A-HIGHERS The System to Produce the High Spatial Resolution Sea Surface Temperature Maps of the Western North Pacific Using the AVHRR/NOAA

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1 Journal of Oceanography, Vol. 56, pp. 707 to 716, 2000 A-HIGHERS The System to Produce the High Spatial Resolution Sea Surface Temperature Maps of the Western North Pacific Using the AVHRR/NOAA FUTOKI SAKAIDA 1 *, JUN-ICHI KUDOH 2 and HIROSHI KAWAMURA 3 1 Ocean Mechanical Engineering Chair, Kobe University of Mercantile Marine, Fukae-Minami-machi, Higashinada-ku, Kobe , Japan 2 Computer Center, Tohoku University, Sendai , Japan 3 Center for Atmospheric and Oceanic Studies, Department of Geophysics, Graduate School of Science, Tohoku University, Sendai , Japan (Received 29 September 1999; in revised form 18 July 2000; accepted 18 July 2000) To study on the oceanic variations in the western North Pacific, we developed a system to produce a high spatial resolution sea surface temperature (SST) map from the data obtained by the Advanced Very High Resolution Radiometer (AVHRR) aboard the National Oceanic and Atmospheric Administration (NOAA) satellites. As the system has been improved on the HIGHERS (Sakaida and Kawamura, 1996), it is called the Advanced-HIGHERS (A-HIGHERS). The A-HIGHERS has been developed on the super computer in the Tohoku University, which is favorable for handling of a large volume of data. Mainly because of improvements in the cloud detection algorithm, the A-HIGHERS can deal with the data obtained at dawn and dusk around the year, and at daytime in summer more effectively. The A-HIGHERS are used to produce SST maps spanning from (60 N, 120 E) to (20 N, 160 E) with a grid size of 0.01 degree. Keywords: High spatial resolution SST maps, AVHRR/NOAA, cloud detection. 1. Introduction Observation of sea surface temperature (SST) from satellites has contributed to the production of SST maps with a high spatial resolution. For example, the National Aeronautics and Space Administration (NASA) has compiled weekly global SST maps with a grid size of about 18 km, which is based on the measurement by the Advanced Very High Resolution Radiometer (AVHRR) aboard the National Oceanic and Atmospheric Administration (NOAA) satellites (Olson et al., 1988). Recently, the Ocean Pathfinder project has been progressed by NASA and NOAA. The project provides us daily global SST maps with a grid size of about 9 km (Vazquez et al., 1998), which is also based on the AVHRR/NOAA s SST observation. The SST maps with a high spatial resolution are useful for analyses of detailed structure of the ocean. To investigate the ocean south of Japan, we produced the * Corresponding author. futoki@cc.kshosen.ac.jp Copyright The Oceanographic Society of Japan. HIGHERS data (Sakaida and Kawamura, 1996) from the AVHRR/NOAA data. The HIGHERS data cover the ocean from ( N, E) to (36.05 N, E) with a grid size of degree. They were used to examine the westward propagation of frontal waves in the Subtropical Front region (Kawamura et al., 1995) and the behavior of the anti-cyclonic eddy during the formation period of the Kuroshio large meander (Sakaida et al., 1998). Since the HIGHERS data were effectual only on the investigation of the ocean near Japan, Kawamura et al. (1997) proposed a project to develop the Advanced- HIGHERS (A-HIGHERS), the successor of the HIGHERS. In the project, the objective area is widen to the western North Pacific. The project aims to retrieve SST maps with a grid size of 0.01 degree for more than 10-years from the AVHRR/NOAA data. To promote the project effectively, the system to produce the SST maps has been reformed completely. The reformed system, or, the A-HIGHERS, is described in this paper. In the followings, the characteristics of the A-HIGHERS will be described in Section 2, and the algorithm in the A- HIGHERS, in Section

