SMALL SCALE PROCESSES IN THE SOUTH ATLANTIC OBSERVED IN SYNERGY OF ATSR AND SAR DATA DURING THE TANDEM MISSION

Transcription

1 SMALL SCALE PROCESSES IN THE SOUTH ATLANTIC OBSERVED IN SYNERGY OF ATSR AND SAR DATA DURING THE TANDEM MISSION Ian S Robinson, Joseph Allison School of Ocean and Earth Science Southampton Oceanography Centre European Way, Southampton, SO14 3ZH Ian.S.Robinson@soc.soton.ac.uk ABSTRACT During the ERS1-2 Tandem Mission in , the synthetic aperture radar (SAR) swath on one satellite overlapped the along-track scanning radiometer (ATSR) swath on the other with a 30 min interval, at latitudes greater than 50. A number of matching pairs of SAR and partially cloud-free ATSR images were found in the ERS archive covering the South Patagonian Shelf, the Drake Passage and the Antarctic Peninsula. The SAR.PRI image data were reduced to 200m pixels by rms averaging, and the ATSR.GBT and.gsst 1km image products were warped onto the same 200m grid as the corresponding SAR scene. Following image enhancement, requiring a de-trending of the SAR image to remove the response of normalised radar cross section (NRCS) to the viewing geometry, visual comparison was made between the two types of data. While no strong mesoscale dynamical features were encountered, a number of small scale thermal signatures were detected, close to the coast or near the ice edge. A variety of signature types were found in the corresponding SAR scenes, which closely matched the location of the thermal signatures, including linear NRCS features, step changes of NRCS and changes in NRCS texture. In this remote region, for which there is little systematic oceanographic knowledge, the combination of the radar and thermal signatures facilitates the interpretation of the data, and demonstrates the usefulness of the synergetic approach for studying coastal ocean dynamics. Coupled with previous analyses of north-east Atlantic data, these matched image pairs point to the potential in future for oceanographic applications of synergetic analyses of combined simultaneous AATSR, ASAR (ScanSAR mode) and MERIS data from Envisat. INTRODUCTION The ocean and atmosphere are naturally turbulent fluid environments, subject to fluctuations at a variety of length and time scales. Although the ocean circulation in broad terms is reasonably well known, a detailed description of currents and mixing processes still eludes oceanographers. The first impact of early satellite observations of the ocean was to convince environmental scientists that they can no longer consider the distribution of ocean properties as a static pattern which may be determined once and for all like the geology of the earth s crust. Even dominant features like the major currents are constantly changing and must be measured repeatedly before their mean flux can be determined. In a turbulent environment it is necessary to sample the same location many times to achieve a satisfactory description of the statistics of the variability. Satellites now play a vital role in helping to monitor the ocean because remote sensing offers the chance to instantaneously freeze the complex spatial patterns in the ocean, and to continue to do so regularly with consistency over many years. One of the dominant scales of variability in the ocean is the mesoscale, with length scales in the range 40 km-400 km and a time scale of several hours to days. This is the scale controlled largely by the effect of the earth s rotation on changes which are forced on the ocean by solar radiation at the global scale, by interaction with the atmosphere at many scales or by coastal inputs of fresh water and other materials at the local scale. The sampling capabilities of visible and

