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T. J. Swissler
,
P. Hamill
,
M. Osborn
,
P. B. Russell
, and
M. P. McCormick

Abstract

We compare a series of 85 dustsonde measurements and 84 lidar measurements made in midlatitude North America during 1974–80. This period includes two major volcanic increases (Fuego in 1974 and St. Helens in 1980), as well as an unusually clean, or background, period in 1978–79. An optical modeling technique is used to relate the dustsonde-number data to the lidar-backscatter data. The model includes a range of refractive indices and of size distribution functional forms, to show its sensitivity to these factors. Moreover, two parameters of each size distribution function are adjustable, so that each distribution can be matched to any two-channel dustsonde measurement.

We show how the mean particle radius for backscatter, rB , changes in response to size distribution changes revealed by the dustsonde channel ratio, N r>0.15/N r>0.25. (N r>x is the number of particles with radius larger than x microns.) In early 1975, just after the Fuego injection, N r>0.15/N r>0.25 was ∼3, and the corresponding rB , was ∼0.5 μm; by early 1980, when N r>0.15/N r>0.25 had increased to eight or larger, rB had correspondingly decreased to ∼0.25 μm. Throughout the 1975–76 Fuego decay, rB always exceeded 0.3 μm; thus, lidar backscatter was influenced primarily by particles larger than those that contribute most to N r>0.15 and N r>0.25. This is in accord with the shorter lidar background-corrected, 1/e decay time: 7.4 months, versus 10.4 and 7.9 months for N r>0.15 and N r>0.25.

The modeling technique is used to derive a time series of dustsonde-inferred peak backscatter mixing ratio, which agrees very well with the lidar-measured series. The best overall agreement for 1974–80 is achieved with a mixture of refractive indices corresponding to aqueous sulfuric acid at about 210 K with an acid-weight fraction between 0.6 and 0.85.

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P. B. Russell
,
J. M. Livingston
, and
E. E. Uthe

Abstract

Many theoretical studies have shown that aerosol-induced changes in the earth-atmosphere albedo might be an important climate change mechanism. However, there has been a lack of experimental documentation of albedo changes caused by actual aerosol layers with measured properties. Here we report an incident in which the measured surface-plus-atmosphere albedo was increased by about 0.01 (from 0.11 to 0.12) by a transient aerosol layer. We also report simultaneous measurements of the aerosol by a multi-wavelength sunphotometer, a lidar, a nephelometer and other radiometers, and we use these aerosol measurements to deduce an expected albedo change for comparison to the measurements.

Specifically, we combine the aerosol measurements with several assumed refractive indices to derive a time-dependent aerosol optical model for the day of the incident. We then use this model in a two-stream radiative calculation to compute the expected time-dependent aerosol-layer albedo. Finally, we compute aerosol-plus-surface albedos by modifying a familiar expression to account for changing solar zenith angle and the diffuseness of surface reflectivity. Use of the aerosol model in this expression yields a calculated time-dependent atmosphere-plus-surface albedo that agrees with the measurements, provided an aerosol refractive index of about 1.50−0.01i is assumed. This refractive index value is in accord with the aerosol backscatter-to-extinction ratios measured simultaneously by the lidar and sunphotometer.

To our knowledge, this incident is the first in which an aerosol-induced albedo change and the responsible aerosol have been simultaneously measured to this degree of detail. Although the incident was too brief to be climatically significant, the analysis is significant because it provides a practical methodology for incorporating measured properties of aerosol layers into efficient albedo-change calculations. This methodology, which uses ground-based measurements to characterize elevated aerosol layers, could be applied to more widespread and persistent (hence, climatically significant) aerosol layers. Moreover, the agreement between measured and calculated albedos in this incident provides an initial validation of the methodology for not uncommon surface and aerosol conditions. More general measurements, including better complex refractive index determinations, are required to further validate and apply the methodology.

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G. W. Grams
,
I. H. Blifford Jr.
,
D. A. Gillette
, and
P. B. Russell

Abstract

The angular variation of the intensity of light scattered from a collimated beam by airborne soil particles and the size distribution of the particles were measured simultaneously 1.5 m above the ground. These measurements gave an estimate of the complex index of refraction m=n REn IM i of airborne soil particles, where n RE is the real part and n IM the imaginary part of the refractive index.

