U.S. Department of Commerce NOAA/NESDIS/ORA, Camp Springs, MD, U.S.A.

email: Pablo.Clemente-Colon@noaa.gov

RADARSAT-1 synthetic aperture radar (SAR) images were acquired over the Bohai Sea and Korea Bay region during the 2000 ice season as part of U.S. National Ice Center experimental ice monitoring activities. The waters in this region are characterized by high turbidity. This can make the identification of sea ice with visible sensors ambiguous at times. SAR can be used to reduce this ambiguity while providing sea ice observations under all-weather conditions. SAR images show open ocean conditions over most of the region in early January 2000, except on Liaodong Bay, which already shows significant accumulation. Maximum sea ice cover is observed by the beginning of February with a return to ice free conditions by the third week in March. The most persistent sea ice coverage in the season occurs over the eastern Bohai Sea and to some extent along the coast in the Korea Bay. Significant coverage along most of the Bohai Sea coast is also observed during the peak period. In addition to sea ice, the data show a wide range of atmospheric phenomena affecting the region. These include high winds, atmospheric gravity waves, boundary layer rolls, and atmospheric fronts. Scatterometer wind vector data from ERS-2 and QuikSCAT show good agreement with observed SAR wind patterns. Features related to ocean circulation, bathymetry, and possibly turbidity conditions are also evident. SAR images also show the distribution of hard targets associated with a large number of vessels and oil platforms operating in this important maritime region. Additional coastal features such as harbors and wetland agriculture/aquaculture fields are mapped.

The U.S. National Ice Center (NIC) has the responsibility of mapping sea ice conditions over all ocean regions of the world. RADARSAT-1 synthetic aperture radar (SAR) imagery is extensively used in the production of sea ice analyses in U.S. waters and the Arctic. NIC also relies on visible imagery from the Defense Meteorological Satellite Program (DMSP) Operational Linescan System (OLS) sensor and on imagery from the NOAA Advanced Very High Resolution Radiometer (AVHRR) and the passive microwave DMSP Special Sensor Microwave Imager (SSM/I). The high spatial resolution of the SAR datasets used (100 m to 200 m) is significantly better than that provided by any of these sensors (550 m for OLS, 1 km for AVHRR, and 25 km for SSM/I). In contrast to SAR observations, visible observations of sea ice are limited by the availability of sunlight while visible and IR observations are both further limited by the presence of clouds.

The Bohai Sea and Korea Bay lie north of the Yellow Sea, around 38°N and 122°E, and comprise the southernmost region in the Northern Hemisphere to develop substantial concentrations of sea ice. These shallow basins (the Bohai Sea average depth being less than 20 m) are also characterized by very high water turbidity. The high reflectivity of turbid waters can create difficulties in discriminating sea ice from open water using visible sensors. In order to study the benefits of adding SAR imagery to the local sea ice analysis, NIC scheduled the onboard recording of RADARSAT-1 ScanSAR Wide B imagery over the Yellow Sea region during the 2000 ice season. These data are now available to NOAA/NESDIS for further research, not only on sea ice conditions, but also on a number of other possible applications. This paper provides a review of the available SAR data as well as examples of oceanic, atmospheric, and coastal features imaged during the season.

Interpretation of ocean and atmospheric features in the SAR imagery was aided by the NIC sea ice analyses themselves and the availability of scatterometer wind data from both the ERS-2 and QuikSCAT satellites. Additionally, NOAA AVHRR multi-channel sea surface temperature (MCSST) images of the region were obtained from the Korean Ocean Research and Development Institute server (KORDI, 2000)

The RADARSAT-1 SAR instrument is a C-band (5.3 GHz) horizontal send and receive polarization (HH) multi-mode instrument capable of producing imagery ranging from 9 m resolution and 50 km swath width (Fine Mode) to 100m resolution and 500 km swath width (ScanSAR Wide Mode). The satellite orbits the Earth at an altitude of 798 km with an inclination of 98.6° and a repeat cycle of 24 days. The ScanSAR Wide mode was selected by NIC because it provides the largest spatial coverage. Although this mode could allow, in theory, repeat observations of a site every 2 to 3 days at 40° N, only 21 passes over the Bohai Sea and Korea Bay for the period January 4 to April 3, 2000 were actually acquired and made available. These data thus provide observations of at least some part of the region roughly every four days. The data were recorded onboard the satellite and processed at the Canadian Space Agency (CSA) Gatineau Satellite Receiving Station (GSS). The list of available SAR data is shown in Table 1. The calibration coefficients supplied by GSS to obtain normalized radar cross-section values have not yet been applied to the data discussed here.