2 infrared satellite sensors are such that they are well adapted for monitoring mesoscale variability in the ocean, as long as no cloud obstructs the view. Unfortunately clouds are ubiquitous and in many regions prevent the regular monitoring of mesoscale processes by infrared sensors. Although microwaves can penetrate the clouds passive microwave radiometers have not yet gained sufficient accuracy and resolution to monitor variability at length scales less than 100 km. This is where imaging radars have a role to play. Several years of experience with the analysis of ERS SAR ocean images [1, 2] has demonstrated their capacity to detect a variety of mesoscale ocean dynamical features, including those such as fronts, eddies and upwelling which have a strong thermal signature and are normally monitored remotely by infra-red satellite sensors. The all-weather capability of SAR would make it an attractive complement to infrared sensors if we could interpret with confidence the radar signature of mesoscale features. However, the modulation of surface roughness which creates patterns of radar backscatter in a SAR image is caused by a variety of processes. These include mesoscale ocean dynamical features, but they are also strongly influenced by the local wind and by smaller scale dynamics in the upper ocean or shallow sea. These have length scales between 500 m and 50 km and their coherent structure is not apparent from the deck of a ship. SAR appears to be the most effective way to reveal a scale of variability in the ocean which had previously been neglected, but which may have considerable importance for air-sea interaction and upper ocean mixing processes, such as that associated with internal waves [3] or spiral eddies [4]. Difficulties in interpretation can arise when the patterns on an image are caused by the surface wind variability [5], and may be associated with meteorological features such as rain cells [6]. The ways in which the wind, tide or current may "paint" patterns of roughness on the sea surface are quite subtle and not yet capable of being completely or confidently described by existing theoretical models. One of the difficulties of interpreting SAR images is knowing what the ocean or atmospheric conditions were at the time of the image. The physical phenomena controlling the surface roughness can be short-lived, varying in a matter of minutes as the wind changes. It is very exceptional to have detailed field measurements coincident with a SAR image. In this context a coincident image from a thermal sensor is potentially useful, since it is capable of revealing some, if not all, of the state of the ocean environment. The availability of coincident SAR and thermal images could clarify the extent to which SAR can be used as an alternative or substitute for infrared monitoring of mesoscale dynamics in cloudy conditions. It could also assist with the interpretation of SAR ocean images in general, when the features being imaged are not mesoscale but local or intermediate scale processes. Given the short response time of surface roughness, the ideal would be to have SAR and thermal images sampled at the same instant. Unfortunately a SAR and a thermal scanner have not yet been configured to achieve this. Although ERS-1 and ERS-2 each carried both a SAR and an ATSR, the latter's swath extended only 250 km from nadir, about the same location as the SAR swath's nearest edge. The Tandem Mission of ERS-1 and ERS-2 provided an opportunity to achieve the desired overlap, with a 30 minute time lag. The matching of SAR and ATSR images during the Tandem Mission was investigated earlier for data from the northeast Atlantic [7], where a number of mesoscale eddies and frontal features were encountered. Since then the geographical scope of the use of Tandem Mission data has been extended. This paper describes the search for useful SAR - ATSR synergy in the South Atlantic, on the Patagonian Shelf, in the Drake Passage and the Belingshausen Sea, and presents some of the interesting features found to have signatures in both SAR and ATSR images. OPPORTUNITIES FOR SYNERGY WITH TANDEM MISSION DATA Reference [7] has previously discussed the various opportunities for synergy provided by the Tandem mission when ERS-1 and ERS-2 were flown in the same orbit with a spacing such that their ground tracks were identical but viewed one day apart. Here we recall the cross-satellite SAR-ATSR combinations that are possible. The SAR looks to the right of the direction of travel of the satellite. Therefore on ascending orbits the SAR on ERS-2 looks towards where the ATSR-1 swath was observed 30 minutes earlier. On descending orbits the SAR on ERS-1 looks towards where the ATSR-2 swath will come 30 minutes later. This is illustrated in Fig. 1. It can be seen that the complete overlap of the SAR and ATSR swaths is possible only at latitudes greater than 54, while there is partial overlap between 48 and 54. At lower latitudes, the oblique viewing SAR cannot reach to the track-centred swath of the ATSR on the other satellite. For increasing latitudes above 50, the SAR swath shifts progressively towards the centre of the adjacent ATSR swath. In the southern hemisphere case relevant to this paper the same pattern is repeated apart form the tracks now converging towards the south rather than the north. Thus the possible combinations are ATSR-1 with SAR-2 on ascending passes and ATSR-2 with SAR-1 on descending passes.

