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Some computations concerning the actual light scattering coefficient as measured in haze and fog
No full textThe aerosol light scattering coefficient is a parameter of paramount interest in problems related to diffuse transmission, visibility, remote sensing, and radiative transfer.
Its measurement is actually carried out with sources of angular divergence θd ≠ 0 and detection systems of half-angle acceptance θ0 ≠ 0; the scattering coefficient so measured is called actual (or apparent) scattering coefficient and is denoted as β(θd, θ0). The total (or true) scattering coefficient is defined as the limit case for θd = θ0 = 0 and usually denoted as β; this is the quantity which characterizes the attenuation of a monochromatic radiation propagating through an aerosol and has to be recovered from the measured β(θd, θ0). Experimental works, carried out for different values of θd, θ0 and with different aerosol particles attenuating the light beam, have shown that, even in the case where θd and θ0 can be reduced to few mrad, the actual value of the scattering coefficient β(θd, θ0) is less than the true value β.
In an earlier article we reported computations of the actual Mie scattering efficiency factor K(m,x; θ0), θd = 0, for the following values of the refractive index (m), size parameter (x), and θ0: m = 1.33, x = 0.1 (0.1) 200 and m = 1.55, x = 0.1 (0.1) 100; θ0 = 0.1° (0.1°) 1.0°. Tables and card decks have been arranged and are available from the author. The results show that for aerosol radii r 3 μm the ratio decreases, and at r ~= 20 μm and θ0 = 1.0° it is 0.58.
In this Letter we compute β and β (θ0), θd = 0, on the basis of 89 size spectra of condensation nuclei (CN), haze, and fog, with the aim of determining the limits between which the ratio R(θ0) = β (θ0)/ β can range in different situations
Condensation nuclei supersaturation spectrum: analysis of the relationship between the saturation droplet radius and the critical supersaturation in the Laktionov isothermal chamber
No full textComputations testing the uniqueness of the relationship between the saturation droplet radius r100 and the critical supersaturation S, have been performed for condensation nuclei (CN) of different chemical composition: pure salts, mixtures of salts and partly insoluble nuclei; indeed, the reliability of this relationship is the key-point in determining the CN supersaturation spectrum by an isothermal chamber. The results show that in the case of typical natural inorganic aerosols the dependence of Sc, on r100 is practically unique for all the CN considered, as the uncertainty in the CN electrolytic composition can lead to a maximum error of about 8% on the activated nuclei counts; by using Laktionov's relationship this error can reach about 12%. When surface-active organic materials are present, the relationship between r100 and S,. is significantly modified ; the activated nuclei counts can differ, for continental aerosols, up to +100% from the corresponding counts of pure inorganic aerosols. Also a decrease in nuclei counts can be predicted ; values up to — 80% have been determined. In conclusion, when the Laktionov isothermal chamber is used, simultaneous measurements characterizing the organic materials are required for an accurate determination of the CN supersaturation spectrum
The parameterization of the gravitational water flux in fog models
No full textThe parameterization of the mean terminal fallspeed of the fog droplets v has been analyzed in terms of the ratio between the liquid water content and the droplet concentration, W/N, as linked to the gravitational water flux G via G = vW. Computations have been carried out on the basis of 239 experimental droplet size distributions selected from the literature and grouped according to the fog type. The results show that the (v,W/N) correlation is poor; the values of the correlation coefficients are 0.46, 0.44, 0.48 and 0.56 for valley, advection and radiation fog, and for all the fog types together, respectively.
On the other hand, G turns out to be virtually determined by the contribution of the fog droplets (Gz) only, while the contribution to G of the haze droplets (G1) is negligible. This result leads us to consider the parameterization of G2 only through a new (v,W/N) relationship, where v, W and N are due only to droplets grown on activated condensation nuclei; this new relationship is shown, as a whole, to be slightly better than the first one and, in particular, to be reliable only for radiation fogs (the correlation coefficients are 0.60, 0.64, 0.92 and 0.71 for the above reported fog groups)
Absorption and liquid water content relationship in fog, at thirteen IR wavelengths
No full textThe relationship between the optical absorption coefficient, σa, and the fog liquid water content, W, has been examined at thirteen IR wavelengths from 5 to 30 μm, on the basis of 239 experimental fog droplet size spectra selected from the literature.
The results show that the correlation coefficients R between σ and W lie in the intervals 0.996-0.968, 0996-0971, 0.997-0.946 and 0.995-0.931 for valley, advection, radiation fog and for all the tog types together, with maxima at λ = 5, 9, 10 μm and the minimum at λ = 14/μm.
The behaviour of R with λ and with the fog type is discussed, while reference is made to the use of the results in fog models
A method for recovering the true scattering coefficient from the apparent one measured in fog
No full textThe apparent light scattering coefficient, β(θ0), measured in fog by optical detection systems of finite acceptance angle θ0, generally has to be corrected in order to recover the true scattering coefficient, β. To this end 22 curves have been derived, from which the value of β can be correctly extrapolated, by grouping 81 curves of the ratio β(θ0)/β(1.0°) computed for droplet size spectra of valley, advection and radiation fogs, at a wavelength of λ = 0.6328 μm. Some examples show the procedure to be effective in practice
Discussion on a method for recovering the true scattering coefficient from the apparent one measured in fog – AUTHOR’S REPLY
No full textThe aim of the proposed method is to determine β(θ0=0°) avoiding the troublesome experimental determination of the f(r) function (see 2nd section, 7th line). The value of β(θ0=0°) has to be extrapolated along one of the curves given in Fig. 1 and the appropriate curve has to be located by comparing its measured right part with the corresponding side of the curves of Fig. 1 (see second section, 14th; 36th and 39th line). Thus, β(θ0) has to be measured at more than one θ0 value, in order to be able to choose properly, among the curves of Fig. 1, the β(θ0)/ β (1.0°) curve which has to be used
On the reliability of the parameterization of microphysics in fog models
No full textThe relationships between the mean terminal speed of fog droplets (ve) and the liquid water content (W), and between the integral volume droplet absorption coefficient (σE) and W, have been examined on the basis of 107 experimental fog droplet size spectra selected from the literature, in order to investigate the reliability of parameterizing the microphysics, in fog models, through W.
The results show that a weak correlation exists between vE and W, the values of the linear correlation coefficients (R) being 0.12, 0.30, —0.19 and 0.67, for valley, advection, radiation and artificial fog, respectively. Thus parameterizations of vE through W only are generally unreliable, and other parameterizations, in terms of more than one parameter, should be developed.
On the other hand, a good correlation between σE and W has been found, with R =0.98, 0.87, 0.91 and 0.99 for the fog types indicated above. Comparisons between the values of σE and those obtained by the best-fit lines and by the parameterizations used in fog models, together with the evaluation of the effect of such approximations on the radiative cooling profile, have been carried out
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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