The NRC Decadal Survey recommended the Aerosol-Cloud-Ecosystem (ACE) mission. NASA's ACE Science Working Group has further developed the mission concept, scientific rationale, and a traceable implementation strategy for this Tier-2 mission that directly addresses key climate change issues. The results are summarized here. ACE is designed to substantially advance the state of aerosol, cloud and ocean ecology science by providing global data sets of unmatched accuracy and substantially improved information content to enable a major step forward compared to what is presently known, or possible with present capabilities. ACE will be the successor to the A-Train and EarthCare (and JPSS/VIIRS) as regards global aerosol and cloud data products. Driving the approach for ACE is the goal to substantially reduce the uncertainty in climate forcing associated with aerosol-cloud interactions, including precipitation, and to markedly advance knowledge of ocean ecosystem, and its CO2 uptake, though synergistic use of advanced aerosol characterization in the retrieval of ocean ecosystem parameters. ACE is the only Decadal Survey Mission in Tier 1 or Tier 2 that is focused on Aerosols and Clouds, and their interactions, including effects on precipitation. To do these things, ACE is designed to take advantage of new and emerging capabilities to dramatically enhance knowledge of the contents of these components, specifically aerosol properties, cloud microphysics, and marine biosphere properties that are not presently available, on a global basis. The present ACE mission implementation concept is envisioned to include a multi-angle spectral polarimeter, a multi-wavelength High Spectral Resolution Lidar, dual-frequency Doppler cloud radar, a multi-band spectral ocean ecology radiometer, and other passive radiometers.

The polarimeter is primarily focused on aerosol and cloud requirements while the spectral radiometer is mostly focused on ocean ecology measurements. The polarimeter will provide wide-swath imager coverage of aerosols and clouds. Polarization will be measured only in select channels, and possibly only over a more limited spatial domain. The driving science requirement is to provide greatly improved measurements of aerosol and cloud physical and radiative properties, including more definitive measures of particle size and type. The ocean color radiometer will provide forward- and aft-viewing capability so that sun-glint can be minimized by switching the modes during an orbit. It will also include UV channels, as well as channels in the visible part of the spectrum. The spectral sampling markedly exceeds present capabilities. The science goal is to quantify the amount of dissolved organic matter, carbon, and other biogeochemical species to define the role of the oceans in the carbon cycle (e.g., uptake and storage), and specifically to enable quantitative characterization of ecosystem speciation.

The lidar provides a major step forward for aerosol science and also offers new capabilities for ocean ecology. The driving science requirement is to provide accurate measurement of aerosol physical and radiative properties. This will impact the accuracy of the ocean ecosystem measurements by enabling more physically retrieval techniques, i.e., integrated aerosol and ocean color retrieval. Capabilities to directly provide subsurface measurements of ecosystem properties are also emerging. The dual-wavelength radar is essential to quantifying microphysical profiles, where the effects of aerosol-cloud interaction are most explicit, and will provide new insights into global cloud processes. The radar will be comprised of 95 and 35 GHz systems that will share some components, e.g., antenna. The 95 GHz system will provide similar capabilities to CloudSat in the A-Train (cloud properties), with limited swath coverage (a few fov's). The 35 GHz system will provide measurements into denser clouds including precipitation. The 35 GHz system is planned to give narrow swath coverage (~30 km or greater), yielding definitive information on cloud system context that is highly challenging for present nadir-pointing systems. Both radars are planned to provide Doppler measurements of in-cloud vertical velocity, with useful accuracy and sensitivity, which is essential for understanding the physics of clouds. The profile information provided by the lidar and radar sensors is essential for unraveling the complex linkage and interaction among aerosol, clouds and precipitation. The ACE instrument suite provides coverage over the full range of atmospheric particles.

The current ACE concept also calls for wide-swath radiometer measurements in the infrared, microwave and submillimeter spectral regions to quantify cloud properties at sufficient accuracy and to provide a relatively complete picture of the local components of the atmospheric hydrologic cycle, necessary to describe cloud processes and their interactions. These measurements are also important for providing column constraints on profile retrievals, such as cloud liquid and ice water content and particle size, in order that the corresponding accuracy becomes more useful for science investigations and models. The present global observing system provides useful measurements, but with significant limitations on accuracy and/or sampling that limit their impact.

The Intergovernmental Panel on Climate Change (IPCC, 2007) identified the largest uncertainty in our understanding of physical climate as that due to aerosols and clouds. ACE is specifically designed to advance our ability to observe and predict changes to the Earth's hydrological cycle and energy balance to climate forcings, especially those changes associated with the effects of aerosol on clouds and precipitation. ACE will acquire global, cloud-resolving, coincident measurements of cloud microphysical profiles in the context of their aerosol environment. To achieve the required accuracy and fidelity, passive and active remote sensing measurements are required at multiple frequencies (and Doppler), responding to different moments of the particle size distribution. Coincident measurements of integrated quantities, such as cloud liquid and ice water paths will greatly enhance the accuracy of cloud profile retrievals by providing integral constraints of sufficient accuracy.

At the same time, ACE will provide a dramatic step forward in terms of accuracy and information content for retrievals of ocean ecosystem properties, distinguishing between the amount of dissolved organic matter and species active in carbon uptake and storage. This will significantly improve the accuracy of estimates of the ocean carbon cycle and its role in global change. The motivation and concepts for ACE are documented in our 2010 report, available at: http://dsm.gsfc.nasa.gov/ace/documents.html >