Standard microscopic analysis procedures were employed to determine n RE. Although a wide range of values was observed, the value 1.525 was taken as representative. By applying Mie scattering theory to each of the observed distributions of particle size, the expected angular variation of the intensity of the scattered light was calculated for a fixed value of n RE and a wide range of values of n IM. For each set of simultaneous measurements, the value of n IM was taken to be that value which provided the best fit to the experimental data. The upper limit of the value of n IM for the airborne particles studied in the experiment was determined to be 0.005 with an uncertainty factor of about 2. The estimate of n IM was found to be fairly insensitive to the assumed value of n RE.

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Petr Chýlek
,
G. W. Grams
,
G. A. Smith
, and
P. B. Russell

Abstract

Hemispherical backscattering cross sections σb of spherical particles are calculated using a recently derived analytic expression. Results are compared with σb values obtained by numerical integration. It is found that the analytic formula gives exact values of the hemispherical backscattering cross sections and also saves computer time. The behavior of σb in the limits of very small and very large spheres is discussed. As an aid in utilizing simple models of climate change due to aerosols, the percentage of incident solar radiation scattered into the backward hemisphere is calculated for a range of particle sizes and complex refractive indices. Similar results are also presented for the ratio of absorption to hemispheric backscattering, a critical parameter in many aerosol climate models.

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P. B. Russell
,
J. M. Livingston
,
T. J. Swissler
,
M. P. McCormick
,
W. P. Chu
, and
T. J. Pepin

Abstract

We present a model of stratospheric aerosol optical properties (refractive index and relative size distribution) and their variability. The model's purposes are 1) providing flexible, efficient means for converting between different aerosol macroproperties (e.g., number or mass concentration, extinction or backscatter coefficient), and 2) quantifying the uncertainties in the conversion process. The latter purpose is achieved by including the results of a sensitivity analysis in the model output products.

The model has three layers, the boundaries of which are defined by tropopause height. Each layer includes a set of empirically based refractive indices and relative size distribution types. In contrast to previous models, this model allows for a range of sulfuric acid and ammonium sulfate refractive indices within the “inner stratospheric” layer (∼2 to 20 km above the tropopause, where the major peak in aerosol mixing ratio occurs). We show that nine different analytical types of size distribution previously recommended for this layer can be parameterized in terms of channel ratio—i.e., the relative size distribution indicator that has been extensively measured by dustsondes.

When so parameterized, all nine inner stratospheric function types give very similar results for the several conversion ratios of interest. This parameterization allows considerable saving of computer time while preserving the flexibility to handle certain types of size distribution change. We show that the inner stratospheric parameterization works because all nine inner stratospheric size distribution types are relatively narrow, and their optical integrals of interest are determined primarily by a size range that is well characterized by channel ratio.

Data from previous measurements made near the tropopause are used to demonstrate that, in that region, size distributions are broader than any of the inner stratospheric types, and that their optical integrals are strongly influenced by particles too large to be characterized by channel ratio. Hence, in the layer near the tropopause, conversion ratios can differ significantly from the inner stratospheric values; consequently, parameterization by channel ratios is not successful.

We develop methods for deriving vertical profiles of several conversion ratios and their uncertainties. We also demonstrate an application of the model: deriving profiles of number density and its uncertainty from satellite-measured profiles of extinction and its uncertainty. A companion paper applies the model to the task of validating satellite measurements of stratospheric aerosol extinction.

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P. B. Russell
,
M. P. McCormick
,
T. J. Swissler
,
J. M. Rosen
,
D. J. Hofmann
, and
L. R. McMaster

Abstract

A large satellite validation experiment was conducted at Poker Flat, Alaska, 16–19 July 1979. Instruments included the SAM II and SAGE satellite sensors, dustsondes impactors, a fitter collector and an airborne lidar. We show that the extinction profiles that were measured independently by SAM II and SAGE agree with each other. We then use a generalized optical model (which agrees with the Poker Flat optical absorption and relative size distribution measurements) to derive extinction profiles from the other measurements. Extinction profiles thus derived from the dustsonde, fitter and lidar measurements agree with the satellite-measured extinction profiles to within the combined uncertainties. (Individual 1 σ uncertainties are, at most heights, roughly 7 to 20% each for the satellite, dustsonde and filter measurements, 30 to 60% for the lidar measurements, and 10 to 20% for the process of converting one measured parameter to another using the optical model.)

