13 th World Renewable Energy Congress (WREC XIII), London, UK August 3-8, 2014 Abstract—This paper presents empirical regression models for estimating the monthly average daily global horizontal irradiance in the capital city of Abu Dhabi, United Arab Emirates (UAE) based on nine-year measurement data. It also addresses selected models available in the literature for estimation of diffuse horizontal irradiance and their correlation to measured data. The effect of atmospheric turbidity, due to humidity, sandstorms and aerosols, on the solar irradiance is also addressed. Index Terms—Empirical model, clearness index, global horizontal irradiance, diffuse horizontal irradiance, diffuse fraction, turbidity factor I. INTRODUCTION Solar radiation received at the surface is of primary importance for the purpose of building solar energy systems, crop productivity and soil erosion studies, and hydraulics. Empirical and satellite-derived weather models are a viable alternative in the absence of the measuring stations in the regions where solar data are required. El Chaar and Lamont [1] performed measurements for the global horizontal irradiance (GHI) in five locations of the Abu Dhabi Emirate for 2008. They also measured the clearness index between 0.14 (minimum value January) and 0.803 (May) with the monthly average clearness index in the range 0.6- 0.7 throughout the year. They indicated that winter season (Dec-Jan-Feb) observes the lowest radiation due to dust storms, and scattered muddy showers. Spring season exhibits the highest radiation due to the pleasant ambient temperature and acceptable humidity levels. The high temperatures and humidity levels near the coastal areas, where 80 % of the population resides, may cause a decrease in GHI due to higher turbidity factor resulting from the high temperature, humidity levels, and effect of aerosols and dust particles. Abdalla and Feregh [2] performed GHI measurements, in Abu Dhabi, for 10 years (1971-1980) to establish estimation models. The maximum monthly mean daily GHI measured during the 10-year period is 6.55 kWh/m 2 during May which corresponds to a clearness index of 0.6, and with a mean maximum sunshine hours per day of 11.3 hours in May and June. The lowest values of GHI and sunshine hours being in January with 3.38 kWh/m 2 and 7.6 hours, respectively. They employed empirical models by Page [3] and Liu/Jordan [4] to estimate the diffuse horizontal radiation in the absence of measurement data. Elnashar [5] conducted global (GHI) and diffuse horizontal irradiance (DHI) measurements in Abu Dhabi during 1987. The author reported that the estimated instantaneous values of clearness index, diffuse radiation, atmospheric transmittance and extinction coefficient are strongly dependent on the air mass and month of the year. It was noted that both the clearness index and diffuse fraction have diurnal and seasonal variations, with higher values in summer than in winter season. The clearness index reaches its lowest value in July and the highest value in January. This was attributed to the fact that the level of fine dust particles in the air and relative humidity are higher during summer than winter months. Elnashar suggested an improvement to the models of Page and Lui/Jordan by incorporating the effect of air mass in the empirical formulas. More recently, Masdar Institute researchers [6] have performed corrections to the Heliosat-2 model in order to adapt it to the climatic environment of the Arabian Peninsula which is characterized by high concentrations of airborne dust particles and high humidity levels. The authors benchmarked remote estimation of GHI by Heliosat-2 model against ground-based measurements in different locations of the emirate of Abu Dhabi. They noted higher turbidity levels in the summer months (May- July) for inland ground-measurement stations compared to the near-coastal stations. The high values of turbidity are attributed to high dust generation in the desert areas which experience a hot and dry summer. The annual average daily sunshine hours and clearness index for Abu Dhabi are 9.87 h and 0.66, respectively. This data was derived from the measured NCMS data for years 2002-2008. The data indicates that Abu Dhabi enjoys clear skies for most of the year with lower values of clearness index resulting during rain fall or sandstorms. The monthly mean of the daily sunshine hours is similar all over the Emirate of Abu Dhabi, UAE, and ranges between 8.4 h in winter, to 11.6 h in summer (daily average of 9.7 h), which corresponds to an average annual solar radiation of approximately 2300 kWh/m 2 , i.e. 6.3 kWh/m 2 per day. Referring to the climatic atlases published by the WMO, the Arabian Peninsula and the African desert are the two areas that receive the highest amount of the incoming solar radiation [8]. Section II presents a comparison between measured data available from ground stations, literature, and satellite website databases. Section III addresses the estimation models for GHI and DHI and results discussion followed by conclusions. II. COMPARISON OF THE AVAILABLE METEOROLOGICAL DATA Estimation of Global and Diffuse Horizontal Irradiance in Abu Dhabi, United Arab Emirates Hassan A. N. Hejase, Department of Electrical Engineering, United Arab Emirates University, P.O. Box 15551, Al-Ain, UAE and Ali H. Assi, Department of Electrical and Electronic Engineering, Lebanese International University, Beirut, Lebanon