As part of the NIC West Arctic regional-scale analyses, sea ice analysis charts are prepared for fifteen ocean regions, including the Yellow Sea, every week. The Yellow Sea chart covers the Bohai Sea and Korea Bay region. The charts are based on several data sources available to the NIC analyst at the time of their production. As already mentioned, these may include SAR, OLS, and AVHRR, in addition to visual reconnaissance data. SSM/I is not use in this chart due to its coarse spatial resolution. The charts provide an estimate of both the location of the ice edge and information on the type and concentrations of the sea ice. NIC sea ice analysis charts are routinely available to the public from the NIC web server (NIC, 2000).

Wind data acquired by both the ESA ERS-2 and the NASA/JPL SeaWinds (QuikSCAT) scatterometers over Bohai Sea and the Korea Bay region were obtained for the January-April 2000 period. Ocean wind vectors (level 2 products) from both satellites are routinely produced using the NASA NSCAT2 empirical model and provided in near real-time through the NESDIS web server (NESDIS, 2000). The wind vector maps depict the ocean surface winds at a 10m height under neutral stability conditions and are produced at a spatial resolution of 25 km.

The ice regime in the Bohai Sea and the Korea Bay region is relatively short with ice thickness rarely reaching a stage of development beyond first-year-thin (30 –70 cm). SAR observations of the ice conditions in the Bohai Sea during early, peak, and near the end of the 2000 season are shown in Figs. 1a-1c. The NIC ice chart produced during peak conditions with the relevant geographical areas indicated is shown in Fig. 1d. Initial SAR observations in early January 2000 show significant ice accumulation already present in the eastern part of the Bohai Sea (Liaodong Bay, not shown) with ice developing along the northern coast and along the Korea Bay coast. The maximum ice coverage observed by SAR over the Bohai Sea occurred on February 4, 2000 (Fig. 1b). Although full freezing of the Bohai Sea is not reached, some freezing along most of the coastline is observed on that day. Maximum ice coverage over the Korea Bay was observed on February 15, 2000 (Fig. 2a). By early March, sea ice is restricted to narrow coastal bands in the Liaodong Bay and Korea Bay.

Most of the large-scale salient features imaged by SAR over the ice-free ocean surface during the observing period appear to be of atmospheric origin, although, several feature are clearly the signature of ocean processes. In order to interpret or separate them, feature analysis including spatial scale analysis and temporal scale analysis is required. Thus, repeat observations and supplementary data, such as SST, vector winds, and tides during the observation period are usually required.

A wide range of atmospheric phenomena affecting the region are observed in the imagery including wind speed variability, atmospheric fronts, atmospheric internal gravity waves (AGW), boundary layer rolls (wind rolls), and convective activity. A good indication of an atmospheric front is shown in Fig. 1a with relatively higher wind speeds (higher radar backscatter/brighter image) in the northern part of the Bohai Sea and lower speeds to the south. Persistence of high winds over the northern Bohai Sea and low winds to the south during this day are supported by the available scatterometer data. Remarkably, indication of at least some degree of AGW activity was found during all but two of the twenty-one days of available SAR observations. Similar wave-like patterns have been identified as lee waves in the region (Zheng et al.). AGW’s can provide information about atmospheric stratification, instability conditions, and the complexity of the atmospheric inversion layer. The most complex pattern of AGW was observed on March 23, 2000 with multiple AGW wavelengths and propagation orientations imaged over the Bohai Sea. Additional patterns usually associated with convective instability in the atmosphere were also observed during January and February. AGW, boundary layer rolls, wind shadowing, and the strongest convective activity patterns observed were imaged over the Korea Bay and northern Yellow Sea during strong northwesterly wind conditions on February 15, 2000. Figs. 2a and 2b show the SAR image acquired on that day at 21:48 and QuikSCAT observations from a descending pass acquired just over three hours earlier, respectively. The wind direction suggested by SAR linear features (such as wind rolls and wind shadows extending from land) is consistent with the direction of the scatterometer vectors. A high wind region with speeds over 10 m/s is dominated by multiple large convective cell-like patterns with a spatial scale of the order of 10 km in diameter.