The wire impactor-derived results are also consistent with the other results, but the comparison is coarse because of the relatively large uncertainties (±35% to a factor of 4) in impactor-derived mass, extinction, N 0.15 and N 0.25 (Nx is the number of particles per unit volume with radius greater than x μm.) These uncertainties apply to background stratospheric aerosol size distributions, and result primarily from relatively small uncertainties (±8 to ±20% for confidence limits of 95%) in radii assigned to impacted particles, combined with the steepness of background size distributions in the radius range that contributes most to mass, extinction, N 0.15 and N 0.25. Polar nephelometer-measured asymmetry parameters (0.4 to 0.6) agree with a previous balloon photometer inference, but are significantly less than the value (∼0.7) obtained from Mie scattering calculations assuming either model or measured size distributions.

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P.B. Russell
,
M.P. McCormick
,
T.J. Swissler
,
W.P. Chu
,
J.M. Livingston
,
W.H. Fuller
,
J.M. Rosen
,
D.J. Hofmann
,
L.R. McMaster
,
D.C. Woods
, and
T.J. Pepin

Abstract

We show results from the first set of measurements conducted to validate extinction data from the satellite sensor SAM II. Dustsonde-measured number density profiles and lidar-measured backscattering profiles for two days are converted to extinction profiles using the optical modeling techniques described in the companion Paper I (Russell et al., 1981). At heights ∼2 km and more above the tropopause, the dustsonde data are used to restrict the range of model size distributions, thus reducing uncertainties in the conversion process. At all heights, measurement uncertainties for each sensor are evaluated, and these are combined with conversion uncertainties to yield the total uncertainty in derived data profiles.

The SAM II measured, dustsonde-inferred, and lidar-inferred extinction profiles for both days are shown to agree within their respective uncertainties at all heights above the tropopause. Near the tropopause, this agreement depends on the use of model size distributions with more relatively large particles (radius ≳0.6 μm) than are present in distributions used to model the main stratospheric aerosol peak. The presence of these relatively large particles is supported by measurements made elsewhere and is suggested by in situ size distribution measurements reported here. These relatively large particles near the tropopause are likely to have an important bearing on the radiative impact of the total stratospheric aerosol.

The agreement in this experiment supports the validity of the SAM II extinction data and the SAM II uncertainty estimates derived from an independent error analysis. Recommendations are given for reducing the uncertainties of future correlative experiments.

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P. B. Russell
,
J. Redemann
,
B. Schmid
,
R. W. Bergstrom
,
J. M. Livingston
,
D. M. McIntosh
,
S. A. Ramirez
,
S. Hartley
,
P. V. Hobbs
,
P. K. Quinn
,
C. M. Carrico
,
M. J. Rood
,
E. Öström
,
K. J. Noone
,
W. von Hoyningen-Huene
, and
L. Remer

Abstract

Aerosol single scattering albedo ω (the ratio of scattering to extinction) is important in determining aerosol climatic effects, in explaining relationships between calculated and measured radiative fluxes, and in retrieving aerosol optical depths from satellite radiances. Recently, two experiments in the North Atlantic region, the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) and the Second Aerosol Characterization Experiment (ACE-2), determined aerosol ω by a variety of techniques. The techniques included fitting of calculated to measured radiative fluxes; retrievals of ω from skylight radiances; best fits of complex refractive index to profiles of backscatter, extinction, and size distribution; and in situ measurements of scattering and absorption at the surface and aloft. Both TARFOX and ACE-2 found a fairly wide range of values for ω at midvisible wavelengths (∼550 nm), with 0.85 ≤ ω midvis ≤ 0.99 for the marine aerosol impacted by continental pollution. Frequency distributions of ω could usually be approximated by lognormals in ω maxω, with some occurrence of bimodality, suggesting the influence of different aerosol sources or processing. In both TARFOX and ACE-2, closure tests between measured and calculated radiative fluxes yielded best-fit values of ω midvis of 0.90 ± 0.04 for the polluted boundary layer. Although these results have the virtue of describing the column aerosol unperturbed by sampling, they are subject to questions about representativeness and other uncertainties (e.g., thermal offsets, unknown gas absorption). The other techniques gave larger values for ω midvis for the polluted boundary layer, with a typical result of ω midvis = 0.95 ± 0.04. Current uncertainties in ω are large in terms of climate effects. More tests are needed of the consistency among different methods and of humidification effects on ω.

