论文设计基于固有光学特性的北部湾透明度遥感反演及其检验44246

专业好文档基于固有光学特性的北部湾透明度遥感反演及其检验魏国妹1 商少凌1* 李忠平2 兰健3(1厦门大学近海海洋环境科学国家重点实验室，福建 厦门 3610052 Northern Gulf Institute, Mississippi State University, MS 395293 中国海洋大学海洋系，山东 青岛 266100)投稿：高技术通讯通讯作者：slshang ; slshangTel.0592-2184781; Fax. 0592-2184781第一作者：weiguomeiTel.0592-218687113799288374摘要综合了固有光学量的反演和透明度与固有光学量的关系，构成了基于固有光学特性的透明度遥感算法。将此算法应用于北部湾的MODIS遥感数据，得到了该区域的透明度，并利用实测透明度对遥感反演结果进行了验证。通过对时间窗口为 48 h，空间窗口为1 km的30组实测与遥感匹配数据的统计分析(实测透明度在1.826.0 m 范围)，得到平均百分比误差()为22%，对数均方根误差(log_RMSE)为0.121。而基于叶绿素的经验算法反演的透明度，为42，log_RMSE为0.185。上述结果说明，至少对于北部湾这类近岸水体，海色遥感结合基于固有光学量的算法，可以实现较高准确度的透明度遥感反演。关键字：北部湾 透明度 半分析算法 MODIS13An IOP-based remote-sensing algorithm for Secchi depth and its validation for the Gulf of Tonkin Wei Guomei 1 Shang Shaoling 1* Lee Zhongping 2 Lan Jian3(1 State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361005, China2 Northern Gulf Institute, Mississippi State University, MS 395293 Department of Oceanology, Ocean University of China, Qingdao 266100, China)By combining the retrieval of inherent optical properties (IOP) with the relationship between IOP and Secchi depth (Zsd), an IOP-based algorithm was formulated to estimate Zsd from remote sensing of water color. This algorithm was further applied to the moderate-resolution imaging spectroradiometer (MODIS) data over the Gulf of Tonkin to get Zsd from satellite observations, and the calculated results were evaluated with the data from the in situ measurements. With a time window of 48h and a spatial range of 1 km, 30 match-up points were found (Zsd was in a range of 1.8 26.0 m). By comparing the Zsd from the IOP-based algorithm with the Zsd from in situ measurements, the average percentage error () was 22%, and the root-mean-square-error in log scale (log_RMSE) was 0.121. When the satellite Zsd is calculated with the traditional chlorophyll-based approach, however, is 42% and the log_RMSE is 0.185. Clearly, these results demonstrate that, at least for waters in the Gulf of Tonkin, much more accurate Zsd estimation is achieved when it is calculated with the IOP-based algorithm.Key words: Gulf of Tonkin, Secchi depth, quasi-analytical algorithm, MODIS水体透明度（Secchi depth，Zsd）是指放入水中的黑白相间的塞氏盘（Secchi disk）开始看不见时所到达的深度。它是描述海水特性的一个重要参数，它可以辅助水团、流系识别1，在军事上是确定潜艇潜没的深度和布设水雷的重要参考参数2。在海洋水质监测中，它是一种直观的指示参数，它可粗略反映水体中悬浮物的多少和水体的污染情况3。此外，在渔业上可根据透明度来判断鱼类的活动范围和数量4。尽管透明度的实时测量操作简单，但要获得大范围海域的海水透明度的时空分布，通过船舶出航来完成是不现实的，而利用海色遥感反演则是十分重要的手段。已有不少学者建立了用于不同水域和不同遥感系统的透明度遥感算法5-7，如Morel等5在Preisendorfer8和Tyler9的理论的基础上，建立了透明度（或真光层深度）和叶绿素浓度之间的经验关系模式，但该算法可能仅适用于一类水体。王晓梅等6建立了黄海、东海二类水体透明度统计反演模式，但此类算法需要预先取得大量的现场数据来调试模式参数，区域性要求比较强，相对而言缺乏普适性。何贤强等7建立了海水透明度的半分析（quasi-analytical）模式，它在一类水体中可取得较好的反演结果，相对平均误差为23%，但作者没有给出透明度小于10m的浑浊水体的检验结果，且指出，对于此类水体，反演误差较大，反演结果一般也偏大。Preisendorfer8 通过理论推导指出，除了测量时的环境条件和人员因素外，Zsd主要取决于水体的固有光学特性（inherent optical properties，IOP）。因此，在Preisendorfer8和Tyler9的理论的基础上，Doron等10给出了基于490 nm波段垂直漫衰减系数和光束衰减系数的Zsd计算方法。为了获得490nm波段的漫衰减系数和光束衰减系数，参考半分析算法（quasi-analytical algorothm，QAA）11，Doron等10以709nm为起始波段开始推导水体的固有光学量，该起始波段在高浊度水体中固然更有效，但对于大洋和大部分的近岸水体，由于水分子在长波处的强烈吸收，以相对较短波段作为起始波段则是必需的11。故此，我们尝试采用已被广泛应用的QAA算法12，由遥感反射率反演吸收与散射系数，而后结合Doron等10的Zsd计算公式，形成一个基于固有光学量的透明度遥感算法（为表述简便，以下称之为IOP算法），然后应用中分辨率成像光谱仪（moderate-resolution imaging spectroradiometer，MODIS）遥感反射率数据反演Zsd，采用北部湾的实测Zsd数据进行检验。