2 Table 1. The specification of the A-HIGHERS data. Image size pixels North-West corner 60 N, 120 E South-East corner 20 N, 160 E Grid size 0.01 degree Data size 30.5 MB Fig. 1. The example image of the A-HIGHERS data. 2. The Characteristics of the A-HIGHERS Since a system to receive the NOAA-High Resolution Picture Transmission (HRPT) data, which contain the full resolution AVHRR data, has started its operation at the Center for Atmospheric and Oceanic Studies (CAOS) in the Tohoku University in 1988, the NOAA-HRPT data have been actively collected (Kawamura et al., 1993; Kawamura and Saitoh, 1995). The A-HIGHERS as well as the HIGHERS are planned for the retrieval of SST maps by use of the NOAA-HRPT data library in the Tohoku University. Although the HIGHERS data were produced on a MS-4170 computer produced by NEC Corporation in CAOS, the A-HIGHERS has been developed on a super computer in the computer center of the Tohoku University (Kawamura et al., 1997). The super computer can rapidly process a large volume of data. Therefore, the grid size of 0.01 degree can be executed and the data area can be widen to the western North Pacific. The A-HIGHERS is used to produce the A-HIGHERS data, whose specification is shown in Table 1. Figure 1 shows the example image of the A-HIGHERS data. It takes about 5 minutes to produce one scene of the A-HIGHERS data under the present computer environment (SX4 produced by NEC Corporation). In the A-HIGHERS, there are a number of improvements in the cloud detection algorithm. The northern limit that the cloud can be detected effectively moves to 60 N, as shown in Fig. 1. In addition, the effective cloud detection can be performed for the data received at dawn and dusk around the year and at daytime in summer. The details of the algorithm will be described in Section 3. It should be mentioned that the A-HIGHERS keeps to be empty (correctly speaking, empty means filled by dummy value ) the grid points flagged as cloudy or outside of the AVHRR observation swath. In other word, neither the composite nor the averaging technique is used in the A-HIGHERS to fill each of all grid points. The reason why we do not adopt the composite and averaging technique in producing the SST map with a high spatial resolution is as follows. Figures 2 and 3 show the SST pattern difference between the original image and the images produced by the composite and the averaging technique. Roughly speaking, the images shown in Fig. 3 correspond to the daily average image (Fig. 3(a)) and the daily composite image (Fig. 3(b)) because these are produced from four images in Fig. 2. The daily composite image (Fig. 3(b)) is constituted by the maximum values among the images in Fig. 2. In Fig. 2, the Kuroshio is found as the higher SST belt. As indicated by arrows in the right images in Fig. 2, a small prominence is found at the north edge of the Kuroshio and it moves to the Hachijyo-jima Island with transformation. However, this prominence is difficult to find in the daily average image (Fig. 3(a)). It is also recognized that the unreal pattern appears at the north edge of the Kuroshio in the daily composite image (Fig. 3(b)). Figure 3 shows that images produced by the composite and the averaging technique are useful to grasp the Kuroshio as a whole because of the decrease of cloudy pixels in images. However, they are unfavorable for the investigation of small scale eddies or undulations of the Kuroshio. However, one of advantages in a high spatial resolution SST map is that it can be utilized to investigate a small scale eddy and undulation in the oceans. Therefore, neither the composite nor the averaging technique is adopted in the A-HIGHERS. 3. Algorithm Description The A-HIGHERS comprises three processes, i.e., the SST calculation, the cloud detection, and the quality control (QC). This section explains the algorithm in each process. 708 F. Sakaida et al.

