-atmosphere boundary layer, microwaves, these structures become visible in SAR such as sea surface temperature, suspended matter and imagery. This allows to monitor sea dynamics processes chlorophyll-a maps, mesoscale water dynamics charts via their surface manifestations. Taking advantage of this (NOAA AVHRR, Terra/Aqua MODIS). two-stage process, we are able to improve and extend the information that can be inferred from satellite imagery of coastal zones by analysing signatures of marine surface 3. BIOGENIC FILMS films [8]. In particular, due to the presence of surfactant films, SARs are capable of registering eddy structures. 3.1. Radar manifestations of natural films on the Surfactant films get entrained in the eddy motion and, sea surface under low to moderate wind conditions, the structure Regular SAR observations have demonstrated that becomes outlined in the radar image [9]. We will biogenic films can be found virtually everywhere on the illustrate this by the following examples. vast sea surface, mostly in warm seasons. Biogenic films An Envisat Advanced SAR (ASAR) image obtained on are the result of life activity of marine organisms and 15 May 2006 at 19:10 GMT (Figure 2a) reveals an early seaweeds, primarily phytoplankton and zooplankton, as formation stage of a dipole composed of a jet and a pair well as bacteria. They are produced in the sea during of cyclonic and anticyclonic eddies. At the moment of complex biochemical processes of life and decay of sea image taking the centre of the eddy pair was organisms. On the sea surface, under low winds organic approximately at 37°35’E, 44°15’N. The jet direction films are retained in the form of slicks for a considerable time and start to be disrupted at wind speeds of 6-7 m/s was about 10° to the east and its length was 110-120 km. and higher. When the strong wind subsides, organic The image was acquired under weak southwest wind of substances come up to the surface again and form slicks. 2-3 m/s and weak surface waves. Relative spatial uniformity of the wind field and its small amplitude Biogenic films cannot be considered pollutants in strict coupled with the presence of biogenic films on the sense, however, their presence in coastal waters is surface made possible the visualization of the highly undesirable for the tourist industry. convergence and divergence zones of the dipole in the Such films, due to non-uniform surface tension in areas SAR image. The eddy is hardly discernable as in a of accumulation, are capable of smoothing the gravity- NOAA AVHRR IR image, as in an Aqua MODIS image capillary component of surface waves thus diminishing obtained the same day. It appears more distinct in the radar backscattering cross-section [5, 6]. AVHRR and MODIS data acquired on the next day, 16 May, at 10:39 GMT and 10:40 GMT, respectively According to our observations, large phytoplankton (Figure 2b,c,d). populations are present for a considerable period of time (mid-May - September) along the Black Sea shore and at a distance of 160-200 km from it. SAR data analysis

Figure 2. a) Envisat ASAR VV-polarization image (100x100 km) obtained on 15.05.06 at 19:10 GMT (© ESA, 2006). Eddy dipole is manifested through bands of surfactant slicks; b) SST field derived from Aqua MODIS data (11 micron channel); c) SST field derived from NOAA AVHRR data; d) Eddy dipole evolution schematic superimposed on georeferenced Envisat ASAR image of 15.05.06; e) Water leaving radiance chart derived from Aqua MODIS data (composite of seven channels); f) Chlorophyll a concentration chart derived from Aqua MODIS data

It is easy to notice the presence of mushroom-like Two Envisat ASAR images were acquired 19.06.06 with structures, similar in location and size, in the sea surface an interval of about 11 hours. The image obtained at 07: temperature field (SST) charts derived from IR data (Fig. 52 reveals a nascent eddy dipole (Fig. 3a). The 2b,c). A less distinct structure can be detected in the dimensions of the dipole in the first image are 66 km water leaving radiance chart derived from Aqua MODIS (width) to 62 km (length). The dipole seen in the second data (Fig. 2e). In the chlorophyll-a density chart (Fig. 2f), image (Fig. 3b) is bigger in size: 92 km (width) to 78 the maximal contrast is measured along the dipole jet. km (length) and its axis is slightly displaced clockwise. Maximal chlorophyll-a concentrations appear to nearly The analysis of these two images allows a reconstruction correspond to the convergence zones of the dipole. In the of the local velocity field and retrieval of drift parameters ASAR image, they are visualized as dark filaments due of the biogenic films driven by the eddy dipole (Fig 3c). to ripple damping by biogenic surfactant films. Figure 2d The calculated velocity of its cyclonic fraction is 0.05- presents a schematic co-location of radar, thermal and 0.1 m/s in northwest direction, anti-cyclonic 0,2-0,3 m/s optical data obtained over the two consequent days and in southeast direction. revealing the evolution of the eddy dipole. The dipole Previously, satellite observations of vortex structures in retained its shape and position over a considerably long the northeastern part of the Black Sea have been time, drifting slowly in the northwestern direction due to performed using IR or optical data, together with in-situ the effect of the Rim Current. measurements [10]. The spatial resolution of such images of about 1 km makes it possible to study vortex Dynamic vortex structures of this kind regularly structures of 40 km or larger in size, but only under observed in this region of the Black Sea are known to cloudless conditions. induce not only horizontal, but also vertical mixing of water. They contribute to hydrodynamic instability of the alongshore current and intensify coastal water transport to the open sea. Figure 3. a) ASAR Envisat image, obtained 19.06.2006 at 07:52 GMT (© ESA, 2006); b) ASAR Envisat image, obtained 19.06.2006 at 19:10 GMT(© ESA, 2006); c) a reconstruction of the local velocity field

