4. Verification and evaluation (monitoring) 4.1 Verification using visual information

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1 4. Verification and evaluation (monitoring) 4.1 Verification using visual information Information on sightings of debris reported by ships is available on the web site of the Japanese government ( The debris locations predicted with ocean currents by MOVE-WNP and winds by JCDAS (Fig ) agree well with sightings of debris reported by ships in April In addition, the debris dispersal spreading offshore in the Pacific Ocean (Fig. 4.1) is in good agreement with sightings of debris reported by ships during 2011, which indicates a comparatively high accuracy of prediction to investigate the movement of debris. 1

2 Fig. 4.1 Ship-visual information (left) is well similar to particle diffusion calculated by MOVE- NP and JCDAS data of currents and winds (right). 1

3 Maximum velocity V max [km/h] 4.2 Verification using satellite observations For efficient ocean debris monitoring by spaceborne SAR sensors, several key elements such as swath width, incidence angle, resolution, and sea state need to be considered. Note that any time and date shown in this section is expressed in coordinated universal time (UTC). (1) Swath width Let us assume that an object moves in the cross-track direction as shown below. This situation corresponds to the fastest case of debris cutting across the swath of the SAR. X[km] V[ km/ h] Along track direction Cross track direction Then the maximum observed velocity of the debris, V max km/h, can be derived from the time interval T h as 20 V X T max. The velocity is plotted in terms of swath width X km by varying the time interval. 20 Spotlight mode Stripmap mode ScanSAR mode days/time Swath width 500 X [km] 1

4 On the other hand, velocity maps of debris inferred from images obtained by the COSMO-SkyMed (CSK) and RADARSAT-2 (RS2) commercial SAR satellites are shown below for reference. (c)esri Japan, RADARSAT-2 Data and Products MacDONALD, DETTWILER AND ASSOCIATES LTD All Rights Reserved COSMO-SkyMed Product - (c)asi - Agenzia Spaziale Italiana All Rights Reserved Overlapped area 1.0km/h 08:24 on 12 March 2011 RS2 09:34 on 12 March 2011 CSK 09:34 on 12 March 2011 CSK 20:38 on 12 March 2011 RS2 08:36 on 15 March 2011 RS2 12:56 on 15 March 2011 PALSAR These maps indicate that the velocity is around 2 km/h at most so that under the assumption of V max =2 km/h then observations a few times per week with a swath width of more than 200 km should be acceptable. (2) Incidence angle We shall compare SAR images of Onagawa bay obtained by L-band PALSAR with those of Utatsuzaki obtained by X-band CSK at different incidence angles. PALSAR (L-band) 19 Mar i = Mar i = Mar 2011 i = [dB] 2

5 CSK (X-band) 09:10 12 Mar i = :34 12 Mar i = [dB] COSMO-SkyMed Product - (c)asi - Agenzia Spaziale Italiana All Rights Reserved Aerial photos have confirmed the SAR data in showing that various debris features remained in the area specified by the yellow rectangle during the period. These SAR images show that the visibility of the debris increases with incidence angle (i.e. off-nadir angles) at any wavelength. Note that the theoretical description of the incidence angle dependence is comprehensively discussed in [2, 3]. (3) Resolution When resolution decreases (or equivalently, the illuminated area after synthetic aperture processing is enlarged relative to a fixed size of object), the radar sea-clutter signal increases. Hence, the signal-clutter ratio (SCR) in SAR imagery can be reduced owing to the constant signal received back from the object. Based on this concept, we shall compute the SCR of small debris and clusters in terms of spatial resolution. High Resolution Debris Low Resolution SCR:HIGH Sea SCR:LOW The backscattering cross sections of clusters, sea-clutter and cluster areas for this calculation were extracted from a PALSAR image at 10 m resolution that was obtained on 15 March 2011 and in which various marine debris features were recognized. 3

