Tropical Thin Cirrus from MISR: Detection, Validation and Trends

Abstract

The Multiangle Imaging SpectroRadiometer (MISR) on the Terra satellite has been measuring the altitudes of clouds consistently since 2000, but thin cirrus clouds are difficult to detect, and the operational cloud-height product from MISR may miss thin cirrus when it overlies thicker, lower-level cloud. This thesis focuses on the detection, validation and fluctuations of tropical thin cirrus clouds from MISR. To detect thin cirrus over clear skies with MISR, cloud-heights above 10 km were retrieved using a stereo matching technique on the oblique camera pairs and re-projected onto the nadir camera after correction for parallax and winds. The merged datasets from the CERES broadband radiometer, also on Terra, were used to extract Outgoing Longwave Radiation (OLR) measurements and compared to the OLR from a column model of longwave radiative transfer that uses MISR cloud-top heights and reanalysis data as input parameters. After accounting for uncertainties in modeled OLR of 4Wm 2 due to uncertainties in the input properties, especially known high cloud, surface temperature and specific humidity, the average difference of 17 W m 2 cannot be directly explained, and is compensated with the addition of thin cirrus of coverage 60%. MISR misses thin cirrus of optical depth < 0.3 with its operational cloud-height product whereas oblique analysis improves detection to an optical depth of 0.1. When applied to all tropical scenes, the coverage of thin cirrus found by oblique-stereo analysis with optical depths in the range 0.1-0.3 is 10%. Based on the overall difference between model and the measurement, the remaining coverage of subvisual cirrus with optical depths < 0.1 is 67%. The analysis of tropical MISR orbits with standard processing shows a decline in cloud-top height since 2000. After validation of oblique-detected thin cirrus with coincident MODIS data, and the cloud radar and lidar data from the TropicalWestern Pacific (TWP) Atmospheric Radiation Measurement (ARM) sites, the additional cirrus detected over the tropics also shows a decline in cirrus-top height. Furthermore, comparisons with CALIOP and MISR show similar cloud climatology and trends. These trends would be significantly important to cloud feedback if they continue over the next decade.