NATURE GEOSCIENCE | VOL 5 | OCTOBER 2012 | www.nature.com/naturegeoscience 691 E arth's climate is determined by the flows of energy into and out of the planet and to and from Earth's surface. Geographical distributions of these energy flows at the surface are particularly important as they drive ocean circulations, fuel the evaporation of water from Earth's surface and govern the planetary hydrological cycle. Changes to the surface energy balance also ultimately control how this hydrological cycle responds to the small energy imbalances that force climate change 1 .The seminal importance of Earth's energy balance to climate has been understood for more than a century. Although the earliest depictions of the global annual mean energy budget of Earth date to the beginning of the twentieth century 2,3 , the most significant advance to our understanding of this energy balance occurred after the space age in the 1960s. Among the highlights obtained from early satellite views of Earth was the measurement of Earth's albedo (the ratio of outgoing flux of solar energy to incoming flux from the Sun) at approximately 30% (ref. 4), thus settling a long-standing debate on its magnitude -values ranged between 89% and 29% (ref. 5) before these measurements. The sign and magnitude of the net effect of clouds on the top-of-atmosphere (TOA) fluxes 6 was also later established with the space-borne observations of the scanning instrument on the Earth Radiation Budget Experiment (ERBE) 7 , which better delineated between clear and cloudy skies. ERBE, and later the Clouds and the Earth's Radiant Energy System (CERES) 8 and the French Scanner for Radiation Budget 9 , confirmed that the global cloud albedo effect was significantly larger than the greenhouse effect of clouds. Although this was a major advance at the time, determining the influence of clouds on atmospheric and surface fluxes had to wait until the recent satellite measurements of the vertical structure of clouds became available from the A-train 10 .Climate change is governed by changes to the global energy balance. At the top of the atmosphere, this balance is monitored globally by satellite sensors that provide measurements of energy flowing to and from Earth. By contrast, observations at the surface are limited mostly to land areas. As a result, the global balance of energy fluxes within the atmosphere or at Earth's surface cannot be derived directly from measured fluxes, and is therefore uncertain. This lack of precise knowledge of surface energy fluxes profoundly affects our ability to understand how Earth's climate responds to increasing concentrations of greenhouse gases. In light of compilations of up-to-date surface and satellite data, the surface energy balance needs to be revised. Specifically, the longwave radiation received at the surface is estimated to be significantly larger, by between 10 and 17 Wm -2 , than earlier model-based estimates. Moreover, the latest satellite observations of global precipitation indicate that more precipitation is generated than...

[1] This paper reports on the early mission performance of the radar and other major aspects of the CloudSat mission. The Cloudsat cloud profiling radar (CPR) has been operating since 2 June 2006 and has proven to be remarkably stable since turn-on. A number of products have been developed using these space-borne radar data as principal inputs. Combined with other A-Train sensor data, these new observations offer unique, global views of the vertical structure of clouds and precipitation jointly. Approximately 11% of clouds detected over the global oceans produce precipitation that, in all likelihood, reaches the surface. Warm precipitating clouds are both wetter and composed of larger particles than nonprecipitating clouds. The frequency of precipitation increases significantly with increasing cloud depth, and the increased depth and water path of precipitating clouds leads to increased optical depths and substantially more sunlight reflected from precipitating clouds compared to than nonprecipitating warm clouds. The CloudSat observations also provide an authoritative estimate of global ice water paths. The observed ice water paths are larger than those predicted from most climate models. CloudSat observations also indicate that clouds radiatively heat the global mean atmospheric column (relative to clear skies) by about 10 Wm À2 . Although this heating appears to be contributed almost equally by solar and infrared absorption, the latter contribution is shown to vary significantly with latitude being influenced by the predominant cloud structures of the different region in questions. Citation: Stephens, G. L., et al. (2008), CloudSat mission: Performance and early science after the first year of operation,