1 透明度水色反演方法Preisendorfer8和Tyler9提出，可见光在均匀介质中传播时，Zsd可表达为 (1)式中，D0是水表面的透明度盘的表观对比度， Dmin为透明度盘的表观对比度下降到肉眼的灵敏度阈值。根据Preisendorfer8，是一个常数，其变化范围在5到10之间。和分别表示可见光波段光的垂直漫衰减系数和光束衰减系数，Doron等10根据一个主要针对欧洲二类水体（地中海、波罗的海、北海、英吉利海峡、亚得里亚海）、由7个航次调查构成的数据集（COASTLOOC dataset），发现与之间高度相关，可以一个多项式拟合二者的关系，因此Zsd可以下式估算： (2)这里 (3)和是水的吸收系数、后向散射系数和总散射系数的函数13,14。北部湾水体受河流影响较大，水中悬浮无机质较多15，因而颗粒后向散射系数和颗粒总散射系数的比值考虑为0.0216，由此 (见 Lee等14 ) 和表示为 (4) (5)式中, 和采用QAA算法由遥感反射率计算（见Lee等12）。 为纯水的散射系数, 取0.0030 m-1 17。2 北部湾透明度Zsd的计算及检验2.1 数据来源现场观测数据来自“908-01-ST09”项目2006年7月15日至8月7日和2006年12月25日至2007年1月22日两个北部湾航次。遥感数据为美国NASA提供的MODIS归一化离水辐射率(nLw) L2级数据，空间分辨率1 km。由于受云的影响较大，在卫星过境短时间间隔内的实测与卫星匹配数据极少，为增加匹配数据组，我们将时间窗口扩大到 48 h，由此获得遥感与实测匹配数据34组，分布位置见图1。2.2 计算及误差分析方法在整个Zsd计算流程中唯一需要特定选择的参数是，它的取值在5-10之间，它可能随着观测时间、环境及观测人的视力发生变化。Morel等5采用108323个现场数据（美国海洋数据中心，USNODC）进行分析，认为取5.5为最佳平均值。因而对北部湾区域，我们同样选择5.5的经验系数进行计算。为作比较，我们同时采用了Morel提出的经验算法5进行计算，简称为Chl算法： (6) (7)式(6)中，角标5.5是常数的取值；式(7)中的Chl是MODIS标准产品，由OC3M算法18获得。为了比较反演结果与实测结果，我们计算了平均百分比误差（）和对数均方根误差(log_RMSE)。定义如下： (8) (9)xdir表示反演值，xmea表示实测值。图1 2006年夏季（）及2007年冬季（）北部湾航次站位图；左图小方框指A、B、C站的范围，放大图见右下角2.3 结果与讨论IOP算法与Chl算法的反演值和实测值进行比对的结果见表1和图2。由表1结果可知，对于本文所研究的34组数据，实测透明度在0.7-34.0 m的范围内变动，反演误差为0-157%（IOP算法）和10%-443%（Chl算法）不等，Chl算法的反演误差显著高于IOP算法。对于IOP算法，高误差出现在A（109.5656E，21.3021N）、B（109.3820E，21.3402N）和C（109.1521E，20.9912N）站（具体位置见图1），误差范围为55%-157%，相应Chl算法得到的误差范围为39%-443%。A和B站处于近岸位置，水深都是14 m，是仅有的两个实测透明度小于1 m的站位（皆为0.7 m）。测量当天的海况分别是4级和5级，我们认为底层沉积物的再悬浮以及海浪很可能直接影响实测结果，其可靠程度较低。C站和周围站点(C1-2)相比（详细位置参见图1右下角），现场实测叶绿素和浊度相对较高(50%)，但实测透明度却高于(30%)周围站点（图3），由此可以推断，该站点的实测数据存在问题。表1 实测值和两种算法的反演值的比较经度/ E纬度/ N实测Zsd/ mIOP算法Chl算法反演值/ m误差/ %反演值/ m误差/ %109.565621.30210.71.61293.3371109.382021.34020.71.81573.8443109.337421.27231.81.803.9117109.244921.12073.02.3234.240109.152120.99123.11.4554.339109.094620.90392.41.8254.692108.629821.31734.52.3495.318108.602021.21876.09.15210.067108.578021.098315.219.52817.415108.345021.45943.82.9241.853108.091521.35012.02.3153.890108.089321.30084.94.3125.512108.086521.228010.09.468.416108.093521.118411.616.84513.920108.094421.008716.016.9613.714108.090520.907719.018.9116.513108.086420.815521.019.6717.318109.542720.65193.12.3265.165109.499720.38213.02.2274.860108.039220.393626.023.11116.736109.613620.20022.53.1245.5120109.822520.20152.11.6244.091108.094421.008716.020.02514.311108.090520.907719.022.61917.110108.086420.815521.022.5717.019108.078920.201626.020.12313.947109.263517.635634.023.13221.736109.288917.966519.518.9317.510109.625820.39671.91.2372.953109.500620.38942.31.5353.344109.066420.38064.83.2334.213108.766320.38927.25.3264.439108.255019.85647.46.978.211108.456219.76885.03.6286.836(a)IOP算法 (b)Chl算法图2 北部湾Zsd实测值和反演值的比较为夏季，为冬季 (a)浊度 (b)叶绿素浓度图3 C站和周围站点(C1,2)的透明度、浊度和叶绿素a浓度 由图2可见，两种算法的Zsd反演结果与实测值皆呈高度相关。然而，采用基于一类水体建立的Chl算法5计算，在透明度较小时，反演值偏高；透明度较大时，反演值则偏低。IOP算法的反演值则相对均匀地分布在1:1线周围，惟有一个数据点显著偏离1:1线。该点为所研究数据组中的最大透明度点（站点D，109.2635E，17.6356N，具体位置参见图1）。原因何在，尚难有定论，可能是算法在高透明度水体中存在局限性，也可能是深水中洋流造成测量绳的倾斜使得实测数据失真，有待未来加强数据采集工作，做进一步的检验。根据以上分析，我们判定C站的实测数据为异常，同时考虑到A、B两个透明度极低站点（1 m）的反演质量难于把握，最大透明度点偏离1：1线的原因也尚难有定论，故而排除这四个站点，进行误差统计。结果表明，当透明度在126 m之间，Chl算法平均百分比误差()为42，最大误差120%，log_RMSE为0.185。