## 2 Materials and Methods
### Quality Control Procedure
#### 2.5.1 Time Series Plots  
Before proceeding with the quality control routine, a visualization of the time series is required. The graphical presentation of the data gives us the opportunity to detect any major problem with the data. It is possible to examine the type of the data we are dealing with, to identify their trends and to easily examine them in order to check if there are any missing values, lags or spikes. The time series plots of the solar shortwave and Total Ultraviolet (UVT) radiation components for the period June 2020 to May 2021 are shown in Figure 4, without any abnormal values.  
Figure 3: Relationship between the daily clearness index (\(K_{T}\)) and relative sunshine duration (\(\sigma\)) which define the four classes of sky conditions.  
By analyzing these plots, we can notice the extreme values of the measured meteorological and radiation parameters. The highest temperature reached the value of 44.8 \({}^{\circ}\)C which was recorded on the 4 th of September of the year 2020, while the lowest was slightly below 0 \({}^{\circ}\)C (\(-\)0.01 \({}^{\circ}\)C). The highest value of UVT irradiance is slightly above 65 W/m\({}^{2}\) which is occurred in the summer months.  
The highest direct normal irradiance was 968 W/m\({}^{2}\), while the highest values of diffuse irradiances were recorded in spring of the year 2021, when frequent events of dust are observed. Generally, diffuse irradiances are lower than 700 W/m\({}^{2}\) which is the upper threshold given in the literature [5]. Extreme values were recorded for global irradiance reaching values around 1400 W/m\({}^{2}\). These values occur during partially cloudy conditions, when total irradiance may be enhanced due to the reflection of solar irradiance from the base of the clouds and from scattering of direct irradiance due to cloud particles. Such periods of enhanced solar irradiance may last from several seconds to some minutes depending on the cloud motion. [PERSON] and [PERSON] (2014) [40] have detected values of global irradiance exceeding 1500 W/m\({}^{2}\) in Cyprus, that correspond to 150% of the theoretical value computed by clear sky models. An example of such partially cloudy conditions is recorded on the 10 th of April 2021. The daily sunshine duration on that day was about 2 h and the highest global irradiance reached the value of 1426 W/m\({}^{2}\). Similar cases were observed in more than 10 days of April, May and June when the global irradiance exceeded 1200 W/m\({}^{2}\) (enhanced by 125% compared to the theoretical value of clear sky conditions). Such situations were also observed in various areas of the world [40].  
The maximum reflected irradiance is 255 W/m\({}^{2}\), while maximum \(GPAR\) is 470 W/m\({}^{2}\). The maximum value of Near Infrared Radiation (\(NIR\)) is about 710 W/m\({}^{2}\). \(NIR\) is estimated from the difference of global and the sum of \(UVT\) and \(GPAR\) radiation components:  
\[NIR=G-(UVT+GPAR) \tag{16}\]  
Figure 4: Time series plots of solar shortwave radiation components during the period June 2020–May 2021.  
In order to deepen the visual analysis of the data presented in the previous paragraph, we display the measurement in two-dimensional representation, where the \(x\)-axis represents the day of the year, the \(y\)-axis the hour of the day, and the dots color demonstrate the value of the data considered as shown by the color scale from blue to red. This analysis allows visual identification of errors over time, to identify missing values, issues in time reference and abnormal values (Figure 5). In the different cells of Figure 5, the two black lines represent the theoretical sunrises and sunsets. The figure shows no abnormal conditions for global radiation. Similar patterns are also observed for the other variables. The plausibility analysis of the radiation parameters will be carried out in the next section as part of the QC of the measurements.