Cloudless Sky Conditions Research Articles

IMPLEMENTASI DIGITAL IMAGE PROCESSING SEBAGAI PENDETEKSI POSISI MATAHARI PADA PERANCANGAN DUAL AXIS SOLAR TRACKER

One of the uses of new renewable energy (EBT) in Indonesia which has great potential is solar panels. Solar panels are electrical devices that are used to convert solar energy into electrical energy. The limited surface area of the panels requires users to be able to pay attention to the work efficiency of solar panels. The aspect that has the most influence on optimizing the efficiency of solar panel performance is the placement of the surface of the solar panel against incoming solar radiation. Therefore, it is necessary to have a method to detect the position of the sun to provide input to the actuator so that the surface of the solar panel is positioned in the direction of the sun's rays. This study describes the accuracy of detecting the position of the sun using digital image processing methods. This process needs to be done as a reference in designing a dual axis solar tracker. Detection of the sun's position using this method gets 100% accurate results in cloudless sky conditions and gets 80% success results in cloudy sky conditions.

Field Quality Control of Spectral Solar Irradiance Measurements by Comparison with Broadband Measurements

Measurement of solar spectral irradiance is required in an increasingly wide variety of technical applications, such as atmospheric studies, health, and solar energy, among others. The solar spectral irradiance at ground level has a strong dependence on many atmospheric parameters. In addition, spectroradiometer optics and detectors have high sensitivity. Because of this, it is necessary to compare with a reference instrumentation or light source to verify the quality of measurements. A simple and realistic test for validating solar spectral irradiance measurements is presented in this study. This methodology is applicable for a specific spectral range inside the broadband range from 280 to 4000 nm under cloudless sky conditions. The method compares solar spectral irradiance measurements with both predictions of clear-sky solar spectral irradiance and measurements of broadband instruments such as pyrheliometers. For the spectral estimation, a free atmospheric transmittance simulation code with the air mass calculation as the mean parameter was used. The spectral direct normal irradiance (Gbλ) measurements of two different spectroradiometers were tested at Plataforma Solar de Almería, Spain. The results are presented in this article. Although only Gbλ measurements were considered in this study, the same methodology can be applied to the other solar irradiance components.

Evaluation and Calibration of Downward Longwave Radiation Models under Cloudless Sky at Ile-Ife, Nigeria

Downward longwave radiation flux is an important variable for estimating net radiation, in order to investigate the surface energy budget and carry out climatic studies. The flux is usually estimated using empirical models based on the information of meteorological parameters such as relative humidity, surface and air temperature, and water vapor pressure. This paper presents the assessment of 11 widely used empirical models for estimating downward longwave radiation using a ground-based dataset acquired from January 2016 to December 2017 at Ile-Ife, a tropical city in Nigeria. The original Idso and Niemela et al. models performed better than other models with errors less than 5.0% when compared to measured values. The performances of all the models improved greatly after calibration. The Guest model, which gave low errors (MBE = 0.65 Wm–2, RMBE = 0.15%, RMSE = 9.38 Wm–2, RRMSE = 2.14%, MAE = 7.84 Wm–2, RMAE = 1.79%), performed best followed by the Dilley and O’Brien, Idso, Prata, Brutsaert, Garratt, Niemela et al., and Ångström models. The calibrated models presented in this study can be used to estimate the flux under cloudless sky conditions at Ile-Ife and at other places with similar meteorological conditions, where this flux is not measured due to technological problems and the high cost of purchasing and maintaining the needed sensors. The proposed model for estimating the flux showed better performance with lower statistical errors than all the existing empirical models tested, and conform greatly (R2 = 0.88) to the measured data.

