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V. Ramaswamy, W. Collins, J. Haywood, J. Lean, N. Mahowald, G. Myhre, V. Naik, K. P. Shine, B. Soden, G. Stenchikov, and T. Storelvmo

describing the agents driving Earth’s climate change since preindustrial times (1750) and the formulation of the “radiative forcing” (RF) (see section 2 ) of climate change. The central purpose of this paper is to trace the progression in the RF concept leading to our current knowledge and estimates of the major agents known to perturb climate. Below, we give a perspective into the key milestones marking advances in the knowledge of RF. Subsequent sections of the paper focus on the evolution of the

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Sonia M. Kreidenweis, Markus Petters, and Ulrike Lohmann

et al. 2012 ). The interactions of aerosols in the troposphere and stratosphere with incoming solar radiation had been pointed out early in the twentieth century. Charlson et al. (1992) , building on the estimates of Charlson et al. (1990) , argued that the addition of anthropogenic aerosols to the climate system constituted a significant perturbation—a climate forcing—to the natural system. Their analysis focused on sulfate aerosol, due to anthropogenic emissions that could be shown to

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David P. Jorgensen and Tammy M. Weckwerth


From its initial deployment as a research tool following the second World War, radar has played a fundamental role in revealing the forces that initiate and organize severe storms and larger mesoscale convective systems composed of a conglomeration of convective storm cells. Early radar observations were primarily descriptive and showed the tremendous variety of precipitating moist convection types and sizes. Examples include single convective storms, longer-lived multicellular storms, fast-moving squall lines, slower-moving linear and nonlinear convective systems, and long-lived supercell storms. Certain modes or types of convective systems were shown to possess a variety of hazardous weather that includes very heavy rain, large hail, straight-line damaging winds, tornadoes, and lightning. It was soon recognized that the type of convective system was strongly dependent on the environment in which it was embedded. Researchers determined that two variables were particularly important in describing convective behavior: the vertical profile of the horizontal wind and potential instability of the air feeding the system [convective available potential energy (CAPE)]. The types of convective systems are discussed here according to their typical shear and CAPE values. In addition to the knowledge gained from observational radar studies, considerable advancement in understanding of convective system dynamics has resulted from high-resolution numerical simulations.

In addition to being a critical factor in determining the particular structure and organization that convective systems assume once convection is initiated, radar (particularly in clear air mode) has been a leading tool in identifying forcing mechanisms for convective initiation. In particular, the role of “boundary layer forcing” in initiating convection has received much attention in recent years. Boundary layer circulations, which are sometimes precursors to deep convective development, are clearly observed by radar as reflectivity fine lines and/or discontinuities in Doppler velocity. Some of these mesoscale boundary layer mechanisms for producing upward motion include horizontal convective roles, sea-breeze circulations, drylines, gust fronts, orographic circulations (e.g., mountain–valley), and circulations resulting from horizontal inhomogeneities in surface character. Convection initiation sometimes does not occur continuously along boundaries but only at preferred along-boundary locations. Location preferences can sometimes be identified with boundary intersections, such as colliding gust fronts, sea-breeze fronts and rolls, and drylines and rolls. It is not always clear, however, why convection forms at certain locations along boundaries and not others. It is possible that low-level waves, bores, and other features, which may not always be apparent in radar data, may also play an important role in convection initiation processes.

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Pavlos Kollias, Eugene E. Clothiaux, Thomas P. Ackerman, Bruce A. Albrecht, Kevin B. Widener, Ken P. Moran, Edward P. Luke, Karen L. Johnson, Nitin Bharadwaj, James B. Mead, Mark A. Miller, Johannes Verlinde, Roger T. Marchand, and Gerald G. Mace

. Ahmad , and D. Hartmann , 1989 : Cloud-radiative forcing and climate: Results from the Earth Radiation Budget Experiment . Science , 243 , 57 – 63 , doi: 10.1126/science.243.4887.57 . Sassen , K. , C. J. Grund , J. D. Spinhirne , M. H. Hardesty , and J. M. Alvarez , 1990 : The 27–28 October FIRE IFO cirrus case study: A five lidar overview of cloud structure and evolution . Mon. Wea. Rev. , 118 , 2288 – 2311 , doi: 10.1175/1520-0493(1990)118<2288:TOFICC>2.0.CO;2 . Sassen

