Aerosol-cloud-precipitation interactions. Part 1. The nature and sources of cloud-active aerosols. Andreae, M. O.; Rosenfeld, D..
Earth Science Reviews:
Atmospheric aerosol particles serve as condensation nuclei for the formation of both, cloud droplets and atmospheric ice particles. As a result, they exert a substantial influence on the microphysical properties of water and ice clouds, which in turn affect the processes that lead to the formation of rain, snow, hail, and other forms of precipitation. In recent years, considerable progress has been made in understanding the chemical composition of aerosols, their microphysical properties, and the factors that enable them to act as cloud condensation nuclei (CCN) and ice nuclei (IN). The first part of this review article will focus on the nature and sources of CCN and IN. We discuss the fundamentals of the cloud droplet and ice nucleation processes, and the role that the chemical composition and particle size play in this process. We show that, in many instances, the influence of chemical composition can be represented by a simple parameterization, which leaves particle size as the main variable controlling CCN efficiency. Aerosol particles are produced either directly by anthropogenic and natural sources (dust, sea salt, soot, biological particles, etc.), or they are formed in the atmosphere by condensation of low-volatility compounds (e.g., sulfuric acid or oxidized organic compounds). We discuss the magnitude of these sources, and the CCN and IN characteristics of the particles they produce. In contrast to previous assessments, which focused on the aerosol mass, we are emphasizing the number of particles being produced, as this is the key variable in cloud microphysics. Large uncertainties still exist for many aerosol sources, e.g., the submicron part of the seaspray aerosol, the particles produced by the biosphere, and the secondary organic aerosol. We conclude with a discussion on what particle concentrations may have been in the pristine atmosphere, before the onset on anthropogenic pollution. Model calculations and observations in remote continental regions consistently suggest that CCN concentrations over the pristine continents were similar to those now prevailing over the remote oceans, suggesting that human activities have modified cloud microphysics more than what is reflected in conventional wisdom. The second part of this review will address the effects of changing CCN and IN abundances on precipitation processes, the water cycle, and climate. (C) 2008 Elsevier B.V. All rights reserved.
Measuring erosion with the micro-erosion meter – contributions to understanding landform evolution. W.J. Stephenson, B.L. Finlayson.
Earth Science Reviews:
We review the contribution made to understanding landform evolution through the use of the micro-erosion meter (MEM) and the variant, the traversing micro-erosion meter (TMEM). The MEM has allowed the direct measurement of bedrock erosion in terrestrial and coastal settings as well as of building stone. As recorded in MEM measurements, world wide the average rate of surface lowering on shore platforms is 1.486 mm a − 1 , and on terrestrial rock surfaces, measured lowering rates are on average 0.903 mm a − 1 . Reported means obscure significant variation in rates from a range of environmental settings and lithologies. The MEM has in a number of examples allowed the identification of the contribution of individual processes in polygenetic settings, particularly in shore platform studies. The dynamic nature of rock weathering has been highlighted, where the TMEM has revealed rock surfaces swelling and contracting at time scales ranging from hours to years. The MEM will continue to provide useful data on erosion
North American drought: Reconstructions, causes, and consequences. Cook, E. R.; Seager, R.; Cane, M. A.; Stahle, D. W..
Earth Science Reviews:
Severe drought is the greatest recurring natural disaster to strike North America. A remarkable network of centuries-long annual tree-ring chronologies has now allowed for the reconstruction of past drought over North America covering the past 1000 or more years in most regions. These reconstructions reveal the occurrence of past "megadroughts" of unprecedented severity and duration, ones that have never been experienced by modem societies in North America. There is strong archaeological evidence for the destabilizing influence of these past droughts on advanced agricultural societies, examples that should resonate today given the increasing vulnerability of modem water-based systems to relatively short-term droughts. Understanding how these megadroughts develop and persist is a timely scientific problem. Very recently, climate models have succeeded in simulating all of the major droughts over North America from the Civil War to the severe 1998-2004 drought in the western U.S. These numerical experiments indicate the dominating importance of tropical Pacific Ocean sea surface temperatures (SSTs) in determining how much precipitation falls over large parts of North America. Of central importance to drought formation is the development of cool "La Nina-like" SSTs in the eastern tropical Pacific region. This development appears to be partially linked to changes in radiative forcing over that region, which affects the Bjerknes feedback mechanism of the ENSO cycle there. Paradoxically, warmer conditions over the tropical Pacific region lead to the development of cool La Nina-like SSTs there, which is drought inducing over North America. Whether or not this process will lead to a greater prevalence of drought in the future as the world warms due to accumulating greenhouse gases is unclear at this time. (c) 2007 Elsevier B.V. All rights reserved.