海面粗糙度對衛星雷達與視覺影像的異同

Abstract

當內波通過時，在表面發生流速輻合(convergence)與輻散(divergence)的現象，水面流速輻合的地方，就是表面波被聚集的地方，海表面會變的粗糙。衛星雷達可以偵測內波的波長與傳播等現象，但是雷達影像上的最大粗糙度、內波的波峰位置，與人眼所見的最大波高位置相互關係，有待實驗證實。衛星雷達偵測的海表面粗糙度為 4~10cm 之間的短波，但我們肉眼所見的海面粗糙度是最大波高，它是由公尺級的重力波所決定。現場人眼所見的海面粗糙度以及衛星雷達所觀測的海面粗糙度並不相同，它們的量化關係隨當時觀測的角度、高度、方向，陽光照射方向，以及波浪行進的方向及波長而改變。在分析衛星雷達影像時，人們常以肉眼所見的海洋現象，解釋波浪受海流聚散作用的影響，其差異之處，尚待探討。 本文主要研究不同波長的表面波在通過海檻前後作什麼不同的反應，以及表面粗糙度在視覺、各種(船載、山上)光學照片、衛星雷達影像上的不同呈現。在基隆海檻上方海域，潮流受到地形的影響而改變速度，造成輻散與輻合的現象，表面波會受到變速潮流的調制而改變其波長與對地波速。波長小於1.2m的波因其波速過小無法頂流穿越海檻到上游。但人眼感受及影像呈現的粗糙度是海檻上方波高最大，但無短波存在。衛星雷達觀測的是短波，雷達影像在海檻上游呈現暗帶，背向散射雷達波較弱、視其為平滑的海面，亦即海面粗糙度較低。光學影像與衛星雷達影像兩者所顯示最粗糙海面的位置並不相同，衛星雷達所觀測到海面最粗糙的位置為距離雷達影像粗糙度界面約200m的下游處，也是海流輻合效應最強的地方；光學影像感受到海面最粗糙的區域約是海流最強區域，海檻頂部（最淺處）到雷達影像粗糙度界面約16m下游處(在海檻下游的肩部)。顯示人眼所看到的粗糙度不是SAR影像所顯示的亮帶，因此在分析衛星雷達影像時若以人眼所見的海洋現象去解釋，其中會有差異存在。

When non-linear internal wave (NLIW) propagates, its associated current will generate convergence and divergence on sea surface. The convergence will concentrate short surface waves and make sea surface rougher. This roughness is related to the bright patches on satellite radar images, therefore, radar images may be used to study the location, wavelength and propagation of NLIW. But the relation between rough surface on radar images, the location of NLIW peak, and the visual image of surface roughness is still to be verified. One sees roughness by the wave height that is mostly determined by waves longer than 1m, while radar sees mostly the surface wave about 0.04~0.1 meter long. Therefore, sea surface roughness means differently to human eyes and to satellite radars, because they look at different bands of wavelength. Their quantitative relation depends on the viewing angle and height, illuminating direction of the sun, and the direction and wavelength of surface waves. The objective of this study is to resolve the response of surface waves to the bathymetry-induced convergence/divergence, and to differentiate the sea surface roughness viewed by human eyes, by optical sensors, and by satellite radars. As gravity waves propagate upstream across Keelung Sill, they are slowed down by the current, their wavelengths are shortened by the convergence effect, and their wave heights increase. Waves less than 1.2 meters long can not pass over the sill due to the current on the sill. But one sees largest wave height over the sill, there are few short gravity waves and the sea surface is smoother. Satellite radar observes short waves, its image shows dark band over the sill, representing low backscattering cross-section, low surface roughness, and smoother surface. The largest roughness to satellite radar is 200 m downstream from the boundary between dark and bright bands on radar image, where convergence is the strongest. Visually rough surface is in the region where the current speed reaches maximum, or from the sill top to 16 m down stream of above boundary. This dissimilarity on surface roughness raises caution on interpreting satellite radar images with visual experience on ocean phenomena.