3 Fig. 2. The SST images (left) and the Kuroshio path trajectories (right) near the Izu Islands extracted from the A-HIGHERS data. These are obtained at about (a) 03 Universal Time Coordinate (UTC), (b) 08 UTC, (c) 16 UTC, and (d) 21 UTC on November 28, In the left images, the darker gray corresponds to the higher SST; black and white is used for the land and the cloudy region, respectively. The A-HIGHERS System to Produce the High Spatial Resolution SST Maps 709

4 Fig. 3. The images produced by the averaging technique (a) and the composite technique (b). These are produced by use of four data shown in Fig. 2. The darker gray corresponds to the higher SST; black and white is used for the land and the cloudy region, respectively. The algorithm for the A-HIGHERS was developed for the first time when the system was transferred to a super computer environment. To deal with the region north of 36.5 N, the northern limit to the HIGHERS data area, the first version of algorithm was developed. This version was used to produce about 5000 scenes of the A- HIGHERS data. However, we do not explain this version because it has been replaced already by the improved algorithm to be described in this paper. The improved algorithm adopted in the A-HIGHERS can deal more effectively with severe conditions of radiation, such as the large angle of the sun zenith and the existence of the sunglitter. The first version A-HIGHERS as well as the HIGHERS are apt to err in the cloud detection under such severe conditions. The new algorithm, that is second version, will be described in this section. At present, the A-HIGHERS has been examined for data from the NOAA-11, -12 and -14. The AVHRRs on the NOAA-7, -9, and -15 also equip with infrared (IR) channels suitable to the split-window technique for SST estimation, so that we prepare for using these satellite data. Since the same version AVHRR (AVHRR/2) is aboard the NOAA-7, -9, -11, -12, and -14, the A- HIGHERS must be applied to data from the NOAA-7 and -9 although some adjustments might be needed for the present algorithm. On the other hand, the AVHRR/3 aboard the NOAA-15 is considerably changed from the AVHRR/2; the AVHRR/3 provides spectral and gain changes to the visible channels and adds a sixth channel at 1.6 microns (NOAA, 1999). Therefore, many adjustments should be needed for the A-HIGHERS to deal with the data of the AVHRR/ SST calculation The SST calculation is a process for the atmospheric correction to the clear pixels in the AVHRR data. The process can reduce the difference between the SST observed by the in-situ observation and that by the satellite. The Multi-Channel SST (MCSST) method is widely used to estimate the SST from the AVHRR/NOAA data (e.g., McClain et al., 1985). Recently, various methods have been proposed to estimate the SST from the AVHRR/ NOAA (e.g., Walton, 1988; Emery et al., 1994). Almost all the newly-proposed methods have been developed to overcome the defect of the MCSST method. The newlyproposed methods, however, are not popularized at present and there is room for examination on their accuracy. Thus, the A-HIGHERS still uses the MCSST method. The MCSST equation form of the A-HIGHERS can be written as, MCSST = C 1 T 4 + [C 2 + C 3 (1 1/cosθ sa )]<(T 4 T 5 )> + C 4 (1) where the coefficients, C i (i = 1~4), are constant; T 4 and T 5 are brightness temperatures of the AVHRR split-window channels 4 (11 µm) and 5 (12 µm); and, θ sa is the satellite zenith angle. The coefficients of the equation are shown in Table 2. In Eq. (1), only the term of T 4 T 5 is enclosed in angle brackets. This means that the average for clear pixels within a 7 7 array is used. This technique is effective to reduce noise included in brightness temperatures observed by the AVHRR (Barton, 1989; Badenas et al., 1997). It should be noticed that the average for clear pixels within a 7 7 array is not used for the term of T 4. By this treatment, we can keep the original grid size of 0.01 degree in the AVHRR/NOAA data in the A-HIGHERS. T 4 is replaced by the average of clear pixels within a 3 3 array when the center pixel of 3 3 array is flagged as cloudy. This process is applied to repair a small scratch noise, which tends to occur in the cloud detection process. 710 F. Sakaida et al.