结合Doron等10提出的Zsd关系式和QAA后形成的IOP算法，为22%，最大误差52%，log_RMSE为0.121。两种算法差异甚大，最主要的原因在于基于叶绿素浓度的Zsd算法完全是一种经验算法，既依赖于叶绿素的经验反演，又依赖于Zsd和Chl的经验关系。当水的光学系数由非叶绿素成分决定时（如有色溶解有机物或再悬浮的沉积物），或近岸经验反演的Chl受近岸高浓度颗粒碎屑和有色溶解有机物的干扰时，都足以让Zsd产生很大的误差19,20。3 结论透明度是一个具有实用价值的水质参数，实际测量简便，但要实现大面积、多时段的观测只能依靠遥感手段。不少研究者已试图建立各种算法实现透明度的遥感反演，但不同程度地存在局限性。本研究通过结合Doron等10给出的透明度计算公式与反演IOP的QAA算法12，构成了基于IOP的透明度反演算法，并将之应用于北部湾的测量。采用两个航次(2006年夏季和2007年冬季)的实测数据对遥感透明度进行检验，当时间窗口为 48 h，空间窗口为1 km，实测透明度在126 m的范围内，该算法反演的透明度，平均百分比误差为22%，最大误差52% (该点实测Zsd 为 6.0 m)，对数均方根误差log_RMSE为0.121，与Morel等5提出的基于叶绿素的算法相比，反演准确度有了显著的提高。诚然，本研究所采用的检验数据，就时空覆盖度和数据量而言，还是有限的。为了将此算法应用于其他区域和季节的Zsd反演，进一步的测量和研究将是必要的。需要特别指出的是，我们没有利用实测数据来确定或修订算法参数，而是利用独立的实测数据来检验和评价由MODIS数据反演的遥感Zsd, 并由此来衡量算法的适用性。需要指出的是，本研究采用的Zsd计算公式中，参数的取值可能随着观测时间、环境和观测者而变化，这一不确定性是伴随着透明度这一参数本身的定义而产生的。而另一个同样反映水体透明度的参数真光层深度，是科学上定义严格的一个概念，计算之需要的物理量都建立在海洋光学理论的基础上，而且可以通过电子光学仪器准确测量；将之用于衡量水体透光程度，可以在很大程度上降低不确定性12。致谢：国家863计划（2006AA09A302，2008AA09Z108）、国家自然科学基金（40521003）、国家908专项908-01-ST09项目、厦门大学新世纪人才计划联合资助。感谢现场调查航次首席科学家厦门大学李炎与胡建宇教授的支持。参考文献1 张绪琴. 渤海、黄海和东海的水色分布和季节变化黄渤海海洋，1989，7(4)：39452 何贤强，潘德炉，黄二辉等中国海透明度卫星遥感监测中国工程科学，2004，6 (9)：33373 Jamu D M， Lu Z M，Piedrahita R HRelationship between secchi disk visibility and chlorophyll a in aquaculture pondsAquaculture，1999，170：2052144 周雅静，林建国，俞慕耕东海透明度特征探讨J东海海洋，1999，17(3)：67725 Morel A，Huot Y，Gentili B，et alExamining the consistency of products derived from various ocean color sensors in open ocean (Case 1) waters in the perspective of a multi-sensor approachRemote Sensing of 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Follow him on Twitter: jnjonesjr (CNN) - I will always wonder what it was like to huddle around a shortwave radio and through the crackling static from space hear the faint beeps of the worlds first satellite - Sputnik. I also missed watching Neil Armstrong step foot on the moon and the first space shuttle take off for the stars. Those events were way before my time.As a kid, I was fascinated with what goes on in the sky, and when NASA pulled the plug on the shuttle program I was heartbroken. Yet the privatized space race has renewed my childhood dreams to reach for the stars.As a meteorologist, Ive still seen many important weather and space events, but right now, if you were sitting next to me, youd hear my foot tapping rapidly under my desk. Im anxious for the next one: a space capsule hanging from a crane in the New Mexico desert.Its like the set for a George Lucas movie floating to the edge of space.You and I will have the chance to watch a man take a leap into an unimaginable free fall from the edge of space - live.The (lack of) air up there Watch man jump from 96,000 feet Tuesday, I sat at work glued to the live stream of the Red Bull Stratos Mission. I watched the balloons positioned at different altitudes in the sky to test the winds, knowing that if they would just line up in a vertical straight line we would be go for launch.I feel this mission was created for me because I am also a journalist and a photographer, but above all I live for taking a leap of faith - the feeling of pushing the envelope into uncharted territory.The guy who is going to do this, Felix Baumgartner, must have that