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Felix A. Theopold and Jens Bösenberg

Abstract

The method of measuring atmospheric temperature profiles with differential absorption lidar (DIAL), based on the temperature dependence of oxygen absorption lines in the near infrared, is investigated in some detail. Particularly the influence of Doppler broadening on the Rayleigh-backscattered signal is evaluated, and a correction method for this effect is presented. This correction, however, requires an accurate estimate of the molecular- and particle backscatter contributions, which is hardly achievable by usual lidar inversion techniques. Under realistic conditions, resulting errors may be as high as 10 K. First range-resolved measurements using this technique are presented, using a slightly modified DIAL system originally constructed for water vapor measurements. Temperature profiles in the planetary boundary layer are obtained with a resolution of 82 m vertical and 30 min in time, showing an absolute accuracy of 4 K and an error in the temperature gradient of 0.5 K (100 m)−1. While much better resolution can certainly be achieved by technical improvements, the errors introduced by the uncertainty of the backscatter contributions will remain and determine the accuracy that can be obtained with this method.

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Christoph Senff, Jens Bösenberg, and Gerhard Peters

Abstract

A remote-sensing method to retrieve vertical profiles of water vapor flux in the convective boundary layer by using a differential absorption lidar and a radar-radio acoustic sounding system is described. The system's height range presently extends from 400 to 700 m above the surface, and flux data can be sampled with a height resolution of 75 m and a time resolution of 60 s. The results of a first measurement in July 1991 under predominantly convective conditions are presented. The resolution of the remote-sensing system apparently is sufficient to resolve the major contributions to the flux in the convective mixed layer. In addition, the advantages and limitations of this method are discussed.

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Ralph A. Kahn, John A. Ogren, Thomas P. Ackerman, Jens Bösenberg, Robert J. Charlson, David J. Diner, Brent N. Holben, Robert T. Menzies, Mark A. Miller, and John H. Seinfeld

We briefly but systematically review major sources of aerosol data, emphasizing suites of measurements that seem most likely to contribute to assessments of global aerosol climate forcing. The strengths and limitations of existing satellite, surface, and aircraft remote sensing systems are described, along with those of direct sampling networks and ship-based stations. It is evident that an enormous number of aerosol-related observations have been made, on a wide range of spatial and temporal sampling scales, and that many of the key gaps in this collection of data could be filled by technologies that either exist or are expected to be available in the near future. Emphasis must be given to combining remote sensing and in situ active and passive observations and integrating them with aerosol chemical transport models, in order to create a more complete environmental picture, having sufficient detail to address current climate forcing questions. The Progressive Aerosol Retrieval and Assimilation Global Observing Network (PARAGON) initiative would provide an organizational framework to meet this goal.

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Albert Ansmann, Jens Bösenberg, Gérard Brogniez, Salem Elouragini, Pierre H. Flamant, Karlheinz Klapheck, Holger Linn, Louis Menenger, Walfried Michaelis, Maren Riebesell, Christoph Senff, Pierre-Yves Thro, Ulla Wandinger, and Claus Weitkamp

Abstract

Four lidars located roughly 75 km from each other in the inner German Bight of the North Sea, were used to measure geometrical and optical properties of cirrus clouds during the International Cirrus Experiment 1989 (ICE '89). A complete cirrus life cycle was observed simultaneously with three lidan during a case study on 18 October 1989. Time series of particle backscatter, depolarization-ratio height profiles, cloud depth, optical thickness, and of the cirrus extinction-to-backscatter, or lidar, ratio describe the evolution of the cloud system. A two-wavelength lidar measurement was performed and indicates wavelength independence of ice-crystal scattering. The optical and geometrical depths of the cirrus were well correlated and varied between 0.01 and 0.5 and 100 m and 4.5 km, respectively. Although the evolution of the cloud deck was similar over the different observation sites, cirrus geometrical, scattering, and microphysical properties were found to vary considerably within the lidar network. A statistical analysis of ice-cloud properties is performed based on 38 different cirrus cases sampled during ICE '89. Cirrus formation was found to start at the tropopause in most cases. Ice clouds, measured at high midlatitudes (around 54°N), were thin with mean optical and geometrical depths mainly below 0.4 and 2 km, respectively. A good correlation between mean cloud optical and geometrical thickness, and a weak decrease of the mean optical depths with temperature was observed.