6 Signal Clutter Ratio [db] Resolution [m] Small Debris Cluster 50 Large Debris 9 漂流物小クラスタ Cluster Small Debris Resolution [m] Resolution [m] One can readily deduce that both SCR's decrease in terms of resolution. The SCR of a cluster decreases more slowly than that of small debris due to its wider area on the sea surface. Assuming a desired SCR of 9 db, the required resolution is approximately 10 m for small debris and 50 m for clusters, respectively. (4) Polarization HH, HV and VV polarized images of Onagawa bay obtained by PALSAR on 8 April 2011 are shown below. PALSAR (L-band) 8 Apr HH 8 Apr HV 8 Apr VV -11.9dB -25.5dB -10.6dB -30-5[dB] HH and HV polarized images of Utatsuzaki obtained by RS2 on 12 March 2011 are also shown. 4

7 RS2 (C-band) 12 Mar HH 12 Mar HV [db] RADARSAT-2 Data and Products MacDONALD, DETTWILER AND ASSOCIATES LTD All Rights Reserved As mentioned in (2), various debris features remained in the area specified by the yellow rectangular for the period under consideration. These images show that HH and VV polarizations can be easily affected by sea-clutter whereas HV achieves the highest visibility. Averaged sea-clutters are calculated from the pixels surrounded by the red rectangle in the PALSAR imagery. The values indicate that VV is the most sensitive to sea-clutter. Note that a theoretical treatment of the polarization dependence is given in [2, 3]. (5) Sea State Two ScanSAR images of Matsushima bay obtained by PALSAR are shown below. The sea state of this area was calm on 14 March 2011 whereas it was rough on 31 March 2011, so that these two images are suitable to study the effect of sea state. 14 March 2011 HH i =27.0 Sea state: LOW 31 March 2011 HH i =27.0 Sea state: HIGH 5

8 Both images were heavily affected by sea clutter due to the small off-nadir angle. However, as shown in areas specified by the yellow rectangle, a certain amount of debris can be recognized with higher visibility. This suggests that when one is aware of bad weather conditions beforehand, HV polarization is more effective at suppressing the effects of sea state [2, 3]. The future direction of our marine debris monitoring program can be summarized as follows: When monitoring large debris features or clusters, one should highlight swath width, so that observations a few times per week having a swath width of more than 200 km are recommended. The resolution must be 50 m or less. On the other hand, when detecting small isolated debris, one should select a fine resolution of less than 10 m. However, note that the observation interval should then be increased due to the limited swath width. For both cases, HH polarization should be used at higher off-nadir angle, or HV polarization should be selected when one is aware of bad weather conditions beforehand. Note that debris speeds of less than 2 km/h and SCR values of around 9 db are assumed for detection. (6) Debris Observation by Foreign Satellites To establish of a SAR observation program based on satellites other than Daichi (which terminated on April, 2011) C band and X band SAR images were purchased of the Tohoku coastal zone from the image archives of foreign satellites, which enabled us to maintain observations of marine debris and also evaluate Daichi images. From the results of high-resolution image analysis, it was revealed that the debris outflow area extended from Aomori-prefecture to Ibaragi-prefecture and that debris very close to the coast tended to stagnate. On this basis our prediction results were improved. 4.3 Research cooperation with overseas bodies, and dissemination of information to overseas authorities Cooperation through intergovernmental consultation between Japan and the United of States The Chief of NOAA Marine Debris Program, U.S., visited Japan on January , and participated in the intergovernmental consultation within the Ministry s liaison conference, of which Headquarters for Ocean Policy is the secretariat. At the consultation we introduced the present status of forecast research for the floating debris Japan-U.S. expert meeting A meeting was held in Honolulu on 9-10 February 2012 between experts from Japan and from the USA to mainly discuss issues associated with forecasting techniques. 6

