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Steven C. Sherwood, Sandrine Bony, Olivier Boucher, Chris Bretherton, Piers M. Forster, Jonathan M. Gregory, and Bjorn Stevens

More intensive analyses of climate simulations reveal a need to revise definitions of forcing and feedback and to recognize the new concept of rapid adjustments. The traditional and now ubiquitous framework for understanding global climate change involves an external forcing, a response whereby the climate system opposes the forcing in order to regain equilibrium, and feedbacks that amplify or damp the response ( National Research Council 2005 ). The concept is most often applied to the global

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Fiammetta Straneo, Patrick Heimbach, Olga Sergienko, Gordon Hamilton, Ginny Catania, Stephen Griffies, Robert Hallberg, Adrian Jenkins, Ian Joughin, Roman Motyka, W. Tad Pfeffer, Stephen F. Price, Eric Rignot, Ted Scambos, Martin Truffer, and Andreas Vieli

An interdisciplinary and multifaceted approach is needed to understand the forcings and mechanisms behind the recent retreat and acceleration of Greenland's glaciers and its implications for future sea level rise Mass loss from the Greenland and Antarctic ice sheets tripled over the last two decades, from 100 ± 92 Gt yr −1 (0.28 ± 0.26 mm yr −1 sea level equivalent) during 1992–2000 to 298 ± 58 Gt yr −1 (0.83 ± 0.16 mm yr −1 ) during 2000–11 [see Shepherd et al. (2012) and references

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Anders Persson

The Coriolis force, named after French mathematician Gaspard Gustave de Coriolis (1792–1843), has traditionally been derived as a matter of coordinate transformation by an essentially kinematic technique. This has had the consequence that its physical significance for processes in the atmosphere, as well for simple mechanical systems, has not been fully comprehended. A study of Coriolis's own scientific career and achievements shows how the discovery of the Coriolis force was linked, not to any earth sciences, but to early nineteenth century mechanics and industrial developments. His own approach, which followed from a general discussion of the energetics of a rotating mechanical system, provides an alternative and more physical way to look at and understand, for example, its property as a complementary centrifugal force. It also helps to clarify the relation between angular momentum and rotational kinetic energy and how an inertial force can have a significant affect on the movement of a body and still without doing any work. Applying Coriolis's principles elucidates cause and effect aspects of the dynamics and energetics of the atmosphere, the geostrophic adjustment process, the circulation around jet streams, the meridional extent of the Hadley cell, the strength and location of the subtropical jet stream, and the phenomenon of “downstream development” in the zonal westerlies.

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J. E. Penner, R. J. Charlson, J. M. Hales, N. S. Laulainen, R. Leifer, T. Novakov, J. Ogren, L. F. Radke, S. E. Schwartz, and L. Travis

Anthropogenic aerosols are composed of a variety of aerosol types and components including water-soluble inorganic species (e.g., sulfate, nitrate, ammonium), condensed organic species, elemental or black carbon, and mineral dust. Previous estimates of the clear sky forcing by anthropogenic sulfate aerosols and by organic biomass-burning aerosols indicate that this forcing is of sufficient magnitude to mask the effects of anthropogenic greenhouse gases over large regions. Here, the uncertainty in the forcing by these aerosol types is estimated. The clear sky forcing by other anthropogenic aerosol components cannot be estimated with confidence, although the forcing by these aerosol types appears to be smaller than that by sulfate and biomass-burning aerosols.

The cloudy sky forcing by anthropogenic aerosols, wherein aerosol cloud condensation nuclei concentrations are increased, thereby increasing cloud droplet concentrations and cloud albedo and possibly influencing cloud persistence, may also be significant. In contrast to the situation with the clear sky forcing, estimates of the cloudy sky forcing by anthropogenic aerosols are little more than guesses, and it is not possible to quantify the uncertainty of the estimates.