5 Table 2. The coefficients of the MCSST equations used in the A-HIGHERS. Satellite Node MCSST coefficients Source C 1 C 2 C 3 C 4 NOAA-11 A* Sakaida and Kawamura (1992a) D** NOAA-12 A Kidwell (1998) D NOAA-14 A Kidwell (1998) D *A: Ascending node. **D: Descending node. As shown in Table 2, coefficients of the MCSST equation for the NOAA-12 and -14 are derived by NOAA/ NESDIS (Kidwell, 1998). According to Sakaida and Kawamura (1992a, b), the NOAA s equation can be applicable to the ocean around Japan, except for a coastal area. The accuracy of the NOAA s equation on a coastal area is suspected in the Mutsu Bay (Tanba et al., 1988) and the Seto-Inland Sea (Tsukamoto et al., 1997). Further investigation to obtain the high accuracy for a coastal area should be developed. 3.2 Cloud detection The cloud detection is a process for the detection of the clear pixels in the AVHRR data. The clouds always influence radiation emitted from the sea surface, so that the pixels affected by clouds are useless for the SST retrieval. Table 3 shows threshold tests for the cloud detection of the A-HIGHERS. The cloud detection in the A- HIGHERS is performed by means of the combination of the threshold tests, which is same as that in the HIGHERS. The threshold tests can be written as inequality equations, as shown in Table 3. All the coefficients and the thresholds for the tests are determined empirically. Each pixel in the AVHRR data is examined by some threshold tests and then, the pixels passing through all the threshold tests are flagged as clear. The combination of the threshold tests depends on the availability of the visible and the reflected IR channels of AVHRR and the existence of the sunglitter. The combination of the threshold tests is named scheme in this paper. As shown in Table 3, three schemes are considered in the A-HIGHERS. The scheme-1 is used for the case of the daytime without the sunglitter. The scheme-2 is for the sunglitter region in the daytime image. The scheme-3 is used for the nighttime image. The A-HIGHERS deals with the data obtained at dawn and dusk, such as the data from NOAA-12. Therefore, it is necessary to make a border of the daytime and the nighttime scheme clear. This border is set at the sun zenith angle (θ sn ) of 86.5 degree, which is determined empirically according to the affection of the sunlight in the AVHRR data. When θ sn > 86.5 degree, the channel 1 (0.6 µm) and the channel 2 (0.9 µm) of the AVHRR become invalid and the sunlight affection can be ignored in the channel 3 (3.7 µm). As shown in Table 3, thus, the channels 1 and 2 are used only in the daytime scheme (scheme-1 and -2) and the channel 3 is used only in the nighttime scheme (scheme-3). In the case of the daytime without the sunglitter, the channels 1 and 2 of the AVHRR provide the effective data to detect clouds. Especially, the ratio of the channel 2 to the channel 1 is useful for the cloud detection (Saunders and Kriebel, 1988), so that this ratio is used in a NIR/VIS test in both the HIGHERS and the A-HIGHERS. According to Hutchison and Hardy (1995), the ratio of the channel 2 to the channel 1 depends on the sun zenith angle. Therefore, the NIR/VIS test in the A-HIGHERS changes its threshold at the sun zenith angle of about 72.5 degree (cosθ sn ~ 0.3) and this test is not used when θ sn > 81.5 degree (cosθ sn > 0.147), as shown in Table 3. This adjustment of the NIR/VIS test is to deal with the large sun zenith angle case. As mentioned above, the daytime scheme in the A- HIGHERS gains the sunglitter scheme (scheme-2). The sunglitter region appears when the satellite observes the reflection of the sunlight on the specular sea surface. As the sunglitter region has a character similar to the cloudy region, the number of mis-flagged clear pixels as cloudy would be increased if there is no sunglitter scheme. In the AVHRR image around Japan, the sunglitter tends to appear at dawn around the year and in the daytime in summer. Therefore, the sunglitter scheme is favorable for the SST retrieval around Japan at dawn around the year and in the daytime in summer. The A-HIGHERS System to Produce the High Spatial Resolution SST Maps 711