Our results show that observation of vortex structures of The shape, concentration and location of surface film smaller sizes (less than 30 km) at short time scales (days pollutants depend on the dynamics of water masses: to weeks) can be supplemented by SAR imagery of • currents, near-shore wind and waves force floating higher spatial resolution. The use of SAR data allowed to litter, foam and films to accumulate in convergence find out of intense small-scale vortex activity, which zones; researchers were unaware of before. Our observations • in a cyclonic eddy, surface pollutants accumulate showed that many small vortices with dimensions of 10 along its periphery; km and less appear when the Rim Current slackens. The • in a near-shore anti-cyclonic eddy, pollutants cumulative contribution of this small vortices into the accumulate along its spiral convergent curves; transport of pollutants and water cleaning is comparable • in an eddy dipole, pollutants accumulate along the to that of the Rim Current and large anti-cyclonic eddies. perimeter of its head and at the edges of the jet. Figure 4 shows an Envisat ASAR image of two small cyclonic eddies. 4. TYPES OF POLLUTION DISTRIBUTION The analysis of data received allows to outline three types of film pollution distribution depending on the water circulation in the northwestern part of the Black Sea: First type is conditioned by strong persistent northerly winds and a stable Rim Current passing close to the coast. Pollutants accumulate near the coast and are transported along it. Second type is conditioned by a subsiding wind or a change in its direction and an unstable Rim Current passing far from the coast. The Rim current may be split into several branches and anti-cyclonic eddies are very probable to occur near the coast. The distribution of pollutants is irregular. Partly, they are transported across the Rim Current via its interaction with the eddies, partly, they are accumulated inside the eddies. Third type is conditioned by a long-term calm, a weak Rim Current and the absence of the near-shore branch of Figure 4. 25km × 25 km fragment of an Envisat ASAR the Current. Large near-shore anti-cyclonic eddies develop and interrupt the alongshore transport, small image of 15 August 2006 (© ESA, 2006). Two cyclonic cyclonic vortices occur and contribute to the cleaning of eddies with diameters of 3.75 km (A) and 3 km (B) are coastal waters. clearly seen 5. SHIP POLLUTION Over the period of observations from April to October 2006, around 50 oil spills from ships were registered. The size of spills ranges from 0,1 to 13 km2. The integral area of spills detected over the period is around 120 km2. Several examples from the oil spill gallery are presented in Figs. 5-6.

Figure 7. Map of oil spills detected in the northeastern Black Sea

6. COASTAL POLLUTANT OUTFLOWS

The dynamics of pollution in the coastal region is conditioned by local circulation, sewage discharge rate Figure 5. A release of oil from the ship moving northeast and river outflows. Coastal outflows are composed (white dot) on September 19, 2006, 19:19 GMT. Length mainly of industrial and household wastewater from 2 of the spill is 50 km, surface 9.2 km (© ESA, 2006) tourist facilities, ground rainwash and accidental runoffs from industrial plants situated within the river catchment areas.