same feeling, at a level I will never reach. However, it did not stop me from feeling his pain when a gust of swirling wind kicked up and twisted the partially filled balloon that would take him to the upper end of our atmosphere. As soon as the 40-acre balloon, with skin no thicker than a dry cleaning bag, scraped the ground I knew it was over.How claustrophobia almost grounded supersonic skydiverWith each twist, you could see the wrinkles of disappointment on the face of the current record holder and capcom (capsule communications), Col. Joe Kittinger. He hung his head low in mission control as he told Baumgartner the disappointing news: Mission aborted.The supersonic descent could happen as early as Sunday.The weather plays an important role in this mission. Starting at the ground, conditions have to be very calm - winds less than 2 mph, with no precipitation or humidity and limited cloud cover. The balloon, with capsule attached, will move through the lower level of the atmosphere (the troposphere) where our day-to-day weather lives. It will climb higher than the tip of Mount Everest (5.5 miles/8.85 kilometers), drifting even higher than the cruising altitude of commercial airliners (5.6 miles/9.17 kilometers) and into the stratosphere. As he crosses the boundary layer (called the tropopause), he can expect a lot of turbulence.The balloon will slowly drift to the edge of space at 120,000 feet (22.7 miles/36.53 kilometers). Here, Fearless Felix will unclip. He will roll back the door.Then, I would assume, he will slowly step out onto something resembling an Olympic diving platform.Below, the Earth becomes the concrete bottom of a swimming pool that he wants to land on, but not too hard. Still, hell be traveling fast, so despite the distance, it will not be like diving into the deep end of a pool. It will be like he is diving into the shallow end.Skydiver preps for the big jumpWhen he jumps, he is expected to reach the speed of sound - 690 mph (1,110 kph) - in less than 40 seconds. Like hitting the top of the water, he will begin to slow as he approaches the more dense air closer to Earth. But this will not be enough to stop him completely.If he goes too fast or spins out of control, he has a stabilization parachute that can be deployed to slow him down. His team hopes its not needed. Instead, he plans to deploy his 270-square-foot (25-square-meter) main chute at an altitude of around 5,000 feet (1,524 meters).In order to deploy this chute successfully, he will have to slow to 172 mph (277 kph). He will have a reserve parachute that will open automatically if he loses consciousness at mach speeds.Even if everything goes as planned, it wont. Baumgartner still will free fall at a speed that would cause you and me to pass out, and no parachute is guaranteed to work higher than 25,000 feet (7,620 meters).It might not be the moon, but Kittinger free fell from 102,800 feet in 1960 - at the dawn of an infamous space race that captured the hearts of many. Baumgartner will attempt to break that record, a feat that boggles the mind. This is one of those monumental moments I will always remember, because there is no way Id miss this.