A modified version of the kernel-driven model for correcting the diffuse light of ground multi-angular measurements

When using the kernel-driven bidirectional reflectance distribution function (BRDF) model to process multi-angular measurements, the input multi-angular measurements must be corrected for atmospheric effects. However, in current databases, a significant number of ground-based multi-angular measurements contain either no corrections or only approximate corrections for atmospheric effects. Thus, the blended diffuse light in the total incident irradiance will result in considerable smoothing of the reflectance anisotropy retrieved by the kernel-driven model unless an atmospheric correction process is conducted. In this study, we propose a diffuse-light correction (DLC) form of the kernel-driven model that improves its ability to process multi-angular measurements blended with hemispherical diffuse light. The DLC form of the kernel-driven model can be used to retrieve the intrinsic reflectance anisotropy of the observed target from atmospheric-uncorrected multi-angular measurements. This study used multi-angular data simulated by the PROSAIL and Radiosity Applicable to Porous IndiviDual objects (RAPID) BRDF model, atmospheric-corrected Polarization and Directionality of the Earth's Reflectances (POLDER), Cloud Absorption Radiometer (CAR) multi-angular measurements and their simulated data based on the Second Simulation of the Satellite Signal in the Solar Spectrum (6S) tools to validate the effectiveness of the DLC form of the kernel-driven model. The results indicated that the reflectance factors directly retrieved by the kernel-driven model are considerably smoothed by the blended diffuse light, especially in hotspot regions. Even under clear and cloudless sky conditions, the retrieved hotspot reflectance in the red band is still underestimated by an average of 9.25%, 7.72%, 11.0% and 13.8% for the PROSAIL, RAPID, POLDER and CAR data, respectively. In contrast, the hotspot reflectance retrieved by the DLC form of the kernel-driven model is very close to the intrinsic reflectance anisotropy of the targets; the average relative error of the DLC form of the kernel-driven model is only 1.99%, 1.50%, 4.57% and 3.42%, respectively. Although the reflectance reconstructed by the DLC form of the kernel-driven model in the hotspot region represents a considerable improvement compared with the reflectance retrieved by the original kernel-driven model, its improvement on the root mean square error (RMSE) and the bias of the entire datasets is not very apparent. Using the DLC form of the kernel-driven model can significantly improve the ability of the kernel-driven model to process multi-angular measurements blended with hemispherical diffuse irradiance.

Mapping diffuse photosynthetically active radiation from satellite data in Thailand

In this paper, calculation of monthly average hourly diffuse photosynthetically active radiation (PAR) using satellite data is proposed. Diffuse PAR was analyzed at four stations in Thailand. A radiative transfer model was used for calculating the diffuse PAR for cloudless sky conditions. Differences between the diffuse PAR under all sky conditions obtained from the ground-based measurements and those from the model are representative of cloud effects. Two models are developed, one describing diffuse PAR only asa function of solar zenith angle, and the second one asa multiple linear regression with solar zenith angle and satellite reflectivity acting linearly and aerosol optical depth acting in logarithmic functions. When tested with an independent data set, the multiple regression model performed best with a higher coefficient of variance R2 (0.78 vs. 0.70), lower root mean square difference (RMSD) (12.92% vs. 13.05%) and the same mean bias difference (MBD) of −2.20%. Results from the multiple regression model are used to map diffuse PAR throughout the country as monthly averages of hourly data.

Controlling sunbathing safety during the summer holidays - The solar UV campaign at Baltic Sea coast in 2015

Information regarding the intensity of surface UV radiation, provided for the public, is frequently given in terms of a daily maximum UV Index (UVI), based on a prognostic model. The quality of the UV forecast depends on the accuracy of column amount of ozone and cloudiness prediction. Daily variability of UVI is needed to determine the risk of the UV overexposure during outdoor activities. Various methods of estimating the temporary UVI and the maximum duration of UV exposures (received a dose equal to minimal erythemal dose – MED), at the site of sunbathing, were compared. The UV indices were obtained during a field experiment at the Baltic Sea coast in the period from 13th to 24th July 2015. The following UVI calculation models were considered: UVI measurements by simple hand-held biometers (Silver Crest, Oregon Scientific, or more advanced Solarmeter 6.5), our smartphone models based on cloud cover observations at the site and the cloudless-sky UVI forecast (available for any site for all smartphone users) or measured UVI, and the 24h weather predictions by the ensemble set of 10 models (with various cloud parameterizations). The direct UV measurements, even by a simple biometer, provided useful UVI estimates. The smartphone applications yielded a good agreement with the UV measurements. The weather prediction models for cloudless-sky conditions could provide valuable information if almost cloudless-sky conditions (cloudless-sky or slightly scattered clouds) were observed at the sunbathing site.