3 Fig. 1. Typical ATSR and SAR swaths for the Tandem Mission showing the overlap zones. The ATSR-2 and adjacent SAR-2 (not shown) swaths are covered about 30 minutes after the ATSR-1 and SAR-1 swaths. (from [7]) The objective of the study described here was to explore the oceanographic insights to be gained from the 30 minute near-coincidence SAR - ATSR matched data from parts of the world ocean less well known than the previously studied N.E. Atlantic. This pointed to the southern hemisphere, where SAR data are sparse at latitudes 50 S to 80 S except in the region between South America and the Antarctic where ground stations have been able to receive ERS transmissions. This therefore defined the study area to lie between 50 W and 80 W. SEARCHING FOR COINCIDENT ERS SAR AND ATSR DATA The specification for data to satisfy the requirements of this study are as follows: 1. A combination of SAR on one satellite and ATSR on the other, acquired within the study region. 2. The ATSR image to be cloud -free in the region of overlap with SAR. 3. There to be interesting dynamical features on either or both the SAR and ATSR image. In order to obtain a dataset for synergy, a lot of work was required in searching the data catalogues before data could be ordered. In practice, since ATSR images are acquired almost every time the satellite passes, the search for specification 1 was started with the available SAR data which were much less frequently acquired. For this the Display Earth remote sensing Swath Coverage for Windows (DESCW) software produced by ESA was found to be most useful. Fig. 2 provides an example of the map produced by DESCW, representing descending passes on two particular days. From this it is simple to identify not only which ATSR frames contain near-simultaneous SAR frames, but where on the ATSR image the SAR is located. On searching though all the available SAR scenes during the tandem phase, a few hundred ATSR frames were identified as candidates for the matched data. The next stage was to determine which of these ATSR scenes were cloud-free in the relevant region. Full examination of each image would have been prohibitively time consuming, but the use of browse files simplified the process. Since the previous study [7] ESA has introduced new browsing software called Open Distributed Information Systems and Services on Earth Observation (ODISSEO) which proved to be very effective, enabling cloud cover to be estimated online from the browse images. Subsequent analysis confirmed that most of the cloud cover had been detected from the browse images; few of the ATSR images ordered at this stage turned out to be of no value. The main problem was that no browse images for ATSR-1 data had been entered into the system when the study was being performed in late Thus any possible matches between ATSR-1 and SAR-2 ascending passes have not been investigated.

4 Fig.2. DESCW output for 9 th Nov 1995 and 17 th Nov 1995 showing overlap of SAR-1 and ATSR-2 image frames in the study area. Unfortunately ATSR-2 was operational only from September to December 1995 during the Tandem mission, severely reducing the possible pool of data for matches with SAR. In the end just 8 ATSR-2 frames containing coincident SAR were found to be cloud-free in the right places. Corresponding to these were 21 SAR scenes and together they formed the dataset for further analysis. The selected images are defined in Table 1. Table 1. Matched data of ATSR-2 and SAR-1 data used in the study. (* Subsequently proved to be too cloudy) ERS-2 Orbit No. ATSR-2 Frame. ERS-1 Orbit No. SAR Frame No. Date SAR centre W, 50.6 S W, 51.5 S 4671 (fig 3a) 68.1 W, 52.4 S (fig 4a) 'W, 53.2'S 2712* W, 63.1 S W, 63.9 S 2784* W, 67.9 S W, 63.1 S W, 63.9 S W, 64.7 S W, 65.5 S W, 63.1 S W, 63.9 S W, 50.6 S W, 51.5 S W, 52.4 S W, 53.3 S W, 50.6 S W, 51.5 S W, 52.4 S W, 53.3 S

5 IMAGE ANALYSIS FOR SAR - ATSR COMPARISON The scale mismatch between SAR and ATSR data requires some special image processing procedures to be adopted. Although the SAR pixel size is 12.5m, and the resolution is about 30m, most of the dynamical features of interest show up on overview images of the whole 100 km scene. Therefore to facilitate comparison with the infra-red data which has a resolution of 1km, the whole of each SAR scene was reduced to a image having 200m pixels derived as root mean square averages of original pixels. This has the effect of considerably reducing speckle. The ATSR images were then warped onto the same 200m grid as the SAR. This is possible by first using the given locations of the SAR corners to derive a mapping from geographical to image co-ordinates and thus, using the geographical locations given for every ATSR pixel, to locate each in SAR image co-ordinates. Although re-mapping the ATSR into 200m pixels gives a blocky effect, this scale offers an effective intermediate resolution between the two types of data. The pixel values of the SAR image are capable of calibration in terms of normalised radar backscatter cross section (NRCS) although the displays below are based on numbers representing the square root of NRCS. In order to contraststretch the SAR images sufficiently to reveal the subtle but coherent patterns of NRCS modulation, it was necessary to remove first the range gradient of NRCS caused by the varying look angle. This has been done to the images shown below. SAR images are displayed with linearly stretched greyscales, whereas for the temperature images a colour palette has been applied which stretches from light mauve at the coldest through blue, green, yellow and red to pink as the warmest. The temperature range from coolest to warmest values in the linear stretch is stated in the captions. EXAMPLES OF RESULTS Four of the resulting matched pairs of images are presented here to illustrate the benefits of the approach. Figure 3 from ERS-1 orbit on 16 th October 1995 covers the south of the Patagonian shelf. The mouth of the Magellan Strait opens westward at the bottom left of the images. The SAR image reveals features typical of many coastal radar images. The darker regions are indicative of reduced roughness which could simply be regions of lighter winds but in this case are more likely to be associated with surface films, perhaps a by-product of a spring phytoplankton bloom. The structures to the bottom right are characteristic of flow over shallow bathymetry. Neither of these features have a counterpart in the thermal image. SAR features which can be related to the thermal image are the fine, bright lines indicative of convergent fronts. These are particularly marked in the Strait, where they match well the thermal front between warm water apparently flowing out of the Strait along the north shore, and cooler water flowing west to the south of it. Near the N.E. corner of the image there is another frontal signature that also has a thermal counterpart. Fig. 3. (a) SAR image, ERS-1 Orbit 22242, frame 4671 (b) ATSR-2 SST image segment from Orbit 2555 Frame 4635, resampled to image (a). Temperature range 3.0 C to 5.0 C. 16 th October, 1995.

