Eric P. Shettle. 13a. TYPE OF REPORT. Scientific Interim. 13b. ... average of Volz's (1973) and Patterson's (1981) measurements for Imaginary part, which are In.
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11. TITLE (Include Stcurity Clauifieation) Optical and
62101F
Radiative Properties of a Desert Aerosol Mode
14
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Eric P. Shettle 13a. TYPE OF REPORT
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Reprinted from: IRS 84: Current Problems in Atmospheric Radiation Proceedings of the International Radiation Sympos ium, Perugia, Italy 21-28 Aug 84, Edited bv Giorgio Fiocco ^£-salj£~
17.
GROUP
7.Desert Aerosols
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FIELD
SUB. GR.
Transmission Atmospheric Radiation
Optical Properties/ Scattering
ten /^..«^-^
19. ABSTRACT (Continue on reverie if neetuary and identify bv block number)
The Experts Meeting on Aerosols and their Climatic Effects, under the World Climate Research Program reviewed the status of the aerosol models in the Standard Radiation Atmospheres of the IAMAP Radiation Commission. One of the major areas for improving these aerosol models was identified as the need for a separate desert aerosol model. Several recommendations were made for developing such an aerosol model. A desert aerosol model based on these recommendations and other work will be discussed with emphasis on the optical and radiative properties and their variations with wind speed.
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Eric P. Shettle DO FORM 1473, 83 APR
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AFGL-TR -86-000 OPTICAL AND RADIATIVE PROPERTIES OF A DESERT AEROSOL MODEL
i- i.
JAN16W
E.P.Sheale Air Force Geophysics Laboratory Hanscom AFB. Massachusetts. USA
Introduction
Scf
The values of the parameters Kj, o j, and R^ arc aummarlted In Able 1, following VMO, 1983 (their Tkble 4.1). The number density distribution for these models la shown in Figure 1 and the ■cross-sectional area distribution la ahown la Figure 2. It will be noted that the two model alse distributions differ algnlf Icaatly only for the larger aeroaola .
One of the «ajor aourccs of the natural ataoapherlc aeroaola la wind-blown dust and aand. Diese predominantly originate f roa the arid and aeal-arld regions which sake up onethird of the earth* a land area (a < ■ aae Lavttf^->^«fl.; Wdfr) . The aeroaol aodels In the Standard Radiation Ataospheras (SRA) of the lAMAP Radiation CDaalaslon, do not Include a model specifically representative of theae reglona. "For thla reaaon the Experts Meeting on Aerosole and their Climatic Effects (WHO 1983)1 under the World dlaate Research Program, Identified as one of the major areas for Improving the SRA aeroaol models, the need for. a separate desert aerosol model.^ Several recoanendatlons were made for developing auen a^' mt(t aerosol model. This paper presents the optical and radiative properties of a desert aerosol model based on those recommendations . V3 models are dlacussed representing the extremes of background conditions and a severe dust storm.
Optical and Radiative Fropertlea The optical and radiative properties were derived from standard We scattering calculatlona. Figure 3 shows our results for the extinction, scattering and abaorptlon coefficients for the Background Desert Aeroaol Model and Figure 4 shows the corresponding reaulta for the Desert Dust Storm Model. The extinction coefficients for the two models arc ahown In Figure 5. It will be noted that the Desert Dust Storm Aerosol Model extinction excedes the Bsckground Model values by a factor of 40 In the visible and by 3 ordere of magnitude In the far IR. This Is due to the enhanced numbers of very large aerosols with acvere wind conditions. The single scatter albedo (the ratio of acattaring to total extinction) Is shown In Figure 6, for the two models.
Physical Properties of the Desert Aerosols The refractive Index of the aerosols Is based on the work of Volt (1973), Benjamin & Carlson (1980) and Patteraon (1981), as discussed below. The real part of the refractive Index follows Carlson & Benjamin (1980) for wavelengths , X < 2 .5 j» . For longer wavelengths Volt'a (1973T measurements from the real part are uaed. The Imaginary part of the refractive Index Is baaed on Benjamin and Carlson (1980) for X £ 1 «0 10*, and la Joined smoothly Into an average of Volz's (1973) and Patterson's (1981) measurements for Imaginary part, which are In good agreement with recent measurements (Fouquart, et al., 1984).
The aeyametry parameter, which characterizes the angular distribution of the acattered radiation la shown In Figure 7 . References Carlson, TJI. & S.S. Benjamin (1980), 'Radiative Heating Rates for Saharan Dust', J . Atmos . Scl., 37, 193-213. Fouquart, T. B. Bonnell, A. Oerf, M. Chaoul, L. Smith, 4 J.C. Vanhoutte (1984) 'Size Distribution and Optical Fropertlea of Saharan Aerosols during ECLATS', In Aeroaola and Their Qlmatic Effecta, edited by A. Deepak 4 H. Gerber, A. Deepak Publishing, In press .
The size distribution Is based on the review by Jaenlcke (1983). The else distributions for the aerosol models are represented as the aua of 3 log-normal distributions : dN(r) d log r
(log r - log Rj)2
N.
1
1-1
exp STi
log
Jaenlcke. R. (1983), Presented at the Experts Meeting on Aerosols and Their Climatic Effects (HMO 1983), also to be published In 'Aerosol Physics and Chenistry' Meteorology Volume In Landolt-Bomsteln.
2(log OjT
where N(r) - particle concentration for particles with radius > r. th Nj ■ total number of particles In the 1 distribution oi - geometric standard deviation Rj - mode radius
Levin, Z.. J.H. Joseph, 4 Y. Mekler (1980), Properties of Sharav (Khamsin) Dust Comparison of Optical and Direct Sampling Data', J . Atmos . Sei ., 37, 882-891.
y
DESERT AEROSOL MODEL
75
Patterson, ZM. (1981), 'Optical Properties of the Crust«! Aerosol; Relationship to Chemical and Physical Charaeterl «tics*. J. Ceophys. Res«. 86, 3236-3246. Volt, P.E. (1973), 'Infrared Optical Constants of Amoniun Sulfate, Saharan Dust, Volcanic Pumice, and Flyash", üppl. Opt.. 12, 564-567 .
WC (1983) 'Report of the Experts Meeting on Aerosols and their Qisatic Effects' (HHilaasburg, VA. 28-30 March 1983). Hbrld Cliaate Research Prograase, Report No. W(P-55, Published by the World Meteorological Organisation, Dec 1983.
TABLE 1 Paraxeters for Desert Aeroaol Sise DistributIon Sice Distribution
1
\ (.em'3)
Background Desert Model
1 2 3
Desert Dust Storn Model
1 2 3
ci
Ijd»)
9S7 x l6i 8.42 x 10*. 7 30 x 10"*
0 328 0 305 0 377
0.0218 6 34
7 36 x I02 134 x 103 1.78 x 10"1
0 347 0 370 D>38
0.0010 0J0188 10 JB
DESERT AEROSOL IDDELS
OJOOIO
DESERT AEROSOL MODELS 1000 »00+ IT O O
BOO
_i
O ^
700
o Z
600'
3 IS
500-
K
400+
o
»-
IO
900
^