Effects of urban agglomeration on surface-UV doses: a comparison of Brewer measurements in Warsaw and Belsk, Poland, for the period 2013–2015

Abstract. Specific aerosols and cloud properties over large urban regions seem to generate an island, similar to the well-known urban heat island, leading to lower ultraviolet (UV) radiation intensity compared to the surrounding less polluted areas, thus creating a shield against excessive human exposure to UV radiation. The present study focuses on differences between erythemal and UVA (324 nm) doses measured by the Brewer spectrophotometers in Warsaw (52.3° N, 21.0° E) and Belsk (51.8° N, 20.8° E). The latter is a rural region located about 60 km south-west of the city. Ratios between erythemal and UVA partly daily doses, obtained during all-sky and cloudless-sky conditions for the period May 2013–December 2015, were analysed to infer a specific cloud and aerosol forcing on the surface UV doses over Warsaw. Radiative model simulations were carried out to find sources of the observed differences between the sites. It was found that Warsaw urban agglomeration induced 8 and 6 % attenuation of the erythemal and UVA doses respectively. This is mostly due to the lower sun elevation in Warsaw during the near-noon measurements and the larger optical depth of the city aerosols and increased cloudiness. It could be hypothesised that the expected stronger absorption of the solar UV radiation by urban aerosols is compensated for here by a higher surface reflectivity over the city.

An assessment of the ratio of ultraviolet-B to broadband solar radiation under all cloud conditions at a subtropical location

Earlier studies have demonstrated that clouds modulate the intensity of solar spectrum bands reaching the Earth’s surface. However, their effect may appear as wavelength dependence. In addition, cloud effects on solar radiation variability dominate any other variable, and these effects usually mask the influences of any other atmospheric variables. The variability of the ratio of ultraviolet-B, UVB, to broadband solar radiation, G, (UVB/G) under all cloud conditions at Qena, Egypt was discussed for the period 2000–2009. The dependence of this ratio on cloud amount (CA) for different solar zenith angle (SZA) classes has also been analyzed. Four classes were considered (SZA⩽30°, 30°<SZA⩽45°, 45°<SZA⩽60°, and SZA>60°). The results obtained reveal that this ratio increased with increasing CA for all SZA classes except for SZA⩽30°. However, for the SZA class⩽30°, the UVB/G ratio decreased with increasing CA. In addition, the effects of cloud type on the UVB/G ratio were investigated. Under cloudy conditions (7–8octas), on average, both low-level and mid-level cloud increased the UVB/G ratio more than for the other cloud classes (1–2, 3–4, and 5–6octas). In contrast, under high-level cloud, the average UVB/G ratio of the 1–2octas cloud class was greater (7.56%) than for the 7–8octas cloud class. The average UVB/G ratio under low-level cloud was 1.1 and 1.2 times that under high- and mid-level cloud respectively. Finally, an assessment of the UVB/G ratio under all cloud conditions was performed. This ratio was averaged according to a narrow range of sky clearness, ε (the range is equal to 0.1), under all cloud conditions. The results showed that the average UVB/G ratio reached a maximum under cloudy sky conditions (0.31±0.02%), which represented 1.08 times the average UVB/G ratio under cloudless sky conditions.