In view of present concerns over greenhouse gas-induced climate change, this situation dictates the need to quantify the forcing by anthropogenic aerosols and to define and minimize uncertainties in the calculated forcings. In this article, a research strategy for improving the estimates of the clear sky forcing is defined. The strategy encompasses five major, and necessarily coordinated, activities: surface-based observations of aerosol chemical and physical properties and their influence on the radiation field; aircraft-based observations of the same properties; process studies to refine model treatments; satellite observations of aerosol abundance and size distribution; and modeling studies to demonstrate consistency between the observations, to provide guidance for determination of the most important parameters, and to allow extension of the limited set of observations to the global scale. Such a strategy, if aggressively implemented, should allow these effects to be incorporated into climate models in the next several years. A similar strategy for defining the magnitude of the cloudy sky forcing should also be possible, but the less firm understanding of this forcing suggests that research of a more exploratory nature be carried out before undertaking a research strategy of the magnitude recommended for the clear sky forcing.

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Dale R. Durran

It is demonstrated that the inertial oscillation is not produced exclusively by “inertial forces,” and that the inertial oscillation appears as oscillatory motion even when viewed from a nonrotating frame of reference. The component of true gravity parallel to the geopotential surfaces plays a central role in forcing the inertial oscillation, and in particular it is the only force driving the oscillation in the nonrotating reference frame.

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Byung-Ju Sohn and Franklin R. Robertson

Despite the general agreement that clouds cool the earth–atmosphere, there are substantial differences in estimated magnitudes of the annual global mean of cloud radiative forcing. Recent estimates of globally averaged net cloud radiative forcing range from −2 to −27 W m−2. The reasons for these differences have not been clarified in spite of the important role of clouds in maintaining global heat balance. Here, three estimation methods [Earth Radiation Budget Experiment (ERBE), Regression I, and Regression II] are compared using the same data source and analysis period.

Intercomparison has been done for the time period of February and March 1985 over which major satellite radiation budget and cloudiness datasets (ERBE radiation budget, Nimbus-7, and ISCCP cloudiness) are contemporaneous. The global averages of five sets of net cloud radiative forcing by three independent methods agree to within 3.5 W m−2; four of five cases agree to within 1 W m−2. This suggests that differences in published global mean values of net cloud radiative forcing are mainly due to different data sources and analysis periods and a best estimated annual mean among all previous estimates appears to be the ERBE measurement, that is, −17.3 W m−2. In contrast to the close agreement in the net cloud radiative forcing estimates, both longwave and shortwave cloud radiative forcing show more dependence on the chosen method and dataset. The bias of regression-retrieved values between Nimbus-7 and ISCCP cloud climatology is largely attributed to the difference in total cloudiness between two climatologies whereas the discrepancies between the ERBE and regression method appear to be, in part, due to the conceptually different definition of clear-sky flux.

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J. Neumann

The deflecting effect of the earth's rotation on winds has been known since the writings of Hadley in 1735, and observations of the clockwise diurnal directional turning of the sea and land breezes in the Northern Hemisphere have been published in quantity since 1801. But, the first reference associating the turning with the earth rotation through the latter's deflecting effect appears to have been made by the Austrian meteorologist and climatologist von Hann in 1901.

Jeffreys' (1922) conclusion that the earth's rotation is not relevant to the dynamics of the sea and land breezes, was based on an overestimate of the speed and an excessive underestimate of the landward penetration of the sea breezes. In 1934 Brunt suggested that the Earth's rotation may be important. Neither Jeffreys nor Brunt seem to have been aware of von Hann's statement and examples.

Haurwitz (1947) was the first to show dynamically that the Coriolis force can explain the observed diurnal directional turning of the sea and land breezes. Neumann (1977) emphasizes that the observed rate of rotation of direction is not constant over the diurnal cycle, as it should be if only the Earth's rotation were operative; the interaction of the pressure-gradient and frictional forces with the flow has a modulating effect on the rate due solely to the Coriolis force. Kusuda and Alpert (1983) study the dynamics of cases where the directional turning is counterclockwise in the Northern Hemisphere.

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P. J. Sellers, B. W. Meeson, J. Closs, J. Collatz, F. Corprew, D. Dazlich, F. G. Hall, Y. Kerr, R. Koster, S. Los, K. Mitchell, J. McManus, D. Myers, K.-J. Sun, and P. Try

A comprehensive series of global datasets for land-atmosphere models has been collected, formatted to a common grid, and released on a set of CD-ROMs. This paper describes the motivation for and the contents of the dataset.

