Airborne hyperspectral observations of surface and cloud directional reflectivity using a commercial digital camera
Airborne hyperspectral observations of surface and cloud directional reflectivity using a commercial digital camera
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DOI：
10.5194/acp-12-3493-2012
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发表时间：
2012-01-01
影响因子：
6.3
通讯作者：
Gayet, J. -F.
中科院分区：
文献类型：
--
作者：
Ehrlich, A.;Bierwirth, E.;Gayet, J. -F.
利用数码单镜头反光照相机测量光谱辐射率，得出北极地区云层和不同表面的定向反射率。该相机已被校准辐射和光谱，以提供准确的辐射测量与高角分辨率。与光谱模块化机载辐射测量sysTem（智能Albedometer）的光谱辐射测量的比较表明，这两种仪器的不确定性（6%）内的协议。用半球方向反射因子（HDRF）计算了海冰、无冰海洋和云层的方向反射率。海冰的反射系数为0.96（波长530 nm），显示出几乎各向同性的HDRF，而海洋HDRF（= 0.12）观察到太阳闪烁。对于= 0.62的云观测，云弓-一种通常用于液态水滴散射的后向散射特征-被相机覆盖。对于非均匀层积云以上的测量，获得清晰显示云弓的平均HDRF所需的图像数量估计约为50张图像（10分钟飞行时间）。HDRF作为散射角的函数的表示仅将图像数量减少到约10（2分钟飞行时间）。测量的云和海洋HDRF与辐射传输模拟进行了比较。海洋HDRF模拟与观测的表面风速为9 m s（-1）同意最好的测量。对于云HDRF，最好的协议是由一个广泛的和弱的云弓模拟的云滴有效半径R-eff = 4 μ m。该值与从现场测量得到的颗粒尺寸一致，并从SMART Albedometer的光谱辐射率中检索。
Spectral radiance measurements by a digital single-lens reflex camera were used to derive the directional reflectivity of clouds and different surfaces in the Arctic. The camera has been calibrated radiometrically and spectrally to provide accurate radiance measurements with high angular resolution. A comparison with spectral radiance measurements with the Spectral Modular Airborne Radiation measurement sysTem (SMART-Albedometer) showed an agreement within the uncertainties of both instruments (6% for both). The directional reflectivity in terms of the hemispherical directional reflectance factor (HDRF) was obtained for sea ice, ice-free ocean and clouds. The sea ice, with an albedo of = 0.96 (at 530 nm wavelength), showed an almost isotropic HDRF, while sun glint was observed for the ocean HDRF ( = 0.12). For the cloud observations with = 0.62, the cloudbow - a backscatter feature typically for scattering by liquid water droplets - was covered by the camera. For measurements above heterogeneous stratocumulus clouds, the required number of images to obtain a mean HDRF that clearly exhibits the cloudbow has been estimated at about 50 images (10 min flight time). A representation of the HDRF as a function of the scattering angle only reduces the image number to about 10 (2 min flight time).The measured cloud and ocean HDRF have been compared to radiative transfer simulations. The ocean HDRF simulated with the observed surface wind speed of 9 m s(-1) agreed best with the measurements. For the cloud HDRF, the best agreement was obtained by a broad and weak cloudbow simulated with a cloud droplet effective radius of R-eff = 4 mu m. This value agrees with the particle sizes derived from in situ measurements and retrieved from the spectral radiance of the SMART-Albedometer.