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Minghua Zhang, Richard C. J. Somerville, and Shaocheng Xie

and 30%–40% difference in the radiative forcing of greenhouse gases in the radiation codes of climate models. At that time, the U.S. Department of Energy (DOE) had a program to study the climate impact of the increasing amount of carbon dioxide in the atmosphere. Results from these two papers pointed to the major uncertainties in climate forcing and feedbacks of climate models. Radiation and clouds, therefore, emerged as a focus in the DOE ARM Program to improve models. To simulate clouds

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Ulrich Schumann and Andrew J. Heymsfield

detection aspects of contrails ( aufm Kampe 1943 ; Brewer 1946 ; Appleman 1953 ; Ryan et al. 2011 ), and these studies contributed to the detection of ice supersaturation, contrail persistence, the dryness of the stratosphere, the Brewer–Dobson circulation ( Brewer 2000 ), and hollow ice particles ( Weickmann 1945 ). The climate impact got more attention later ( Penner et al. 1999 ). The mean radiative forcing (RF) from contrails is likely positive, possibly contributing to global warming ( Boucher

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David S. Battisti, Daniel J. Vimont, and Benjamin P. Kirtman

to wind stress forcing, and to the response of the atmosphere to changes in SST (see section 4b ). The 1982/83 El Niño event (the warm phase of ENSO) was remarkable for its amplitude and duration. It inspired meteorologists and oceanographers to come together and plan the 10-yr program Tropical Oceans on the Global Atmosphere (TOGA) to study the impact of the oceans on the atmosphere over 1985–94. TOGA significantly enhanced the observing system in the tropical Pacific [for an overview, see

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Guoxiong Wu and Yimin Liu

spatial and temporal distributions of 〈 Q 1 〉 and 〈 Q 2 〉 in the notation used by Yanai et al. (1973) and based on observations from the First GARP Global Experiment (FGGE) (December 1978–November 1979) and the Qinghai–Xizang Plateau Meteorology Experiment (QXPMEX), conducted from May to August 1979 by Chinese meteorologists ( Zhang et al. 1988 ). Since then, great efforts have been made to understand the mechanism concerning how the TP forcing, either mechanical or thermodynamical, can affect the

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Robert G. Fovell, Yizhe Peggy Bu, Kristen L. Corbosiero, Wen-wen Tung, Yang Cao, Hung-Chi Kuo, Li-huan Hsu, and Hui Su

shortwave (SW) radiation, effectively rendering clouds transparent. Track variation with respect to MP virtually disappeared, which demonstrated that the interplay of hydrometeors with radiation—which we term cloud-radiative forcing (CRF)—was a distinguishing factor among microphysics schemes. The interaction between condensed water and radiation is species dependent, and the MPs that generate more radiatively active particles also developed more radially extensive convective activity, different

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W.-K. Tao, Y. N. Takayabu, S. Lang, S. Shige, W. Olson, A. Hou, G. Skofronick-Jackson, X. Jiang, C. Zhang, W. Lau, T. Krishnamurti, D. Waliser, M. Grecu, P. E. Ciesielski, R. H. Johnson, R. Houze, R. Kakar, K. Nakamura, S. Braun, S. Hagos, R. Oki, and A. Bhardwaj

advection terms on the RHS of Eq. (2-1) have been used to force CRMs (or cumulus ensemble models) to study the response of convective systems to large and mesoscale processes ( Soong and Tao 1980 ). This CRM approach to studying cloud and precipitation processes is called cloud ensemble modeling [ Soong and Tao 1980 ; Tao and Soong 1986 ; Tao et al. 1987 ; Krueger 1988 ; Moncrieff et al. 1997 ; also see review papers by Tao (2003 , 2007 ) and Tao and Moncrieff (2009) ]. It allows many clouds

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