全球250 m反照率产品算法及验证

反照率是反映地表能量平衡的重要参数之一,也被运用于全球变化与天气预测研究中。为了更好地获取反照率产品,学者们在不断地更新反照率产品和提出新算法。本文主要是在1 km分辨率的全球陆表特征参量（GLASS）反照率产品基础上进一步开发250 m反照率产品。算法思路是利用中分辨率成像光谱仪（MODIS）传感器的250 m分辨率波段提供纹理信息,利用成熟的GLASS 1 km反照率产品提供均值信息,进行数据融合,实现GLASS 1 km反照率产品的降尺度。相比于GLASS 1km反照率产品,250 m产品首先展现出更为丰富的空间细节信息,并且在与地面站点观测数据（北美地区站点数据）直接对比中显示出稍高的验证精度,GLASS 1 km产品的验证精度为0.0257,而250 m产品验证精度0.0235。最后,以北京市为例,基于2003—2013年的产品分析了北京市周边反照率及其变化趋势的空间分布,可以看到250 m反照率产品视觉效果更好,相比于1 km产品能够揭示反照率变化趋势的更多空间细节信息。分析表明,在北京城市化进程中,由于加大了城市绿化,主城区反照率总体呈降低趋势。

An Algorithm for Producing 250 m Global Albedo Product and Validation

Albedo is an important parameter in surface energy balance, and it is also used in the research of global change and weather prediction. In recent years, due to the abnormal weather changes and the increase of extreme weather, there emerged a great demand for high-quality observation data in environmental and climate research. To better understand albedo, researchers have been constantly updating albedo products. The purpose of this paper is to develop a 250 m albedo product based on the 1 km resolution Global Land Surface Satellite(GLASS) albedo product. Compared with GLASS 1 km albedo product, our product has higher resolution and precision. The main idea is to use the texture information from the 250 m band data of Moderate-resolution Imaging Spectroradiometer(MODIS) to scale down the 1 km resolution GLASS albedo.There are three steps for the algorithm: simple direct inversion, simple combination of primary products, and downscaling fusion algorithm. Followingly, the 250 m albedo product was validated by comparing to the ground data from 30 sites in North America. It is found that the 250 m albedo product data is closer to the ground measurements. The accuracy of GLASS 1 km product is 0.0257, while that of the 250 m product is 0.0235. Finally, an application of the product was demonstrated through analyzing the inter-annual albedo change trend of Beijing based on the250 m albedo product from 2003 to 2013. The results show that the urbanization of Beijing decreased the average albedo rather than increased, and that albedo changes caused by natural processes or changes in planting patterns and varieties were more significant than those caused by urbanization. Besides, the result shows that the 250 m albedo product is not only with better visual presentation and clearer texture, but also capable to reveal the spatial details of albedo changes.