9 Expert team on the Japanese side: Awaji, Executive Vice-President, Kyoto University Kamachi, Director of the Oceanographic Research Department, MRI, Japan Meteorological Agency Nishimura, Executive Assistant to the Director-General, Data Research Center for Marine-Earth Sciences, JAMSTEC Agents from Ministry of Foreign Affairs and Ministry of Environment Attendance on the USA side: University of Hawaii at Mānoa IPRC NOAA Marine Debris Program (1) Significance The Japanese delegation explained details of the technique of modeling and simulation to their U.S. colleagues. The U.S. side evaluated the Japanese simulation techniques highly, since these are are more elaborate than those used in the U.S. The U.S. side forecasts that the floating debris will pass through a region to the north of Hawaii in 2012, approach the U.S. West Coast in 2013 (but will not come ashore along the coast of California due to the offshore currents). It will then mostly reverse toward Hawaii and will later tend to gather and congregate in a focal region in the North Pacific from 2014 to 2016 and may in part drift ashore on the north coast of Hawaii. The NOAA side inquired about the propriety of information that covered aspects such as debris components, a list of missing boats/ships, data on debris expected to have sunk near the coast, and distribution maps and photo data of debris from Japan. Both sides confirmed their intent to cooperate on this project through close and secure communication. The future time line on the Japanese side will be shared with affairs on the U.S. side, (2) Details The IPRC side explained that IPRC's floating debris forecast model experiment is a rather simplified model simulation, which includes statistical information referred to the climatological mean field (about a 10-year mean field) of ocean currents and surface winds. It was explained that the floating debris will pass by the north of Hawaii in 2012, and approach the U.S. West Coast in 2013, although in California 7

10 these debris may not drift ashore due to the offshore current. A greater part of the debris will flow in the reverse direction toward Hawaii during 2014 to 2016, and will tend to gather and congregate in a focal region for marine debris within the North Pacific, with a part of floating debris drifting ashore on the north coast of Hawaii. They also pointed that their predictions are gained using mean values of previous data and highlighted the need to carefully consider the situation of ocean currents in both the present and the following year since ocean currents adopt different states from year to year. The NOAA side explained that the drift routes of floating debris are forecasted using several existing ocean models including the usual experimental rule used by NOAA, the Navy and the Coast Guard, and that NOAA's results are basically similar to results forecasted by IPRC. In contrast, the Japanese side explained that it is necessary to forecast the wind field comprehensively, as this has a major influence on the drift of floating debris. Hence an ocean model alone is unable to make the forecasts of floating debris required in this work. However, the forecasting of floating distributions using sample particles (more than 1 million) with motions simulated using coupled atmosphere and ocean models is a more realistic approach. An introduction to such forecasting results has been introduced at present, while comprehensive forecasting results will be reported at a later time. Hence the experts on the U.S. side evaluated the more elaborate Japanese modeling highly in comparison with the approach taken by the U.S. The Japanese side agreed with the need to bear in mind that the impression given by drift-forecast data is greatly dependent on the indicators and/ or parameters chosen, such as between the density or the evolution of the diffusion. The forecast data made by the Japanese side presently suggest a minor drift of debris into the Japan Sea. However, to prevent a misleading impression, a color scale for density is being introduced to replace the original mono-color display of particle diffusion. With regard to the settings used in the floating debris forecast, the NOAA side asked the Japan side to perform further experiments, because the strength of the wind effect changes depending on the ratio of floating debris above sea surface. In addition to the two cases of debris 10% above sea surface and 0% above sea surface (i.e. sunk 100% under the sea), they requested a further simulation under the condition of 90% above sea surface. On this point, Japanese side responded that the 10% and 0% experiments represent first steps and that ensemble forecasting under other conditions will be performed. (3) Future Cooperation 8

11 In response to an enquiry from NOAA, the Japanese side reported a time frame in which the total amount of debris will be estimated by the end of February 2012, while the drift path will be forecasted by the end of March 2012 and the results then published. Also the Japan side reported that the Headquarters for Ocean Policy intend to publish a Q&A section on their web site and that in future the Government of Japan will consider the required correspondence. (Note that MOE reported an estimation of the total amount of debris on March 9.) They also noted that the achievements of this project will be published on the web site of MOE and the Headquarters for Ocean Policy. (Furthermore, we stressed that the reported items for Ms. Wallace are not shared fully throughout NOAA.) 9