6 Table 3. The cloud detection tests of the A-HIGHERS. The tests checked by are used in each scheme. Name Test Scheme-# Gross cloud (1) T 4 < 15C (0 < φ < 25N) (2) T 4 < 14C (φ < 30N) (3) T 4 < 2C (φ < 35.4N) (4) T 4 < 2C (φ < 40N) (5) T 4 < 3C (φ < 90N) NIR radiance (1) A 2 /cosθ sn > 25/cosθ sa 8.5 NIR/VIS (1) A 2 /A 1 > 0.7 (cosθ sn > 0.3) (2) A 2 /A 1 > 0.85 (cosθ sn > 0.147) (3) A 2 /A 1 > 0.9 (4) A 2 /A 1 > cosθ sn TIR (1) 0.3T 4 T 5 > 8 and T 4 T 5 > /cosθ sa (2) 0.3T 4 T 5 > φ and T 4 T 5 < 1.6 (3) T 4 T 5 < 0.0 (4) T 5 2T 4 + T 3 < 2.0 (5) T 5 2T 4 + T 3 > 1.0 Uniformity (1) DMM(A 2 ) > 0.2 and DMM(T 4 ) > 0.2 (2) DMM(T 4 ) > 0.2 and STD(T 4 T 5 ) > 0.15 (3) LAP(T 4 ) > 0.8 A 1, A 2 : Albedos of AVHRR channels 1 and 2. T 3, T 4, T 5 : Brightness temperatures of AVHRR channels 3, 4, and 5. DMM(x): Maximum-minimum difference of a 2 2 pixel array of parameter x. STD(x): Standard deviation on a 3 3 pixel array of parameter x. LAP(x): Laplacian value of a 3 3 pixel array of parameter x. φ: Latitude. θ sn : Sun zenith angle. θ sa : Satellite zenith angle. To use the sunglitter scheme, the sunglitter region in the image should be specified. In the A-HIGHERS, the reflection angle, θ, is used to specify the sunglitter region. θ can be calculated by, cosθ = (cosθ sn + cosθ sa )/2cosω (2) where θ sa is the satellite zenith angle. ω can be derived by, cos2ω = cosθ sn cosθ sa sinθ sn sinθ sa cos(φ sn φ sa ) (3) where φ sa and φ sn are the azimuthal angles of the satellite and the sun, respectively. The reflection geometry is shown in Fig. 4. The scheme-2 is used at the region of 0 < θ < 18 degree, which is determined empirically. Physically, θ is tilt of the surface facet satisfying the condition of the specular reflection (Fig. 4). The A-HIGHERS uses the constant limit for the reflection angle to specify the sunglitter region (θ = 18 degree). The limitation could be estimated more precisely if the surface wind speed data were available, because the tilt distribution of the surface facet strongly depends on the wind speed (e.g., Cox and Munk, 1954). However, the A-HIGHERS uses the constant limit to specify the sunglitter region because it is difficult to obtain simultaneously the AVHRR/NOAA data and the wind speed data. The examples of the cloud detection obtained by the method mentioned above are shown in Figs. 5 and 6. Fig- 712 F. Sakaida et al.

7 Fig. 4. The reflection geometry between the sun, the satellite, and the surface facet. The <N> is the normal vector of the surface facet. ure 5 is a case of the sunglitter appearance. According to the channel 2 image (Fig. 5(a)), the sunglitter appears near the scene center. As shown in Fig. 5(c), the scheme-2 applies to the sunglitter region correctly. In Fig. 5(d), the cloudy pixels are painted black. According to Fig. 5, the cloudy region is well detected in the sunglitter region. The AVHRR data in Fig. 6 is obtained at dusk, so that the clouds appear in the half of the channel 2 image (Fig. 6(a)). This is a case of the large sun zenith angle and the scheme-1 is changed to the scheme-3 as indicated in Fig. 6(c). According to Fig. 6, the cloudy region is well detected by both the schemes. As mentioned above, all parameters (coefficients and thresholds of the cloud detection tests and borders of the schemes) required in the cloud detection algorithm are determined empirically. More than hundred scenes obtained by the NOAA-11, NOAA-12 and NOAA-14 were used to examine the cloud detection algorithm and then, we find some problems in it. Compared with the daytime scheme, the nighttime scheme is apt to mistake in the cloud detection. The reasons why the cloud detection is difficult in the nighttime are that the visible channels of the AVHRR is useless and that the noise in the AVHRR Fig. 5. The images of (a) the AVHRR channel 2 albedo, (b) the AVHRR channel 4 brightness temperature, (c) the cloud detection scheme distribution, and (d) the cloud detection result. The AVHRR data were obtained by the NOAA-14 at about 04 UTC May 9, The A-HIGHERS System to Produce the High Spatial Resolution SST Maps 713

