A. de Miguel

Diffuse solar irradiation model evaluation in the North Mediterranean Belt area

The performance of daily and hourly diffuse horizontal solar irradiation models and correlations is examined using an assembled data set of multivariate meteorological time series from countries in the North Mediterranean Belt area. The correlations reviewed use only daily global, hourly global or daily diffuse irradiation as input, for the daily or hourly time scale. The best overall performance was presented by the Frutos correlation for the estimation of the daily diffuse radiation by an adapted version of the Liu and Jordan correlation for the mean daily diffuse radiation profile, and by the Hollands and Crha model for estimation of hourly diffuse values from the corresponding global values. The results show that the best correlation for each site varies. Two empirical piecewise correlations were also developed by the authors with the help of the data bank available, yielding models that showed even better fits to the data. The results show some seasonal and location dependence.

Evaluation of the desert dust effects on global, direct and diffuse spectral ultraviolet irradiance

ABSTRACT This paper presents a study of a strong desert dust episode over the Iberian Peninsula, and its effect on the spectral ultraviolet (UV) irradiance in Granada, Spain. Remote sensing measurements, forecast models, and synoptic analysis are used to identify a Saharan desert dust outbreak that affected the Iberian Peninsula starting 20 July 2009. Additionally, a Bentham DMc150 spectroradiometer is employed to obtain global, direct and diffuse spectral UV irradiances every 15 minutes in Granada. The desert dust caused a large attenuation of the direct UV irradiance (up to 55%), while the diffuse UV irradiance increased up to 40% at 400 nm. The UVSPEC/LibRadtran radiative transfer model is used to study the spectral dependence of the experimental UV irradiance ratios (ratios of spectral irradiance for the day with the highest aerosol load to that measured in days with low–moderate load). The spectral increase or decrease of the UV direct irradiance ratios depends on a new parameter: a threshold wavelength. The spectral dependence of the UV diffuse irradiance ratio can be explained because under the influence of the intense dust outbreak, the Mie scattering by aerosols at shorter wavelengths is stronger than the Rayleigh scattering by gases. Finally, the sensitivity analysis of the aerosol absorption properties shows a substantial attenuation of UV spectral irradiance with a weak spectral dependence.

Solar radiation incident on tilted surfaces in Burgos, Spain: Isotropic models

Abstract A number of hourly measurements of global solar radiation on a horizontal surface, taken during the period 1981–1986 in Burgos, Spain, have been analyzed into diffuse and beam components. The components of global solar radiation were used to calculate the monthly average hourly and daily values on an inclined surface. The results obtained using isotropic models have been compared, tabulated and plotted against the angle of tilt for summer, winter and all year. The optimum tilt angle and the solar radiation on a south facing tilted surface have been calculated for different periods of time and by three different models. The optimum tilt angle values range from 7° in June to 70° in December and January.

Solar radiation and sunshine hour maps in Castilla and León region (Spain)

From global solar radiation and sunshine hour data of Castilla and Leon region (Spain), six different Angstrom-type correlations have been performed using the least square technique and in each of them the parameters a and b have been calculated. In two of these correlations we have taken into account the optic effects of atmosphere and not burning the sunshine recorder chart. The results have been compared using statistical tests based on MBE, RMSE and CC and from this calculation we have obtained the best correlation for Castilla and Leon locations. Finally from experimental and calculated data the monthly mean solar global radiation and sunshine isoline maps have been plotted for each month of the year and for the year as a whole.

Observed influence of liquid cloud microphysical properties on ultraviolet surface radiation

Measurements of different UV quantities (UV index, ozone photolysis rates, global and diffuse irradiances, and actinic flux spectra) and cloud properties were collected during a field campaign carried out in Southern Italy in May–June 2010. Independent measurements of cloud liquid water path and optical depth allowed retrieving the cloud effective radius. The cloud modification factor (CMF) is used to investigate the influence of liquid cloud properties on the UV radiation under overcast conditions. CMF was also simulated using a 1-D radiative transfer model. Experimental and simulated CMF values for UV index (under overcast conditions) show a normalized root-mean-square error around 11%. Clouds with small effective radius determine a higher UV radiation attenuation than clouds formed by large particles. The CMFs for the UV index and the global spectral irradiance show a very weak dependence on the solar zenith angle (SZA), while the CMFs for actinic flux (both integrated and spectral) and diffuse spectral irradiance show a variation with SZA. The irradiance is more effectively attenuated at low SZA, while the actinic flux at high SZA. These effects are due to the different weight given to the direct and the diffuse components.

Ozone Dry Deposition and Resistances onto Green Grassland in Summer in Central Spain

Measurements of ozone concentrations, and meteorological and surface parameters were carried out over a flat green grassland in northwest Spain, in July 1995. Turbulent parameters and sensible and latent heat fluxes were calculated using the gradient technique. Fluxes and deposition velocity and resistances were evaluated assuming that the diffusivity for heat fluxes was equal to pollutant diffusivity. The daily average value of dry deposition velocity was 6 mm s-1 but it was influenced by wind velocity and atmospheric stability. Resistances have been calculated according to a simple resistance model and a comparison between theoretical and measured values has been made.

Contribution to the study of solar radiation in Burgos, Spain

Abstract A series of daily measurements of global solar radiation on horizontal surface realized during the period 1978–1982, in Burgos, Spain, is analysed. The mean decadic values show fluctuations, in the spring and at the beginning of the summer especially. The absolute maximums generally appear in July with values that oscillate between 23 and 26 MJ m−2, and the minimums in January or December with values between 4 and 5 MJ m−2. Averaging the analogous months we obtain the maximums in July with a value of 23 MJ m−2. We have studied the elemental statistic characteristics and we remark that the interquartile range is small in the winter months and increases in the spring and summer. The number of the days in which the radiation has remained inferior to a given value, has been calculated in the frequency analysis, remarking that in the days corresponding to the period of the winter appear radiation values inferior to 10 MJ m−2 and only 3% of July days are below this value. We have defined and determined the potential radiation and have calculated the extraterrestrial radiation in order to know the attenuation of global radiation in its passage through the atmosphere. We remark that the energy percentage transmitted by the atmosphere increases from winter to summer, a maximum value of 59% is obtained in July and a minimum of 30% in December. The atmospheric transparency without clouds oscillates between 71% in the spring and 62% in the winter.

COMPUTATION OF UTILIZABILITY FROM CUMULATIVE FREQUENCY CURVES

ABSTRACT For certain practical applications it is necessary to know for each month the number of hours per day in which the irradiance exceeds a given value, as well as to know the uti1izabi1ity O. From the hourly measurements of global solar radiation on horizontal surface, we have determined the monthly cumulative frequency curves and we remark that the maximum irradiance levels range between 410 W m−2 in December and 950 W m−2 in June.With this information the uti1izabi1ity O for all twelve months of the year has been calculated. We remark that the dimensionless -parameter J, defined by J = A/(dj.Imax), ranges between 0.28 in January -and 0.51 in July.