Trends in Equatorial Cloud-top Heights from MISR Since 2000

The MISR instrument on the Terra satellite is a Multiangle Imaging SpectroRadiometer measuring solar radiances in four spectral bands at nine discrete viewing angles. MISR has been making continuous measurements since early 2000, in a constant sun-synchronous orbit corresponding to 10:30 am local time. It uses stereo techniques to retrieve cloud-top height and height-resolved cloud motion vectors. These stereo techniques are insensitive to radiometric calibration and provide extremely stable time series that can also be analyzed to study interannual variations in cloud properties over most of the past decade. While the measurements are global, this paper focuses on changes observed in equatorial cloud-top heights that appear related to circulation changes. The most obvious interannual variation noted so far is a decrease in cloud height that correlates well with the Southern Oscillation Index yet has an apparently sustained secular trend with a peak zonal average of 40 m/yr. The changes do not appear to indicate changes in the Hadley circulation. The region of maximum correlation is centered to the north of Australia, with a secondary maximum (of opposite sign) over the Central Pacific. At the same time, there is no apparent trend in either total cloud fraction or top-of- atmosphere albedo. The decrease in cloud height thus provides a net cooling mechanism. On a global average basis this turns out to be roughly equivalent to the radiative forcing due to increased concentrations of carbon dioxide over the last decade.