Validation of MODIS Terra and Aqua Ice Surface Temperatures at Summit, Greenland
Abstract:Ice-surface temperature (IST) is used in many studies, for example for validation of model output and for detection of leads and thin ice in sea ice. The MODerate-resolution Imaging Spectroradiometer (MODIS) instruments on the Terra and Aqua satellites are useful for mapping IST of sea ice and the Greenland ice sheet (Hall et al., 2012), and validation of the ISTs derived from MODIS has been an ongoing effort (e.g., Koenig & Hall, 2010; Shuman et al., 2014). Recent results call into question the calibration of the MODIS-derived ISTs at very cold temperatures that are characteristic of the Greenland ice sheet high interior during winter (Shuman et al., 2014). In the present work, we investigate the calibration of MODIS IR bands 31 (10.780 – 11.280 µm) and 32 (11.770 – 12.270 µm) under very cold conditions.
MODIS IR bands are calibrated using a quadratic algorithm. In Collection 6 (C6), the offset and nonlinear calibration coefficients are computed from data collected during the blackbody cool-down vs the warm-up data used in Collection 5 (C5). To improve the calibration accuracy for low-temperature scenes, the offset terms are set to 0. In general, Aqua MODIS bands 31 and 32 perform better than Terra MODIS bands 31 and 32. One of the reasons is that the Aqua bands have a lower saturation temperature (~340 K) than the Terra (~380 K) bands, and lower saturation or smaller dynamic range means better resolution.
As compared to ~2-m NOAA air temperatures (TA) at Summit, Greenland, Shuman et al. (2014) show a small (~0.5°C) offset in Terra MODIS-derived IST vs TA near 0°C, and an increasingly larger offset (up to ~5°C) as TA drops to -60°C. To investigate this further, we compare Terra and Aqua C5 and C6 ISTs with TA data from Summit. This work will document the calibration of bands 31 and 32 at very low temperatures in C5 and C6.
Hall, D.K., et al., 2012: Satellite-Derived Climate-Quality Data Record of the Clear-Sky Surface Temperature of the Greenland Ice Sheet, Jour. Clim., 25(14):4785-4798.
Koenig, L.S. and D.K. Hall, 2010: Comparison of satellite, thermochron and station temperatures at Summit, Greenland, during the winter of 2008/09, Jour. Glaciol., 56(198):735-741.
Shuman, C.A., et al., 2014: Comparison of near-surface air temperatures and MODIS ice-surface temperatures at Summit, Greenland (2008-2013), JAMC, in press.