[1] A global multisensor satellite examination of aerosol indirect effects on warm oceanic clouds is presented. The study centers on the water path response of cloud to aerosol burden. A unique element of the study is a rigorous rain screening methodology that is utilized to separate the responses of nonraining and raining clouds. It is demonstrated that high aerosol environments are associated with reduced liquid water path in nonprecipitating clouds and that the reduction in liquid water path reduces the albedo enhancement expected from decreasing effective radius. Furthermore the reduction in liquid water path is greater in thermodynamically unstable environments than in stable environments suggesting a greater sensitivity of cumulus cloud than stratiform cloud liquid water path to aerosol. In sharp contrast with nonprecipitating clouds, the cloud liquid water path of transitional and precipitating clouds increases dramatically with aerosol, which may be indicative of an inhibited coalescence process. The evidence further indicates that increasing aerosol requires greater amounts of cloud condensate before the onset of precipitation. Additional support for this hypothesis is found in a reduction in the probability of precipitation by as much as 10% depending on the thermodynamical state of the environment. Independent estimates of the broadband cloudy-sky albedo are used to confirm that the cloud albedo responds to the trends that are identified in the liquid water path. In particular, it is found that the water path effect dominates the cloud albedo response for precipitating and transitional clouds. Finally, regional analysis demonstrates that the magnitude of the relationship between aerosol and cloud albedo is greatest in the extratropics and subtropical stratus regions primarily in the winter hemisphere. These relationships are used to estimate the magnitude of the first indirect effect as À0.42 W/m 2 .

More than 10 years of observations jointly collected by CloudSat and CALIPSO satellites have resulted in new ways of looking at aerosol, clouds, and precipitation, and new discoveries about processes that connect them. CLOUDSAT AND CALIPSO WITHIN THE A-TRAIN Ten Years of Actively Observing the Earth SystemGraeme StephenS, daVid Winker, JacqueS pelon, charleS trepte, deborah Vane, cherYl YuhaS, triStan l'ecuYer, and mattheW lebSock T he International Geophysical Year (IGY) in 1957/58 was a watershed moment in Earth sciences. It brought together many disciplines and marked a major change in the study of Earth. What evolved out of the IGY was an appreciation that Earth was a dynamic system exemplified by the revolutionary new, emerging model of Earth plate tectonics. Dramatic advances in oceanic and atmospheric sciences also occurred during this period. We saw a deeper appreciation of the importance of the global interactions between oceans and the atmosphere and, more recently, the appreciation for the importance of interactions and feedbacks between land and atmosphere and the role of biogeochemical cycles. Today we fully understand and embrace the concept of an Earth system that is complex with interactions occurring between its many components.The evolution of Earth system science is reflected in the increasing complexity of climate models and their evolution to Earth system models over time. It also has profound implications when contemplating an approach to observe the evolving Earth system. First, the broad realization has emerged that greater confidence in climate projections requires improved understanding of the processes that govern the feedbacks between Earth subsystems (e.g., Bony et al. 2015). This calls for a focus toward observing processes rather than making unconnected observations of individual variables. Second, the important processes, even very rapid processes like convection 569MARCH 2018 AMERICAN METEOROLOGICAL SOCIETY | in the atmosphere, require monitoring over the long term in order to understand how these processes evolve on a slower trend of environmental change. While there is an unequivocal need for sustained long-term space-based observing systems (National Research Council 2015), such as those provided by operational satellites primarily designed for weather observations (e.g., Simmons et al. 2016), many of the most important Earth system processes involve subsystems not sufficiently observed by operational satellite systems. Thus, the challenge is to establish an affordable and implementable cross-disciplinary strategy that can deliver observations of the most critical Earth system processes and that can sustain these observations over the longer term. Despite the well-understood limitations of the existing operational systems, this strategy ought to draw benefit from, and build on, the core observations produced by operational meteorological satellites.The challenge to observe the interactive Earth system has been recognized for some time. Making joint measurements of multiple pa...

[1] Atmospheric aerosol particles act as cloud condensation nuclei, affording them the ability to influence cloud microphysics, planetary albedo, and precipitation. Models of varying complexity and satellite observations from NASA's A-Train constellation of satellites are used to determine what controls the precipitation susceptibility of warm clouds to aerosol perturbations. Three susceptibility regimes are identified: (i) clouds with low liquid water path (LWP) generate very little rain and are least susceptible to aerosol; (ii) clouds with intermediate LWP where aerosol most effectively suppress precipitation; and (iii) clouds with high LWP, where the susceptibility begins to decrease because the precipitation process is efficient owing to abundant liquid water. Remarkable qualitative agreement between remote sensing observations and model predictions provides the first suggestions that certain regions of the Earth might be more vulnerable to pollution aerosol. Targeted pollution control strategies in such regions would most benefit water availability via precipitation.