8 Fig. 6. Same as Fig. 5, but the AVHRR data were obtained by the NOAA-12 at about 08 UTC May 10, channel 3 makes it difficult to adjust the parameters in the nighttime scheme. In the daytime, the case of the sunglitter occurring at the edge of the AVHRR observation swath is also apt to cause the misjudgment in the cloud detection. In this case, all channels of the AVHRR are hard to use and inaccuracy increases in the cloud detection. 3.3 Quality control (QC) In order to decrease the cloudy pixels remaining after the cloud detection process, the quality control (QC) process is added in the A-HIGHERS. Because of the incompleteness of the cloud detection algorithm, the QC process might be confused with the cloud detection. However, the QC process is defined in this paper as the process to check the SST value after the cloud detection process and the SST calculation (atmospheric correction) process. In the QC process of the A-HIGHERS, the following two checks were adopted. Firstly, temperature T(x, y) at a given grid point (x, y), where the coordinate axis of x is laid along the longitude and y, along the latitude, is compared with the zonal maximum SST [T max (y)]. The grid point value, T(x, y), is regarded as a suspectable when T max (y) and T(x, y) satisfy the following relation, ( ) Tmax y + 3 C [ Tmax( y) < 9 C] 12 C [ 9 C< Tmax( y) < 11 C] Tmax( y) T( x, y) > 23 C Tmax( y) 11 C< Tmax( y) < 18 C 5 C [ 18 C< Tmax ( y) ]. [ ] ( 4) The zonal maximum is derived using the data collected under the condition that the time difference is within two days. The zonal maximum is possibly contaminated by the erroneous high value, so that it is replaced by the sum of the zonal average [T ave (y)] and the standard deviation [σ(y)] when T max (y) > T ave (y) + σ(y). The second check uses the number of the cloudy pixels in a 7 7 pixel array. This check catches the center pixel of the 7 7 array when the percentage of the cloudy pixels in the array exceeds 70%. 714 F. Sakaida et al.

9 The QC process is effective for the detection of the cloudy pixels that the cloud detection process misses. However, the pixels misjudged in the clear-cloudy classification still remain in the A-HIGHERS data after the QC process. There are rooms for further improvement in the QC process, as well as the cloud detection process. The work for the algorithm revision must be continued in future and the data will be produced again whenever the algorithm is revised. As we noted, the A-HIGHERS has the advantage in computer environment. Using the computer that can rapidly process a large volume of data, it will take not so much time to produce all the data over again. 4. Summary and Conclusion This paper describes the A-HIGHERS system developed to produce the high spatial resolution SST maps of the western North Pacific. The A-HIGHERS succeeding to the HIGHERS (Sakaida and Kawamura, 1996) has been carried out on the super computer in the computer center of the Tohoku University. The super computer is very useful for the production of the SST maps with a high spatial resolution. Compared with the HIGHERS, the A- HIGHERS can cover a wider area with a finer spatial resolution. In addition, new methods are adopted in the cloud detection process for cases of the large sun zenith angle and the sunglitter appearance. Consequently, the A- HIGHERS can deal with data obtained at dawn and dusk around the year and at daytime in summer more effectively. The A-HIGHERS are used to produce the A- HIGHERS data, which cover the area from (60 N, 120 E) to (20 N, 160 E) with a grid size of 0.01 degree. The A- HIGHERS data produced up to now have been used in the studies for the SST variations of the Japan Sea in winter (Kawamura and Wu, 1998), the recent Kuroshio path variations (Toba and Murakami, 1998), the behavior of the warm core ring in the Tohoku Area (Sakurai and Kawamura, personal communication), sea surface cooling by typhoons (Suetsugu and Kawamura, personal communication), and so on. There still remain many rooms for the investigation of the oceanic phenomena by means of the high spatial resolution SST maps. In order to retrieve more precise SST maps from the AVHRR/NOAA data, we will progress the SST map production with the efforts for the system improvement under the A- HIGHERS project. Acknowledgements The authors acknowledge supports from the computer center of the Tohoku University and NEC Corporation. This study is supported by Research and Development Applying Advanced Computational Science and Technology, Japan Science and Technology Corporation. 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