以向日葵8号卫星观测侦测冰雪的初步实验

Primary Experiments of Detecting Snow and Ice by Using Himawari-8 Imagery

本文以NDSI (normalized difference snow index)进行雪地与海冰的侦测,以地球同步卫星向日葵8号的0.51微米与1.6微米频道计算NDSI,当NDSI大于0.6时视为海冰或雪地。但因有些种类的云,其NDSI值亦大于0.6,主要为云顶发展至高层的冰相云顶深厚云层,为区分此类与海冰/雪地光学性质类似的云,我们采用下列测试,以分辨地面冰雪覆盖区与此类的云。第一个测试:以1.6减2.3微米反照率的差值为判断标准,因为发现1.6微米减2.3微米的差值对于冰相云顶深厚云层时小于0,而对冰雪地面覆盖区时其值大于0。第二个测试:当7.3微米减6.2微米亮度温度小于设定阈值时视为冰相云顶深厚云层。第三个测试:当10.4微米减6.2微米亮度温度小于设定阈值时视为冰相云顶深厚云层。第二与三个测试是利用冰相云顶深厚云层为发展较高的云,而7.3微米减6.2微米或10.4微米减6.2微米亮度温度的差值随云顶高度的增加而减小。实验结果与红绿蓝三色合成的日间雪–雾影像产品比对,个案实验发现目前方法提供合理的冰雪侦测结果。

The normalized difference snow index (NDSI) has been used to detect sea ice and snow cover on the Earth. NDSI is calculated by using channel 0.51 μm and 1.6 μm on board geostationary satellite Himawari-8. The pixel is defined as ice/snow when the value of NDSI is higher than 0.6. However, the value of NDSI is higher than 0.6 for some type of cloud either. A primary type of this cloud is the thick cloud with ice top. In order to distinguish this type of cloud, following tests have been adopted to achieve this goal. First, we use the value of 1.6 μm minus 2.3 μm albedo as a parameter to distinguish between the ice/snow on Earth surface and the thick cloud with ice top because we found that the value of 1.6 μm minus 2.3 μm is positive for ice/snow on Earth’s surface and negative for thick cloud with ice top. Second, the pixel is not considered as ice/snow on Earth’s surface when the value of 7.3 μm minus 6.2 μm is smaller than a setting threshold. Third, the pixel did not define as ice/snow on surface of Earth if the value of 10.4 μm minus 6.2 μm is smaller than a setting threshold. The second and third tests are able to remove thick cloud with ice top by the fact that top of this kind cloud is higher than snow/ice on the Earth. Comparison of ice/snow map from NDSI with tests to snow-fog RGB image shows a relevant consistency between them.