6 Fig. 4. (a) SAR image from ERS-1 orbit 22299, Frame (b) ATSR-2 SST image segment from Orbit 2612, Frame 4635, resampled to image (a). The temperature range is 4.7 C to 6.1 C. 20 th October, Fig. 4. shows a region of sea 6 E of the Magellan Strait shown in Fig. 3, and just southwest of the Falkland Is. (Malvinas). The SAR image in Fig. 4a has a clear frontal structure running diagonally across it, most obvious 30 km south of the N.E. corner of the scene, which at first might suggest an ocean front. However, comparison with the ATSR image in Fig. 4.b makes it clear that this is not an ocean feature at all. There is no correspondence between the widespread lineations of the SAR image and the thermal contours which, if anything, run diagonally upwards from bottom right. Instead there is evidence in both images that it represents an atmospheric front. The most obvious clue is the cloud on the thermal image, which displays as black, and which lines up closely with the SAR frontal feature. Moreover there are streaky lines throughout the SAR image which are approximately parallel with the front. At this scale of 200 m pixels, such streaks are believed to be associated with atmospheric roll vortices and taken as evidence of the wind direction. The strong SAR frontal feature at the east of the image is in fact a boundary between enhanced and reduced NRCS, indicative of a fairly abrupt change of wind stress across the atmospheric front. If, instead of looking first for features in the SAR image which might be matched in the ATSR, we start with the ATSR image, the main problem is the way the thermal image is corrupted by cloud. In this case the cloud flags generated by the ATSR processing have not been used to mask the data and it is difficult to determine how much of the image structure represents the distribution of sub-pixel cloud and how much relates to the underlying sea temperature. There is one feature that can be more confidently identified using the available near-coincident SAR scene. At a location 20 km east and 30 km north of the bottom left corner, there is what appears to be a fairly strong thermal front, having a 1 C temperature change across 3-4 km. However, given the cloudiness of the image it would be unwise to conclude this without further evidence. The SAR image in the same location does show clearly a characteristic narrow bright line, indicative of a convergent front, in exactly the same place, which is the evidence needed to confirm this as an oceanic rather than an atmospheric front. It should be noted that the front does not stand out at all on the SAR image where the atmospheric front and the wind streaks are the dominant features. Thus by itself neither the SAR nor the ATSR would be a very helpful oceanographic tool, but taking the two images together leads to a more confident interpretation of a local ocean frontal feature, a good example of the synergy that can be result from this radar-thermal image combination. The third and fourth examples come from further south, on an orbit which tracked the SAR centre about 100km west of the Antarctic Peninsula. Fig. 5 appears to be north of the ice edge and the most interesting feature is the mottled texture to the southwest of the scene which has the characteristic sea surface signature of convective cells. These are formed in the atmosphere when a negative air-sea temperature difference gives rise to unstable stratification of the marine boundary layer, and strong vertical convective cells are produced with a cylindrical form [6]. When the motion at ground level associated with these cells is added to the mean wind, the sea surface roughness is modulated to create the