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David J. Diner, Robert T. Menzies, Ralph A. Kahn, Theodore L. Anderson, Jens Bösenberg, Robert J. Charlson, Brent N. Holben, Chris A. Hostetler, Mark A. Miller, John A. Ogren, Graeme L. Stephens, Omar Torres, Bruce A. Wielicki, Philip J. Rasch, Larry D. Travis, and William D. Collins

A comprehensive and cohesive aerosol measurement record with consistent, well-understood uncertainties is a prerequisite to understanding aerosol impacts on long-term climate and environmental variability. Objectives to attaining such an understanding include improving upon the current state-of-the-art sensor calibration and developing systematic validation methods for remotely sensed microphysical properties. While advances in active and passive remote sensors will lead to needed improvements in retrieval accuracies and capabilities, ongoing validation is essential so that the changing sensor characteristics do not mask atmospheric trends. Surface-based radiometer, chemical, and lidar networks have critical roles within an integrated observing system, yet they currently undersample key geographic regions, have limitations in certain measurement capabilities, and lack stable funding. In situ aircraft observations of size-resolved aerosol chemical composition are necessary to provide important linkages between active and passive remote sensing. A planned, systematic approach toward a global aerosol observing network, involving multiple sponsoring agencies and surface-based, suborbital, and spaceborne sensors, is required to prioritize trade-offs regarding capabilities and costs. This strategy is a key ingredient of the Progressive Aerosol Retrieval and Assimilation Global Observing Network (PARAGON) framework. A set of recommendations is presented.

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David J. Diner, Thomas P. Ackerman, Theodore L. Anderson, Jens Bösenberg, Amy J. Braverman, Robert J. Charlson, William D. Collins, Roger Davies, Brent N. Holben, Chris A . Hostetler, Ralph A. Kahn, John V. Martonchik, Robert T. Menzies, Mark A. Miller, John A. Ogren, Joyce E. Penner, Philip J. Rasch, Stephen E. Schwartz, John H. Seinfeld, Graeme L. Stephens, Omar Torres, Larry D. Travis, Bruce A . Wielicki, and Bin Yu

Aerosols exert myriad influences on the earth's environment and climate, and on human health. The complexity of aerosol-related processes requires that information gathered to improve our understanding of climate change must originate from multiple sources, and that effective strategies for data integration need to be established. While a vast array of observed and modeled data are becoming available, the aerosol research community currently lacks the necessary tools and infrastructure to reap maximum scientific benefit from these data. Spatial and temporal sampling differences among a diverse set of sensors, nonuniform data qualities, aerosol mesoscale variabilities, and difficulties in separating cloud effects are some of the challenges that need to be addressed. Maximizing the longterm benefit from these data also requires maintaining consistently well-understood accuracies as measurement approaches evolve and improve. Achieving a comprehensive understanding of how aerosol physical, chemical, and radiative processes impact the earth system can be achieved only through a multidisciplinary, interagency, and international initiative capable of dealing with these issues. A systematic approach, capitalizing on modern measurement and modeling techniques, geospatial statistics methodologies, and high-performance information technologies, can provide the necessary machinery to support this objective. We outline a framework for integrating and interpreting observations and models, and establishing an accurate, consistent, and cohesive long-term record, following a strategy whereby information and tools of progressively greater sophistication are incorporated as problems of increasing complexity are tackled. This concept is named the Progressive Aerosol Retrieval and Assimilation Global Observing Network (PARAGON). To encompass the breadth of the effort required, we present a set of recommendations dealing with data interoperability; measurement and model integration; multisensor synergy; data summarization and mining; model evaluation; calibration and validation; augmentation of surface and in situ measurements; advances in passive and active remote sensing; and design of satellite missions. Without an initiative of this nature, the scientific and policy communities will continue to struggle with understanding the quantitative impact of complex aerosol processes on regional and global climate change and air quality.

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