Comparative analysis of cloud effects on ultraviolet-B and broadband solar radiation: Dependence on cloud amount and solar zenith angle

The role played by clouds in atmospheric radiative transfer is fundamental. However, studies of their influences on solar radiation at Qena, Egypt, are lacking. This paper discusses the effect of clouds on the ratio of ultraviolet-B (UVB, 295–2800nm) to broadband solar radiation (G, 280–320nm) under all cloud conditions. To achieve this aim, the dependence of cloud modification factors for both UVB and G (CMFUVB and CMFG respectively) on cloud amount (CA) was compared for each category. From 2000 to 2009, the dataset used included hourly values for both UVB and G under all cloud conditions. The results indicated that the average UVB/G ratio for all cloud conditions was 0.25±0.10%. This average represents 76% of the corresponding value under cloudless sky conditions (0.33±0.11%). Accordingly, it indicates that clouds do not transmit UVB and G equally. However, on average, the percentage decrease in CMFUVB from its corresponding value of CMFG was 20±5%. Further analysis for the hourly values of both CMFUVB and CMFG was employed. These hourly values are distinguished by three different behaviors: CMFUVB≌CMFG (9% of all cases), CMFUVB>CMFG (11% of all cases), or CMFUVB<CMFG (80% of all cases). The average UVB/G values for each group were 0.27±0.09%, 0.33±0.12%, and 0.23±0.09% respectively. Based on the results of earlier studies, an attempt was made to explain these outcomes. In a more thorough study of CMFUVB and CMFG, the dependence of these variables on SZA was investigated. This analysis has shown that both CMFUVB and CMFG decrease as the SZA increases. For each group, the sensitivities of CMFUVB to SZA were −0.27±0.03, −0.32±0.06, and −0.18±0.03 (respectively); moreover, the sensitivities of CMFG to SZA were −0.27±0.03, −0.65±0.05, and −0.18±0.02 (respectively).

The influence of sky conditions on the standardized calibration of pyranometers and on the measurement of global solar irradiation

Global solar irradiance measurement is used for applications such as verifying satellite measurements, studying the distribution and variation of the radiation received or estimating the energy yield of solar plants. This measurement is directly influenced by the reference calibration coefficient and uncertainty of the field pyranometer used. The radiometer calibration laboratory of CENER is accredited by ENAC (Spanish National Accreditation Body) since 2010 according to ISO 9847:1992 for the calibration of field pyranometers via comparison to a reference pyranometer. The accreditation’s scope covers calibrations “type Ia” (outdoor horizontal calibrations for meteorological and resource measurements, part 5.2.2, annex B.2). For the outdoor calibration, the methodology takes three different sky conditions into account: stable cloudless sky conditions, unstable sky conditions with some clouds or cloudy sky conditions. The classification criteria described by the standard have been enhanced for an easier application, establishing additional radiometric conditions, especially for unstable sky conditions with some clouds. Detailed criteria are based on CENER’s experience in solar collector testing and on the experimental CENER BSRN station (Baseline Surface Radiation Network) measurements and total sky camera images. In this study, several calibration coefficient results have been analyzed in order to validate the three different methodologies regarding the sky conditions mentioned before. The calibration coefficient can be slightly different depending on the sky conditions and the calibration methodology; therefore global solar irradiance measurements can differ too. The influence of this dependency has been evaluated in order to estimate the monthly and yearly global solar irradiation.

Reconstruction of daily ultraviolet radiation for nine observation stations in China

The aim of this study was to reconstruct daily ultraviolet (UV) radiation (daily cumulation, similarly hereinafter) for nine observation stations in China, using a radiative transfer model combined with empirical relationships. Measurements of global total solar (G) radiation, UV radiation, and their ratio (UV/G) at the nine stations were analyzed for the period 2006–2011. The daily cloud and aerosol attenuation ratios were defined to quantitatively evaluate the effect of cloud and aerosols on radiation, based on measurements and simulated data under cloudless-sky conditions. A semi-empirical reconstruction method for UV radiation was established at each station using cloud modification factors, and by applying an equation that relates G and UV radiation measurements, as well as their corresponding simulations under cloudless conditions. Daily UV radiation values were reconstructed and compared with ground-based measurements for the nine stations, and the results revealed most of the slope values to be close to one, and the determination coefficients ranged from 89.5 to 95.7 %. The values of mean absolute bias error (MABE) and root-mean-square error (RMSE) were below 15 and 18 % for all stations, respectively, and at most stations were even lower than 10 %. A comparison of monthly measured and reconstructed UV radiation for the nine stations was also performed, revealing the values of MABE and RMSE to be even smaller with values of less than 5 %. According to these results, we conclude that the method established in this study has good applicability and transferability.