Figure 6. A weathered oil spill on July 11 ,2006 ,19:19 GMT, area 4.0 km2 (© ESA, 2006) Summarizing satellite data on oil spills from ships, we have identified regions of the most intense ship pollution. They are ship routs to the ports of Novorossiisk and Figure 8. Outflows of rivers and brooks. Tuapse and oil terminal Zhelezny Rog. The largest oil a) ASAR Envisat image of the northeastern part of the spills were registered in August and the greatest number of spills in September, 2006. A cumulative chart of oil Black Sea acquired 11.07.2006, 19:19 GMT (© ESA, spills based on the analysis of SAR data is presented in 2006) Fig. 7. b) map of water state and circulation superimposed on Aqua MODIS image of the same day Heavy rain periods in Caucasus mountains are found to 9. REFERENCES result in multiple rise in the volume of anthropogenic 1. Lavrova, O.Y., Bocharova, T.Y. & Mityagina, M.I. pollution (domestic and industrial wastewater, oil (2003).SAR Observations of Typical Phenomena in products, fertilizers, etc.) coming into the sea with river the Black Sea Shore Area. In Proc. ‘International outflows. Anthropogenic pollution is complimented by Geoscience and Remote Sensing Symposium’ flows of mud from washed ground areas. It particularly (IGARSS’03). 2, 966 – 968. concerns mountain rivers with uncontrolled outflow. Satellite data show considerable expansion of coastal 2. Mityagina, M., Churyumov, A.& Lavrova, O. water strip rich in suspended matter, appearance of (2005). Problems in detecting oil pollution in Black phytoplankton blooms, increase in floating litter, foam Sea coastal zone by satellite radar means. In Proc. and surfactants including oil. Mountain river outflow rise ‘2004 Envisat & ERS Symposium’ (Eds. H. Lacoste is usually a short-term phenomenon (2-3 days), however, & L. Ouwehand ), ESA SP-572 , 113-116 it brings about pollution of coastal waters on a large 3. Shcherbak S, Lavrova, O., Mityagina M., scale. An illustration is presented in Fig. 8 showing Bocharova, T., Krovotyntsev, V, Ostrovskiy, A. fragments of Envisat ASAR and MODIS images of river (2006). Multisensor satellite monitoring of oil plumes. pollution in northeastern coastal zone of the Black sea. In. Proc. ISRS ‘Panocean Remote Sensing Conference’. 2, 989-992. 7. CONCLUSIONS 4. Lavrova, O., Bocharova, T. & Kosianoy, A. (2006). Satellite Radar Imagery of the coastal zone: Operational satellite monitoring of the coastal zone Slicks and Oil Spills. Global Developments in makes it possible to map hydrodynamic characteristics Environmental Earth Observation from Space. and parameters of pollution, determine pollution type Millpress. Rotterdam. Netherlands, pp763-772. and scale, follow the evolution and drift of pollutants and understand the mechanisms of water self-cleaning.. Maps 5. Ermakov, S.A., Salashin, S.G., Panchenko, A.R. of film pollution derived from SAR data are a good tool (1992). Film slicks on the sea surface and some to estimate the distribution of different types of films on mechanisms of their formation. Dyn. Atmos. Oceans. the sea surface, retrieve their parameters and trace the 16(3-4), 279-304. sources of pollution. 6. Alpers, W. & Huhnerfuss, H. (1989). The damping Long term regular regional satellite monitoring provides of ocean waves by surface films: A new look at an old grounds for the determination and analysis of typical problem. J. Geophys. Res. 94(C), 6251–6265. patterns of pollution distribution, detect new elements of 7. Afanasyev, Y.D., Kostianoy, A.G., Zatsepin, water circulation contributing to the transport of A.G., Poulain, P.M. (2002). Analysis of velocity field pollutants and cleaning of coastal waters. The knowledge in the eastern Black Sea from satellite data during the of local pollution distribution mechanisms raises the Black Sea ’99 experiment. J. Geophys. Res. 107(C8), reliability of environmental mapping and forecast of the 3098. drift and evolution of pollutants. 8. .Gade, M., Ermakov, S.A., Lavrova, O.Yu.,

daSilva, J.C.B. & Woolf, D.K. (2005). Using Marine 8. ACKNOLEDGEMENTS Surface Films as Indicators for Marine Processes in the Coastal Zone. In Proc. 7th Int. Conf. Mediterr. This work was partly supported by INTAS project 06- Coast. Environ. (MEDCOAST’05), 1405-1416. 1000025-9091, Black Sea Scientific Network (Contract # 022868) and RFBR grants # 06-05-08072 – OFI. SAR 9. Lavrova, O. Yu. & Bocharova, T. (2006). data were obtained under ESA projects C1P.1027, Satellite SAR observations of atmospheric and AO3.224 and AO Bear 2775. oceanic vortex structures in the Black Sea coastal zone. Advance in Space Res. 38 (10), 2162-2168.

10. Zatsepin, A.G., Ginzburg, A.I., Kostianoy, A.G., Kremenetskiy, V.V., Krivosheya, V.G., Stanichny, S.V. & Poulain, P-M. (2003). Observations of Black Sea mesoscale eddies and associated horizontal mixing. J. Geophys. Res. 108(C8), 3246.