7 patterns shown in the image. The conditions are suitable for this phenomenon to occur. The sea, although just above freezing point, will be considerably warmer than the wind off the ice edge to the south. It is interesting in this case to have the thermal image to compare, and to note that a band of cloud contamination of pixels overlaps the edge of the region of convective cells. This appears not to be complete cloud cover, because some pixels are clear, but is the patchy cloud that might be expected from the convective atmospheric boundary layer instabilities. Fig. 5. (a) SAR image from ERS-1 orbit 22585, Frame (b) ATSR-2 SST image segment from Orbit 2898, Frame 4905, resampled to image (a). The temperature range is -1.0 C to 0.3 C. 9 th November, In Fig. 6, within the same ATSR frame, but a SAR image from further south, the ice edge is shown clearly. In a region such as this where it is not always easy to distinguish by NRCS magnitude alone between what is ice and what is a Fig. 6. (a) SAR image from ERS-1 orbit 22585, Frame (b) ATSR-2 SST image segment from Orbit 2898, Frame 4905, resampled to image (a). The temperature range is -3.0 C to 0.5 C. 9 th November, 1995

8 fairly rough sea surface, the thermal image adds useful additional information. The colour palette of Fig. 6b spans from -3.0 C to 0.5 C so only the green to red colours imply clear water. The mauve and blue colours represent temperatures below 1.5 C and are therefore likely to be partly covered by ice. This is evident in the thermal signatures of the ice filaments which extend from the main ice edge, and which are clearly defined by the SAR. The ATSR cannot adequately resolve such narrow features, but viewing both images side by side helps to clarify what produces the patterns that appear on the SST image. It also appears that some of the anomalously cold individual pixels in the open sea correspond to ice floes in the SAR image, while isolated warm pixels within the main ice zone correspond with what the SAR shows to be leads of open water (lower backscatter). DISCUSSION AND CONCLUSION This paper has selected a few of the more interesting features from the 20 SAR scenes identified as overlapping with cloud-sparse ATSR images in the South Atlantic during the Tandem Mission. Further work is in progress to analyse these quantitatively. Although the numbers of image match-ups was small, this is partly a consequence of the loss of ATSR-2 for 2/3 of the Tandem Mission, and also of the abandoning of ATSR-1 in this study because there was no available browse file. From the examples it is clear that when cloud free thermal images and SAR are obtained simultaneously, or within 30 min in this case, there is usually a benefit that is typically a clarification of interpretation. This benefit flows in both directions, the SAR assisting in ATSR analysis and vice versa. Of particular value is the way in which the thermal image can help to distinguish between features on the SAR image which are evidence of atmospheric, oceanic or ice phenomena. The effort required to search the archives to bring the different data together is large for the small number of matches achieved, but in future there will be some satellites carrying several sensors together. The wide swath ScanSAR mode of the ASAR on Envisat will overlap with ocean colour images from MERIS, and these promise to be very interesting to compare. Unfortunately the AATSR will still only view near to nadir, and a match between SAR and infrared on the same sensor will not be available from other missions in the near future. However it is to be hoped that improved archiving, browsing and access to satellite data will make it easier to perform the type of searches described here, and will lead to more opportunities to explore the synergy between radar and thermal images. REFERENCES [1] J.A. Johannessen et al, "Synthetic aperture radar imaging of upper ocean circulation features and wind fronts," J. Geophys. Res., vol.96, pp , [2] J A Johannessen et al, "Coastal ocean fronts and eddies imaged with ERS 1 synthetic aperture radar," J. Geophys. Res., vol. 101, pp , 1996 [3] J.C. da Silva, S.A. Ermakov, I.S. Robinson, D.R.G. Jeans and S.V. Kijashko. "The role of surface films in ERS SAR signatures of internal waves on the shelf. I. Short-period internal waves". J Geophys Res., vol. 103 (C4), pp , [4] W. Munk, L. Armi, K. Fischer and F. Zachariasen, "Spirals on the sea", Proc. Roy. Soc. London, Vol. A456 (No. 1997), pp , 2000 [5] A. Scoon and I.S. Robinson, "Meteorological and oceanographic surface roughness phenomena in the English Channel investigated using ERS synthetic aperture radar and an empirical model of backscatter" J. Geophys. Res, vol. 105 (C3), pp , [6] S. Ufermann, and R. Romeiser, "Numerical study on signatures of atmospheric convective cells in radar images of the ocean", J.Geophys. Res., vol.104, pp , [7] I.S. Robinson and J.A. Johannessen, "Opportunities for combined SAR and ATSR ocean observations during the Tandem Mission", Proc Third ERS Symposium, ESA-SP-414, vol.3, pp , 1997.