Geographically weighted regression of the urban heat island of a small city

Despite differences in regional climates, cities world-wide have developed one common characteristic - the urban heat island (UHI). Its magnitude is related to city size, especially under cloudless sky conditions on a regional basis, although individual cities may be impacted by such local factors as proximity to large water bodies or prevailing winds. The UHI pattern in the small city of Ljutomer was examined in order to assess its intensity and morphology and to test the utility of the geographically weighted regression (GWR) method in modeling the regression relationships between mean air temperature and related influence factors in this small-scale urban example. Significant differences in mean air temperature between urban and rural areas were measured. It turned out that built-up areas in Ljutomer are on average 1 °C warmer than the rural surroundings in winter time. The regression analyses confirmed the important role of local non-stationary explanatory variables - distance to urban area, topographic position index and land-cover diversity - and global stationary variables - building volume per area and northness - in explaining spatial variation in mean air temperature. The relationships between mean air temperature and these five explanatory variables produced an overall model fit of 91%, utilizing the semiparametric GWR method, which was tested on the smallest scale so far published.

The Influence of Sky Conditions on the Standardized Calibration of Pyranometers and on the Measurement of Global Solar Irradiation

Global solar irradiance measurement is used for applications such as verifying satellite measurements, studying the distribution and variation of the received radiation or estimating the energy yield of solar plants. This measurement is directly influenced by the reference calibration coefficient of the field pyranometer used.The radiometer calibration laboratory of CENER is accredited by ENAC (Spanish National Accreditation Body) since 2010 according to ISO 9847:1992 for the calibration of field pyranometers via comparison to a reference pyranometer. The accreditation's scope covers calibrations “type Ia” (outdoor horizontal calibrations for meteorological and resource measurements, part 5.2.2, annex B.2). For the outdoor calibration, the methodology takes different sky conditions into account: stable cloudless sky conditions, unstable sky conditions with some clouds or cloudy sky conditions.The classification criteria described by the standard have been enhanced by CENER for an easier application, establishing additional radiometric conditions, especially for unstable sky conditions with some clouds. Detailed criteria are based on CENER's experience in solar collector testing and on the experimental CENER BSRN station (Baseline Surface Radiation Network) measurements and total sky camera images. During laboratory activity, several calibration coefficient results have been analyzed in order to validate the three methodologies regarding the before mentioned sky conditions.The calibration coefficient can be slightly different depending on the sky conditions and the calibration methodology; therefore global solar irradiation measurements can differ too. The influence of this dependency has been evaluated in order to estimate the monthly and yearly global solar irradiation.

Estimation of Daily Ultraviolet Radiation in Beijing Using a Semiempirical Method

This study proposes a semiempirical method to reconstruct daily ultraviolet (UV) radiation from global solar (G) radiation measurements using a radiative transfer model. The attenuation ratio and cloud modification factors are calculated based on measured and simulated data under cloudless-sky conditions. A reconstruction method of UV radiation is established using cloud modification factors; based on comparisons among reconstructions and measurements, the reconstruction model is demonstrated to offer high resolution. The bias errors for daily measured and reconstructed UV radiation are maintained within ±20%, the mean absolute bias error (mabe) is 7.7% and the root mean square error (rmse) is 9.7%. Furthermore, the model performance and transferability were tested by comparison with a simple empirical model in Beijing, Eerduosi and Hailun. A comparison of the measured and estimated UV values for the two methods in the aforementioned three locations revealed that smaller mabe and rmse were observed in our method, with both of these values in the three locations being less than 14%. Thus, a better applicability and transferability has been confirmed. The results and analysis should contribute to improving the knowledge about actual UV climate characteristics.

