Cloud-top height fluctuations observed by MISR

The MISR instrument on the Terra satellite now has over 10 years of consistent measurements of cloud-top height at high spatial resolution. These measurements are obtained by applying stereo methods to reflected solar radiance patterns to yield geometric heights, the changes in which are insensitive to assumptions about thermal structure or to radiometric calibration. Overall, there has been a global decrease in cloud-top height over the decade, due in large part to changes in the tropical Pacific. Some of these changes correlate well with El Nino indices. They also correlate well with long-term fluctuations in surface pressure and appear related to changes in the strength of the Hadley Circulation. In addition to describing the observational changes, we apply these measurements as perturbations to a two-dimensional radiative-convective equilibrium model (with detailed cloud distributions and radiative transfer, but parameterized horizontal transport) to illustrate the significance of these fluctuations to surface temperature. Should the current pattern of fluctuations persist, the reductions in cloud-top height would provide an element of negative feedback to the current global warming.