Estimation of atmospheric turbidity over Ghardaïa city

Abstract

International audienceThe atmospheric turbidity expresses the attenuation of the solar radiation that reaches the Earth's surface under cloudless sky and describes the optical thickness of the atmosphere. We investigate the atmospheric turbidity over Gharda¨ıa city using two turbidity parameters, the Linke turbidity factor and the Angstr ¨om turbidity coefficient. Their values and temporal variation are obtained from data recorded between 2004 and 2008 at Gharda¨ıa. The results show that both parameters have the same trend along the year. They reach their maximumaround summer months and their minimum around winter months. The monthly average value varies between 1.3 and 5.6 for the Linke turbidity factor and between 0.02 and 0.19 for the Angstr ¨om turbidity coefficient. We find that 39.8% of the Linke turbidity factor values are less than 3, 47.5% are between 3 and 5 and only 12.7% are greater than 5. For the Angstr ¨om turbidity coefficient, 9.4% of the values are less than 0.02, 75.4% are between 0.02 and 0.15 and 15.2% exceed 0.15

Identification of clear days from solar irradiance observations using a new method based on the wavelet transform

“…We note a maximum of clear days in 2007 for the two methods. This confirms previous results of the Linke turbidity factor obtained from the same data set [8]. We found that it had a lower value during this year.…”

“…We note that this parameter has maximum values during the summer months and minimum ones in winter. This result confirms what was found and stated in our previous paper [8]. It is explained by a hot summer climate and winds of the south sectors (Sirocco) that characterize the region of Ghardaïa.…”

“…We note that the number of clear days is lower during months between July and October. The reason is that the sky in the Ghardaïa region is affected by clouds and sandy winds during these months [8]. The number of clear days per year obtained with the two methods is given in Table 2 and plotted in Figure 5 (right curves).…”

“…The instruments and methods of the data collection are described in detail in Djafer and Irbah [8]. The key to know is that the three components of solar radiation are recorded every 5 minutes since 2004 together with the temperature and humidity.…”

“…First, we present the algorithm based on the k t mean value of the clearness index to distinguish between a clear and a non clear day. Then, we introduce the wavelet transform method that we compare with the k t mean one considering the number of clear days obtained from five years of data recorded at Ghardaïa city in Algeria [8]. Finally, we base our comparison on the errors induced onto the GSR (Global Solar Radiation) model and in the estimation of turbidity parameters.…”