Selection of cloudless sky conditions by applying solar global ultraviolet irradiance measurements

Taking advantage of UV (295–385nm) irradiance measurements is one of the objectives of this paper. A new index termed kt″ is established for this band. This new index works as a ‘zenith angle independent clearness index’ for band measurements and has similar applications to those of kt′ for broadband measurements. The new index may be applied to identify cloudless instants from UV band measurements. Both indexes were correlated throughout the period 1998–2004 with a R2 of 0.85. A selection criterion of kt″UV >1.1 classified cloudless sky conditions with a probability of 95% in comparison with a selection that two criteria-applying broadband measurements would make. This index may be of interest for classifying cloudless sky conditions when only UV band measurements are available. An estimation method from the literature was applied to the period 1998–2004. This method was previously validated for the UV band with a measurement campaign made in Valencia (Spain) in the summer season.

Impact of loading displacements on SLR-derived parameters and on the consistency between GNSS and SLR results

Displacements of the Earth’s surface caused by tidal and non-tidal loading forces are relevant in high-precision space geodesy. Some of the corrections are recommended by the international scientific community to be applied at the observation level, e.g., ocean tidal loading (OTL) and atmospheric tidal loading (ATL). Non-tidal displacement corrections are in general recommended not to be applied in the products of the International Earth Rotation and Reference Systems Service, in particular atmospheric non-tidal loading (ANTL), oceanic and hydrological non-tidal corrections. We assess and compare the impact of OTL, ATL and ANTL on SLR-derived parameters by reprocessing 12 years of SLR data considering and ignoring individual corrections. We show that loading displacements have an influence not only on station long-term stability, but also on geocenter coordinates, Earth Rotation Parameters, and satellite orbits. Applying the loading corrections reduces the amplitudes of annual signals in the time series of geocenter and station coordinates. The general improvement of the SLR station 3D coordinate repeatability when applying OTL, ATL and ANTL corrections are 19.5 %, 0.2 % and 3.3 % respectively, w.r.t. the solutions without loading corrections. ANTL corrections play a crucial role in the combination of optical (SLR) and microwave (GNSS, VLBI, DORIS) space geodetic observation techniques, because of the so-called Blue-Sky effect: SLR measurements can be carried out only under cloudless sky conditions—typically during high air pressure conditions, when the Earth’s crust is deformed, whereas microwave observations are weather-independent. Thus, applying the loading corrections at the observation level improves SLR-derived products as well as the consistency with microwave-based results. We assess the Blue-Sky effect on SLR stations and the consistency improvement between GNSS and SLR solutions when ANTL corrections are included. The omission of ANTL corrections may lead to inconsistencies between SLR and GNSS solutions of up to 2.5 mm for inland stations. As a result, the estimated GNSS–SLR coordinate differences correspond better to the local ties at the co-located stations when applying ANTL corrections.

Effect of Stratospheric Ozone in UVB Solar Radiation Reaching the Earth’s Surface at Qena, Egypt

This study describes the relationship between UVB and total ozone column (TOC) at a subtropical location through the period from 2001 to 2005. To analyze the relationship between TOC and UVB, the dimensionless parameter UVB transmission (KtUVB) and slant total ozone column (Z) were estimated. The results show an opposite seasonal behavior for KtUVB and Z in all sky conditions at Qena, Egypt (26.200N, 32.750E, 96m above sea level). In order to quantify the UV transmission variations produced by slant path ozone changes (Z), a linear regression between these two variables for cloud–free cases were employed. The correlation (r2) of this relationship was equal to 0.7 for the daily values. In addition, this value (r2) was increased for the monthly average (0.8). This study led to the values of UVB Radiation Amplification Factor (RAF) for the daily and monthly mean values in cloudless sky conditions that were equal to 1.0014 and 1.05, respectively.

