Implementation and validation of the snow grain size retrieval SGSP from spectral reflectances of the satellite sensor MODIS

• Main Author: Wiebe, Heidrun
• Other Authors: Notholt, Justus, Heygster, Georg, Lemke, Peter
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Universität Bremen 2011
• Subjects: snow grain size; satellite; retrieval; validation; MODIS; 530; 530 Physics; ddc:530; Antarctic; Arctic; Greenland; The Antarctic; albedo; Antarc*; Sea ice
• Online Access: https://media.suub.uni-bremen.de/handle/elib/159; https://nbn-resolving.org/urn:nbn:de:gbv:46-00102092-12

Snow is part of the cryosphere in the climate system of the Earth. It has a high albedo in the visible, decreasing towards the near-infrared. Snow on ground is a porous medium of ice, air, and possibly impurities like dust or soot. After deposition, it undergoes snow metamorphism changing the grain size, grain shape, and density. In the visible, the reflection characteristics of snow are mainly determined by the amount of impurities, and in the near-infrared by the size of the snow grains. Satellite sensors allow observing the snow in remote areas like the polar regions on a regular basis and on a global scale. A method to compute the snow grain size and impurity amount from optical satellite observations is the Snow Grain Size and Pollution amount (SGSP) retrieval. It uses data of three reflectance channels (here: at 0.47 µm, 0.86 µm, and 1.24 µm), has a reduced dependency on the snow grain shape, and is applicable at solar zenith angles up to 75°. In this work, the SGSP retrieval is implemented in a near-real time processing chain using data from the Moderate Resolution Imaging Spectrometer (MODIS) operating on the satellites Terra and Aqua. A sensitivity analysis reveals that currently only the snow grain size can be determined reliably by the SGSP retrieval, as the uncertainties of the MODIS instrument are too high for the amount of impurities typically occurring in polar regions. Sensitivity studies on the influence of vertically inhomogeneous snow, wet snow, and cirrus clouds show that the SGSP retrieval typically underestimates the grain size by 15% to 25% for those three cases. The SGSP-retrieved snow grain size is validated using six different ground truth data sets from the Arctic, the Antarctic, Greenland, and Japan from the years 2001 to 2009, and various subsurfaces (land, land ice, sea ice, lake ice). In general, the retrieved and ground-measured grain size are in good agreement. 17 cases have small differences (1 14%), 16 cases intermediate differences (18 53%), and four cases large differences ...