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Jielun Sun
,
Steven P. Oncley
,
Sean P. Burns
,
Britton B. Stephens
,
Donald H. Lenschow
,
Teresa Campos
,
Russell K. Monson
,
David S. Schimel
,
William J. Sacks
,
Stephan F. J. De Wekker
,
Chun-Ta Lai
,
Brian Lamb
,
Dennis Ojima
,
Patrick Z. Ellsworth
,
Leonel S. L. Sternberg
,
Sharon Zhong
,
Craig Clements
,
David J. P. Moore
,
Dean E. Anderson
,
Andrew S. Watt
,
Jia Hu
,
Mark Tschudi
,
Steven Aulenbach
,
Eugene Allwine
, and
Teresa Coons

A significant fraction of Earth consists of mountainous terrain. However, the question of how to monitor the surface–atmosphere carbon exchange over complex terrain has not been fully explored. This article reports on studies by a team of investigators from U.S. universities and research institutes who carried out a multiscale and multidisciplinary field and modeling investigation of the CO2 exchange between ecosystems and the atmosphere and of CO2 transport over complex mountainous terrain in the Rocky Mountain region of Colorado. The goals of the field campaign, which included ground and airborne in situ and remote-sensing measurements, were to characterize unique features of the local CO2 exchange and to find effective methods to measure regional ecosystem–atmosphere CO2 exchange over complex terrain. The modeling effort included atmospheric and ecological numerical modeling and data assimilation to investigate regional CO2 transport and biological processes involved in ecosystem–atmosphere carbon exchange. In this report, we document our approaches, demonstrate some preliminary results, and discuss principal patterns and conclusions concerning ecosystem–atmosphere carbon exchange over complex terrain and its relation to past studies that have considered these processes over much simpler terrain.

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J. E. Harries
,
J. E. Russell
,
J. A. Hanafin
,
H. Brindley
,
J. Futyan
,
J. Rufus
,
S. Kellock
,
G. Matthews
,
R. Wrigley
,
A. Last
,
J. Mueller
,
R. Mossavati
,
J. Ashmall
,
E. Sawyer
,
D. Parker
,
M. Caldwell
,
P M. Allan
,
A. Smith
,
M. J. Bates
,
B. Coan
,
B. C. Stewart
,
D. R. Lepine
,
L. A. Cornwall
,
D. R. Corney
,
M. J. Ricketts
,
D. Drummond
,
D. Smart
,
R. Cutler
,
S. Dewitte
,
N. Clerbaux
,
L. Gonzalez
,
A. Ipe
,
C. Bertrand
,
A. Joukoff
,
D. Crommelynck
,
N. Nelms
,
D. T. Llewellyn-Jones
,
G. Butcher
,
G. L. Smith
,
Z. P Szewczyk
,
P E. Mlynczak
,
A. Slingo
,
R. P. Allan
, and
M. A. Ringer

This paper reports on a new satellite sensor, the Geostationary Earth Radiation Budget (GERB) experiment. GERB is designed to make the first measurements of the Earth's radiation budget from geostationary orbit. Measurements at high absolute accuracy of the reflected sunlight from the Earth, and the thermal radiation emitted by the Earth are made every 15 min, with a spatial resolution at the subsatellite point of 44.6 km (north–south) by 39.3 km (east–west). With knowledge of the incoming solar constant, this gives the primary forcing and response components of the top-of-atmosphere radiation. The first GERB instrument is an instrument of opportunity on Meteosat-8, a new spin-stabilized spacecraft platform also carrying the Spinning Enhanced Visible and Infrared (SEVIRI) sensor, which is currently positioned over the equator at 3.5°W. This overview of the project includes a description of the instrument design and its preflight and in-flight calibration. An evaluation of the instrument performance after its first year in orbit, including comparisons with data from the Clouds and the Earth's Radiant Energy System (CERES) satellite sensors and with output from numerical models, are also presented. After a brief summary of the data processing system and data products, some of the scientific studies that are being undertaken using these early data are described. This marks the beginning of a decade or more of observations from GERB, as subsequent models will fly on each of the four Meteosat Second Generation satellites.

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