J. Bilbao

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.

Test Reference Year Generation and Evaluation Methods in the Continental Mediterranean Area

Different meteorological data series called multiyear data, long-term average measured data series, or test reference years (TRYs) are required for solar energy system simulation. It is known that the use of the multiyear data approach requires a great effort in time and computation, long-term average measured data do not have the extreme values of weather data given along the year, and TRYs represent typical references rather than extreme conditions and facilitate the comparison in the performance of energy systems. In this paper, TRYs have been generated, using three different methodologies, from hourly meteorological data measured in two cities, Madrid and Valladolid (Spain). In order to evaluate them, the performance simulation of three solar energy systems (thermal, passive, and photovoltaic) with long-term measured meteorological data has been compared with estimated performance simulations with TRYs. Root-mean-square and mean bias errors and relative differences have been used as estimators to measure the performance deviation of TRYs from long-term measured meteorological data series. Results of the comparison show that the most appropriate method for generating test reference year depends on the characteristics of the station and varies from month to month. The Danish method (TRY5) gives better results in Valladolid than in Madrid for the photovoltaic and passive systems; the Argirious method (TRY6) gives better results in Madrid than in Valladolid for the photovoltaic system. The Pissimanis method (TRY4) is the best for simulating thermal and photovoltaic systems in summer.

Estimation of Daylight Downward Longwave Atmospheric Irradiance under Clear-Sky and All-Sky Conditions

Abstract Daylight downward longwave irradiance data recorded over a flat place for the period between April 2001 and December 2004 in Valladolid, Spain, have been compared with estimates generated using four different schemes. The parameterization schemes of Brutsaert, Swinbank, Idso, and Brunt have been considered and calibrated for the comparison. Root-mean-square errors (rmse), mean bias errors, and linear regression correlations have been used to compare measured and estimated values. The results of this comparison show that, for clear-sky conditions, rmse values range between 19.57 and 8.85 W m−2 for calibrated schemes and between 39.78 and 11.13 W m−2 for original ones. The Idso and Brunt schemes give the best results with calibrated coefficients, and the Brunt scheme performs the best with original coefficients. A new scheme for estimating daylight downward longwave irradiance under “all-sky” conditions has been developed based on clear-sky schemes and solar global shortwave irradiance, and, after ...

Air Temperature Model Evaluation in the North Mediterranean Belt Area

Abstract A comparative assessment of air temperature models, using hourly and daily air temperature measurements in 34 different stations in the north Mediterranean belt, is presented. Four air temperature models were used to estimate hourly and daily mean air temperature from daily maximum, daily minimum, and monthly mean air temperature. Root-mean-square error (rmse), scatter graphs, and cumulative frequency curves were used to determine the performance of each model. The best overall performance for estimating hourly air temperature from monthly mean values was presented by Erbss model; the “standard” model gave the best performance for estimating daily mean air temperature from daily minimum and maximum air temperature values. The results show that the Erbs and standard models are the best for all stations used. A new Climatic Synthetic Time Series for the Mediterranean Belt Temperature Model (CLIMEDTEM) for estimating daily air temperature was developed by the authors with the help of available data...

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.

Uncertainty of different atmospheric ozone retrievals and its effect on temporal trends and radiative transfer simulations in the Iberian Peninsula

Uncertainty in total ozone column (TOC) values is quantified for eight different databases through a direct comparison with ground-based data at three Spanish locations, the maximum uncertainty being about 10.5 Dobson unit. A long-term TOC series is constructed using the mentioned databases from 1950 to 2011 for nine locations in the Iberian Peninsula. The monthly TOC climatology in the nine locations is presented. An exhaustive analysis is performed of TOC series trends and their statistical significance in the periods 1950–2011, 1950–1984, and 1985–2011. A significant ozone reduction between 1950 and 2011 appears in the Iberian Peninsula with a strong reduction from the late 1970s to the mid-1990s and with more or less constant or slightly increasing levels in the last 17 year. These trends are recalculated taking into account of uncertainty in the TOC values of the series, with a decrease in the number of statistically significant trends emerging. The statistically significant trends in annual and in the averaged Iberian Peninsula series are usually still significant even considering the uncertainty. Finally, a study is carried out of the uncertainty caused by TOC uncertainty in total shortwave and erythemal ultraviolet irradiances simulated under cloudless skies using a radiative transfer model.

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.