Lecturer: Steve Cooper
Date: 2013-08-29 10:30
Place: Aniara

On the limitations of satellite passive remote sensing for cloud and precipitation climate process studies

Meaningful advancement in understanding climate response to anthropogenic forcing requires a cross-disciplinary strategy that encompasses the modeling, remote sensing, and in-situ field measurement communities. While these communities are contributing to our knowledge of climate processes, it is not entirely clear that progress is being made toward reducing uncertainties in projected climate response due to increasing greenhouse gas concentrations. The First International Panel on Climate Change (IPCC) I report from 1990 predicted that the globally averaged surface temperature response to a doubling in atmospheric CO2 concentrations likely lies somewhere between 1.5◦ C and 4.5◦ C. The Fourth IPCC report from 2007 states a nearly identically broad range for the climate sensitivity. It is well known that the challenge of adequately representing the cycling of water through the Earth’s climate system is a key contributor to this continuing stalemate. Given the highly uncertain role of active sensors such as cloud radar and lidar in next generation NASA satellite missions, we examine the ability of passive measurements (visible, near-infrared, infrared, microwave used alone or in combination) to discriminate critical atmospheric processes necessary for the understanding of climate. For this work, we exploit an understanding of both vertical sensitivities and retrieval uncertainties associated with these passive measurements to determine their ability to uniquely differentiate between key cloud and precipitation property states. We perform such analyses for climatologically important low-latitude shallow marine clouds, ice clouds, and high-latitude mixed-phase clouds scenarios. 1) We first examine our ability to define the vertical profile of the partitioning of rain and cloud water properties in low-latitude shallow marine clouds. 2) We then examine our ability to define the partitioning of cloud ice mass between ‘suspended’ and ‘precipitating’ particle modes. 3) We discuss our ability to determine the vertical profiles of high-latitude mixed phase cloud properties. For these vertically defined cloud scenarios, we find that we cannot confidently discriminate between key cloud and precipitation states (e.g. ‘are the ice particles precipitating?’ or ‘are marine stratocumulus scenes drizzling?) due to inherent non-uniqueness and lack of sensitivity issues inherent to the passive retrieval approaches. For this talk, we contrast these findings with retrieval results from active sensors. Although current active sensors on satellites cannot entirely discriminate the processes described above either, they do provide a first order sensitivity to cloud and precipitation vertical profiles lacking from the passive instruments. Such considerations will be discussed in context of design of the next generation NASA ACE mission expected to launch in the mid 2020s.

Created 2013-06-07 13:40:45 by Uwe Raffalski
Last changed 2013-08-28 15:08:14 by Uwe Raffalski