Calibration and data elaboration procedure for sky irradiance measurements.

The problems encountered in the elaboration of measurements of direct and sky diffuse solar irradiance are the following: (1) to carry out the calibration for the direct irradiance, which consists in determining the direct irradiance at the upper limit of the atmosphere; (2) to carry out the calibration for the diffuse irradiance, which consists in determining the solid viewing angle of the sky radiometer; (3) to determine the input parameters, namely, ground albedo, real and imaginary parts of the aerosol refractive index, and aerosol radius range; and (4) to determine from the optical data the columnar aerosol optical depth and volume radius distribution. With experimental data and numerical simulations a procedure is shown that enables one to carry out the two calibrations needed for the sky radiometer, to determine a best estimate of the input parameters, and, finally, to obtain the average features of the atmospheric aerosols. An interesting finding is that inversion of only data of diffuse irradiance yields the same accuracy of result as data of both diffuse and direct irradiance; in this case, only calibration of the solid viewing angle of the sky radiometer is needed, thus shortening the elaboration procedure. Measurements were carried out in the Western Mediterranean Sea (Italy), in Tokyo (Japan), and in Ushuaia (Tierra del Fuego, Argentina); data were elaborated with a new software package, the Skyrad code, based on an efficient radiative transfer scheme.

Use of sky brightness measurements from ground for remote sensing of particulate polydispersions.

The software code SKYEAD.pack for retrieval of aerosol size distribution and optical thickness from data of direct and diffuse solar radiation is described; measurements are carried out with sky radiometers in the wavelength range 0.369-1.048 µm. The treatment of the radiative transfer problem concerning the optical quantities is mainly based on the IMS (improved multiple and single scattering) method, which uses the delta-M approximation for the truncation of the aerosol phase function and corrects the solution for the first- and second-order scattering. Both linear and nonlinear inversion methods can be used for retrieving the size distribution. Improved calibration methods for both direct and diffuse radiation, the data-analysis procedure, the results from the proposed code, and several connected problems are discussed. The results can be summarized as follows: (a) the SKYRAD.pack code can retrieve the columnar aerosol features with accuracy and efficiency in several environmental situations, provided the input parameters are correctly given; (b) when data of both direct and diffuse solar radiation are used, the detectable radius interval for aerosol particles is approximately from 0.03 to 10 µm; (c) besides the retrieval of the aerosol features, the data-analysis procedure also permits the determination of average values for three input parameters (real and imaginary aerosol refractive index, ground albedo) from the optical data; (d) absolute calibrations for the sky radiometer are not needed, and calibrations for direct and diffuse radiation can be carried out with field data; (e) the nonlinear inversion gives satisfactory results in a larger radius interval, without the unrealistic humps that occur with the linear inversion, but the results strongly depend on the first-guess spectrum; (f) aerosol features retrieved from simulated data showed a better agreement with the given data for the linear inversion than for the nonlinear inversion.

Aerosol features retrieved from solar aureole data: a simulation study concerning a turbid atmosphere.

The characteristics of the solar aureole were evaluated for several cases of a turbid atmosphere in the 3° ≤θ≤30° interval of scattering angles; for each case, the features of the aerosol were retrieved from the simulated aureole data. Computations were carried out with a recently set up radiative transfer code that uses the approximated delta-M method, corrected further for the 1st and 2nd scattering orders. Results showed that the software tested can work out both the direct and the inverse aureole problems with great accuracy and efficiency in several different situations, so it can reliably be used for handling experimental data measured in the field with an aureolemeter. Furthermore, the input parameters of ground albedo, complex refractive index, aerosol radius interval, and measurement angles were varied within a set of values to examine the sensitivity of the retrieval to improperly assumed values of these parameters and to evaluate the most suitable way of determining their correct values. Only data concerning diffuse radiation were elaborated. Results showed that (1) the scanned scattering angles have to be extended up to 40°; (2) the most suitable radius interval for aerosols appears to be from 0.05 to 15 µm; (3) ground albedo A should be independently determined within 15%; and (4) as to the complex refractive index m˜, the real part should be given within 3.5%, and the imaginary part within from 10% to 50%, according to its value. Finally, a procedure through which it is possible to derive A and m˜ by extending the information content of the aureole data is discussed. Improved calibration procedures are also proposed.

Reliability of the polar nephelometer for the measurement of visibility in fog.

The relation between the extinction coefficient of light at lambda = 0.55 microm and the scattered intensity was studied as a function of the scattering angle and the wavelength, in connection with its use in the measurement of visibility with the polar nephelometer. Computations used 239 spectra of natural fogs; they considered 181 scattering angles, 22 wavelengths from 0.25 to 5 microm, three cases of polarization, and four classes of visibility. Results show that for each wavelength within the interval micro = 0.25-1.06 microm and for each polarization and visibility class, there exists a corresponding well-defined angular interval in which the above relation is linear and quite reliable. The center and the width of the angular intervals, as well as the relative standard deviation concerning the relation at hand, depend strictly on the wavelength and the class of visibility considered; they range between 26 and 56 deg, 6 and 23 deg, and 0.5 and 3.5%, respectively, so that by a proper choice of avelength and related angular interval, visibility can be determined within 0.5%.

Backscattering, extinction, and liquid water content in fog: a detailed study of their relations for use in lidar systems.

A solution to the single-scattering lidar equation to infer optical extinction, liquid water content W, and visibility in a fog through a monostatic pulsed lidar, requires the use of the relations, and combinations of them, between backscattering coefficient beta, extinction coefficient sigma, and W. To this end, beta and sigma have been computed for 239 droplet size spectra and forty wavelengths from 0.25 to 12 microm, together with W, and the three relations beta vs sigma, W vs sigma and sigma(0.55 microm) vs sigma have been determined. The analysis of their behavior with wavelength shows that (1) the relation beta vs sigma is mostly reliable in the lambda = 0. 25-2-microm region, where the dispersion of the beta values around the best-fit curve is within 20%; (2) the relation W vs sigma is generally not well verified (the dispersion is centered around 30%), and when used in connection with the beta vs sigma relation to infer liquid water content, their joint dispersion P(D) is always greater than 20%; (3) the relation sigma(0.55 microm) vs sigma is well verified in the region lambda = 0.25-2 microm (dispersion within 15%), and when used in connection with the beta vs sigma relation to infer visibility P(D) appears to be minimum at lambda congruent with 0.35 microm (P(D) = 4.7%).

Evolution of the fog droplet size distribution observed by laser scattering.

Scattering of a continuous wave He-Ne laser beam in artificial fog is observed by five photometers at different angles. The measured intensity of the light scattered in the different directions is related to a T-type spectral distribution of the droplet sizes during the various evolution stages of the fog. The artificial fog is produced in a 40-m(3) chamber, where the main physical parameters (vapor and nuclei concentration, temperature) can be controlled to reproduce the complete evolution process. The fog droplet size spectrum is determined at intervals of about 1 min. The results show that the size distribution shifts towards larger size droplets during condensation, and that the shift is reversed during fog dissipation.