Detecting Thin Cirrus from MISR with Oblique Camera Analysis

We report improved detection of thin cirrus clouds over clear ocean in the Tropics using oblique camera stereo retrieval of cloud-top heights (CTH) from the Multiangle Imaging SpectroRadiometer (MISR) instrument on the Terra Satellite. MISR misses thin cirrus clouds of optical depth < 0.3 with its standard nadir stereo processing. Thin cirrus is transparent to incoming solar radiation but is sensitive to the outgoing longwave radiation (OLR). The differences in modeled and measured thermal irradiance emitted to space is used to compute the fractional cover and optical depth of thin cirrus over clear ocean using MISR oblique derived heights corrected for winds and parallax. For special cases over clear skies in the Tropics, the differences between modeled and measured OLR (Cirrus Forcing) is ≈ 17W/m. This can be accounted for with the addition of thin cirrus of coverage 60%.