Detecting tropical thin cirrus using Multiangle Imaging SpectroRadiometer's oblique cameras and modeled outgoing longwave radiation

摘要

We report the improved detection of thin cirrus clouds over the Tropics using oblique camera stereo retrieval of cloud top heights from the Multiangle Imaging SpectroRadiometer (MISR) instrument on the Terra satellite. The MISR oblique stereo captures 10% of thin cirrus with mean height of 13 km over all scenes that the standard stereo misses completely, especially when they are over lower-level clouds that provide more contrast. To determine thin cirrus properties missed by MISR, differences between modeled and measured outgoing longwave radiation (OLR) were used to compute its fractional cover and optical depth. The oblique MISR measurements were used as inputs to the model and a merged data set from CERES, MODIS and MISR instruments on the Terra satellite provided the measured fluxes and the cloud properties. For the cases investigated including all clear and cloudy scenes in the Tropics, the difference between modeled and measured OLR (Cirrus Forcing) averaged ≈ 19 W m~(-2). This can be accounted for by the addition of thin cirrus of coverage 77%. However, oblique analysis only detects 10% of thin cirrus (0.1 < τ < 0.3) and misses 67% of cirrus with τ < 0.3. The missed cirrus coverage includes 32% of homogeneous cirrus (0.1 < τ < 0.3) and 35% of subvisual cirrus (τ < 0.1). To improve the detection of homogeneous cirrus with MISR, the current contrast threshold should be decreased. This will increase the number of pixels to be matched stereoscopically that were screened as noise initially.