Influence of shadow on the thermal and optical snow indices and their interrelationship

In snow-covered mountainous terrains, topography-induced different illumination conditions would cause varying influence on the snow characteristics in the sunlit and shadowed sites owing to albedo variation. Since the thermal bands suffer less by the shadows as compared to the optical bands, it is imperative to investigate the effect of shadow on the optical and thermal snow indices. The investigation comprises determination of (a) difference of means between the sunlit snow cover and snow cover under shadow for thermal snow indices, optical snow cover indices and optical snow cover characteristic indices and (b) correlation coefficient of the thermal snow indices with the respective optical snow cover indices and optical snow cover characteristic indices. The study was conducted in the test sites of the Chenab basin, western Himalayas using the Landsat-8 Operational Land Imager and Thermal Infrared Sensor data. The mean values of different snow indices exhibit significant difference between the sunlit and shadow test sites with Z-values ranging between 68.92 and 1220.39 (p < 0.0001). Shadow significantly increases correlation of the thermal snow indices with the optical snow cover indices with r >= 0.81, while r-values lie below 0.29 in the sunlit test site (Student's t-test, p < 0.0001). On the other hand, thermal snow indices exhibit low correlation with both optical snow cover characteristic indices in either site; however, shadow induces negative correlation between them (r = 0.37 to 0.62, p < 0.0001). The results ascertain the varying influence of shadow on the optical and thermal snow indices and their interrelationship, which could be significantly helpful for accurate radiative transfer modelling of snow in the light of the seasonal variation in the earth-sun geometry. (C) 2016 Elsevier Inc All rights reserved.