In June of 1992, an interdisciplinary earth science workshop was convened in Columbia, Maryland, to assess progress in land-atmosphere research, specifically in the areas of models, satellite data algorithms, and field experiments. At the workshop, representatives of the land-atmosphere modeling community defined a need for global datasets to prescribe boundary conditions, initialize state variables, and provide near-surface meteorological and radiative forcings for their models. The International Satellite Land Surface Climatology Project (ISLSCP), a part of the Global Energy and Water Cycle Experiment, worked with the Distributed Active Archive Center of the National Aeronautics and Space Administration Goddard Space Flight Center to bring the required datasets together in a usable format. The data have since been released on a collection of CD-ROMs.

The datasets on the CD-ROMs are grouped under the following headings: vegetation; hydrology and soils; snow, ice, and oceans; radiation and clouds; and near-surface meteorology. All datasets cover the period 1987–88, and all but a few are spatially continuous over the earth's land surface. All have been mapped to a common 1° × 1° equal-angle grid. The temporal frequency for most of the datasets is monthly. A few of the near-surface meteorological parameters are available both as six-hourly values and as monthly means.

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Theodore L. Anderson, Robert J. Charlson, Nicolas Bellouin, Olivier Boucher, Mian Chin, Sundar A. Christopher, Jim Haywood, Yoram J. Kaufman, Stefan Kinne, John A. Ogren, Lorraine A. Remer, Toshihiko Takemura, Didier Tanré, Omar Torres, Charles R. Trepte, Bruce A. Wielicki, David M. Winker, and Hongbin Yu

This document outlines a practical strategy for achieving an observationally based quantification of direct climate forcing by anthropogenic aerosols. The strategy involves a four-step program for shifting the current assumption-laden estimates to an increasingly empirical basis using satellite observations coordinated with suborbital remote and in situ measurements and with chemical transport models. Conceptually, the problem is framed as a need for complete global mapping of four parameters: clear-sky aerosol optical depth f f, radiative efficiency per unit optical depth δ, fine-mode fraction of optical depth f f, and the anthropogenic fraction of the fine mode f af . The first three parameters can be retrieved from satellites, but correlative, suborbital measurements are required for quantifying the aerosol properties that control E, for validating the retrieval of f f, and for partitioning fine-mode δ between natural and anthropogenic components. The satellite focus is on the “A-Train,” a constellation of six spacecraft that will fly in formation from about 2005 to 2008. Key satellite instruments for this report are the Moderate Resolution Imaging Spectroradiometer (MODIS) and Clouds and the Earth's Radiant Energy System (CERES) radiometers on Aqua, the Ozone Monitoring Instrument (OMI) radiometer on Aura, the Polarization and Directionality of Earth's Reflectances (POLDER) polarimeter on the Polarization and Anistropy of Reflectances for Atmospheric Sciences Coupled with Observations from a Lidar (PARASOL), and the Cloud and Aerosol Lider with Orthogonal Polarization (CALIOP) lidar on the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO). This strategy is offered as an initial framework—subject to improvement over time—for scientists around the world to participate in the A-Train opportunity. It is a specific implementation of the Progressive Aerosol Retrieval and Assimilation Global Observing Network (PARAGON) program, presented earlier in this journal, which identified the integration of diverse data as the central challenge to progress in quantifying global-scale aerosol effects. By designing a strategy around this need for integration, we develop recommendations for both satellite data interpretation and correlative suborbital activities that represent, in many respects, departures from current practice.

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T. J. Immel and R. W. Eastes

In late 2019, two ground-breaking NASA missions to explore the conditions in near-Earth space environment are expected to both be on orbit and operational. These missions are designed to observe changes in the state of the neutral upper atmosphere and ionosphere (at 100–500-km altitude) in response to forcing from the sun and solar wind, as well as the terrestrial atmosphere below. Together these observatories will provide a comprehensive view of the near-Earth space environment, the conditions

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