Validation of a method to estimate direct normal irradiance of UVA and PAR bands from global horizontal measurements for cloudless sky conditions in Valencia, Spain, by a measurement campaign

A method is proposed to provide measurement of direct normal solar irradiance of bands with wavelength ranges (315–400 nm, 400–700 nm) from measurements of global horizontal band irradiance for cloudless sky conditions in Valencia. Global and normal direct irradiance data for every air mass were obtained by applying the SMART2 model to the atmosphere of Valencia. The direct normal to global irradiance ratio was parameterized versus the relative optical air mass. A measurement campaign of global horizontal and diffuse irradiance of UVA and PAR bands was carried out in Valencia, after which, the inferred direct normal irradiance was compared with those provided by the method. The result of the comparison shows that the method is acceptably accurate. The proposed model tends to underestimate the direct normal irradiance of the UVA band by 6%, although for values below 25 W/m2 the model overestimates the direct irradiance by 6%, while for values above 25 W/m2 the model underestimates it by 10%. The other two error estimators used ranging from 11% to 15% are similar in the defined interval measurements in relation to the whole UVA band. Regarding the PAR band, the model overestimates the direct normal irradiance of the PAR band by only 2.2%. With this, the results of the PAR band are more conclusive, as it has been found that for direct normal irradiance values higher than 280 W/m2 the MBE error is almost zero and the other two estimator errors are small, about 5%.

Measurement and modelling of global erythemal irradiance on inclined planes

The aim of this paper is the characterization of global erythemal irradiance (UVER) on inclined planes. Different geometric models have been studied, both isotropic and anisotropic, which have been used to estimate the global UVER on inclined planes at 40 in North, South, East and West orientations. This has led to the hypothesis that these models, all of them originally developed to obtain diffuse irradiance in solar spectrum, can be applied in a much more limited range of UVER. The results have been compared with experimental data using the following statistical parameters: mean bias deviation, mean absolute deviation and root mean square deviation. The global UVER was analyzed for all sky conditions and for cloudless sky conditions, and no significant differences were found between the different models in both cases. Overall, the best performing model is Gueymard’s anisotropic model, even though it improves the Isotropic model in less than 2%.

Analysis of atmospheric turbidity levels at Taichung Harbor near the Taiwan Strait

In this study, two universal turbidity parameters, the Angstrom turbidity coefficient and Linke turbidity factor, are applied to study the atmospheric turbidity characteristics of Taichung Harbor. Meteorological parameter values were measured during 2004 and 2005 at the Wuchi weather station of the Taiwan Central Weather Bureau, near the Taiwan Strait. Results based on the Angstrom turbidity models ( β Lou, β Pin, and β Vis) indicated that annual mean values of the Angstrom turbidity coefficients were 0.174, 0.21 and 0.201, respectively. Four sets of Linke turbidity factors ( T Lin, T Lou, T Pin and T Vis) were calculated using the original Linke method and the Dogniaux method, incorporating the computed Angstrom turbidity coefficients ( β Lou, β Pin and β Vis); the resultant values were 4.30, 6.40, 7.10 and 6.95, respectively. The monthly average values, frequency of occurrence, and cumulative frequency distributions were calculated using different models to describe the clear-sky atmospheric conditions at Taichung Harbor. The frequency results show that for over 50% of the dataset, three sets of Angstrom turbidity coefficients fell between 0.15 and 0.18, and four sets of Linke turbidity factors ( T Lin, T Lou, T Pin and T Vis) fell between 4.0 and 6.5. Thus, for 50% of cloudless days, the sky can be between turbid and clear over Taichung Harbor. Furthermore, the results reveal that for 30% of the dataset, three Angstrom sets of turbidity coefficients ( β Lou, β Pin, and β Vis) exceed 0.2 and four sets of Linke turbidity factors ( T Lin, T Lou, T Pin and T Vis) exceed 5.0. This indicates that 30% of cloudless sky conditions can be considered turbid to very turbid.