Comparison of MISR and Meteosat-9 cloud-motion vectors

Abstract

[1] Stereo motion vectors (SMVs) from the Multiangle Imaging SpectroRadiometer (MISR) were evaluated against Meteosat-9 cloud-motion vectors (CMVs) over a one-year period. In general, SMVs had weaker westerlies and southerlies than CMVs at all latitudes and levels. The E-W wind comparison showed small vertical variations with a mean difference of −0.4 m s−1, −1 m s−1, −0.7 m s−1 and corresponding rmsd of 2.4 m s−1, 3.8 m s−1, 3.5 m s−1for low-, mid-, and high-level clouds, respectively. The N-S wind discrepancies were larger and steadily increased with altitude, having a mean difference of −0.8 m s−1, −2.9 m s−1, −4.4 m s−1 and rmsd of 3.5 m s−1, 6.9 m s−1, 9.5 m s−1at low, mid, and high levels. The best overall agreement was found in marine stratocumulus off Namibia, while differences were larger in the Tropics and convective clouds. The SMVs were typically assigned to higher altitudes than CMVs. Attributing each observed height difference to MISR and/or Meteosat-9 retrieval biases will require further research; nevertheless, we already identified a few regions and cloud types where CMV height assignment seemed to be the one in error. In thin mid- and high-level clouds over Africa and Arabia as well as in broken marine boundary layer clouds the 10.8-μm brightness temperature-based heights were often biased low due to radiance contributions from the warm surface. Contrarily, low-level CMVs in the South Atlantic were frequently assigned to mid levels by the CO2-slicing method in multilayer situations. We also noticed an apparent cross-swath dependence in SMVs, whereby retrievals were less accurate on the eastern side of the MISR swath than on the western side. This artifact was traced back to sub-pixel MISR co-registration errors, which introduced cross-swath biases in E-W wind, N-S wind, and height of 0.6 m s−1, 2.6 m s−1, and 210 m.