Features related to oceanic processes such as circulation, bathymetry, biological activity, and possibly high turbidity were also imaged. The linear features observed in the low wind speed region in Fig. 1a are likely associated with the response of the sediment content in these highly turbid waters to the tidal flow. From model predictions, the local tidal current at the time of the SAR observation was estimated at about 1 m/s into the Bohai Sea (Gallegos, 2000). Although a very shallow basin, the only bathymetric signatures observed in the region were limited to a few small areas around river mouths. The first indication of the presence of biogenic activity or natural slicks in the region was observed on March 10, 2000. This coincided with significant warming of the Yellow Sea waters as shown by the MCSST data. The strongest ocean circulation features observed were imaged east of the Korean Peninsula. East Sea (Sea of Japan) fronts and eddies associated with the warm Tsushima current were observed. The presence of these features was also confirmed by the MCSST data. Some indication of smaller scale sea surface circulation inside the Yellow Sea region was provided by patterns of natural slicks advection by small eddy-like features observed in March.

The Bohai Sea and Korea Bay region is characterized by significant year-round maritime traffic as well as the presence of large fishing fleets in the spring. In addition, tens of oil and gas platforms operate in the area. The high resolution of the SAR images allow for the detection of these hard targets and can be useful to assess their distribution. Most of the ships detected were observed in route around the Shandong Peninsula or at major ports and oil fields in the Bohai Bay, Liadong Bay, and the Yellow Sea Delta. Nevertheless, the largest single concentration was observed in the middle of Korea Bay on April 3, 2000 (Fig. 3). The pairing of most of the ships imaged indicate that they were probably engaged in double boat trawling operations, which is a technique commonly used in the region. This incredible fishing fleet is seen to operate in an almost circular region of over 37 km in diameter and is composed of over 350 vessels. Slicks probably associated with the trawling operations are also imaged. Many additional fishing vessels were observed distributed throughout the rest of the region.

Indications of possible bilge pumping or ship dumping activity in the region were observed in half of the available SAR observation days. A clear example of ship dumping signature off the Shandong Peninsula is shown in Fig. 4. The width of the slick features indicates the effect of wind spreading. Unfortunately, as the presence of natural slicks and the intensity of fishing activity increases during March 2000, it becomes difficult to separate possible pollution signatures from biogenic slicks resulting from natural productivity and fishing operations.

SAR imaged many other features around the Bohai Sea and Korea Bay coastal region. These included harbors, agriculture/aquaculture installations, wetlands, tidal flats, rivers, etc. SAR can serve as an ideal tool for mapping such features providing a basis for change-detection monitoring and research.

The ability of SAR data to observe ice conditions in the Bohai Sea and Korea Bay region is clear. These data provided important additional information to NIC in the preparation of sea ice charts during the 2000 season. Unfortunately, SAR datasets themselves are very expensive to acquire and will not be readily available to most potential users in the near future, at least in the frequency and timeliness required (Liu et al., 1998). However, these data can still be extremely useful in non real-time ice research and many other geophysical, environmental, and management areas of study due to the large amount of information and wide range of spatial scales that SAR can provide. While the interpretation of SAR imagery can be at times difficult and ambiguous (Clemente-Colón and Yan, 2000), SAR data may be able to provide links between a diverse number of observations and physical processes that cannot be attained by any other sensor. A case in point is the clear connection between the observation of natural slicks in March 2000 (associated with enhance biological activity) and the simultaneous increase in the vessels operating in the region (almost certainly engaged in fishing activities). Several areas of study already identified by the authors include the analysis of prevalent atmospheric wave patterns, traffic and offshore dumping patterns in the region, spring season biogenic activity, fishing fleets distribution and operations, mapping of wetland agriculture/aquaculture installations, wind-mapping, and a SAR study of ocean circulation in the East Sea, to name a few. A long-term goal that should also be considered, particularly if SAR data become more accessible, is the eventual integration of SAR observations into regional coastal monitoring systems as part of a strategy for detecting and tracking other time-critical events such as the progression oil spills or toxic red tide blooms.

This research is supported by the NOAA Ocean Remote Sensing Program (NORS). All RADARSAT-1 data are copyrighted by the Canadian Space Agency.