## 1 Introduction  
Morocco declared at the Conference of the Parties (COP21) of the United Nations Framework Convention on Climate Change (UN-FCCC) that it intends to increase its renewable energy capacity to 52% (20% solar, 20% wind, and 12% hydro) by 2030. Between 2018 and 2030, the country expects to install around 10 GW of renewable energy (RE) volume (4.5 GW of solar, 4.2 GW of wind, and 1.3 GW of hydroelectric) [1]. The purpose of this study is to calibrate and validate global horizontal irradiance (GHI) clear sky models in order to contribute to the establishment of a solar resource data bank and solar energy infrastructure in Morocco. Clear sky models are used to estimate solar radiation under clear sky conditions, meaning when there are no visible clouds in the atmosphere, using solar geometry and other atmospheric parameters. Solar energy technologies for photovoltaic (PV) or thermal applications necessitate an evaluation of the available solar energy resource at the selected location [2; 3]. These measurements are frequently unavailable for the targeted areas and must therefore be estimated in clear sky conditions [4; 5; 6; 7]. Even when in situ observations are available, there may be gaps in the observation records that must be filled in order to estimate the solar resource [8; 9; 10]. It is important to know how much solar energy is available at any given location, and clear sky irradiance, specifically GHI, provides that information. This information is essential for predicting or estimating the performance of solar energy technologies [11]. As a result, determining the clear sky GHI at a given location is important for projecting the optimum performance of solar technologies before they are implemented. Various clear sky models with varying complexities of input meteorological factors have been developed and verified throughout time to estimate clear sky GHI for a variety of solar energy systems, particularly in areas where observations are unavailable. The clear sky models are either empirically based [12; 13; 14] or can be broadband [15; 16]. Empirically based irradiance models need geometric parameters, such as solar zenith angle position, and/or key climatological parameters, such as sunshine duration, relative humidity, surface pressure, clearness of the atmosphere, and air temperature as input parameters [17; 18], whereas broadband models need inputs that fully define the atmospheric state in detail, such as aerosol optical depth, amount of precipitable water, and ozone column [4; 19; 20; 21]. Specific atmospheric or meteorological characteristics are not always available or are of low quality, limiting the model outputs' accuracy [21; 22]. GHI at the ground surface can also be accurately estimated using satellite images [23; 24; 25; 26; 27]. The aim of this study is to determine the overall best model to estimate clear sky GHI in Morocco. The models Bird [13; 28], Simplified Solis [29; 30], Ineichen and Perez [31; 32], Berger-Duffie [33], Adnot-Bourges-Campana-Gicquel (ABCG) [34], Haurwitz [35; 36], and SOLCAST [37] were calibrated and validated against McClear clear sky model data [38]. The study's clear sky models were chosen based on the availability of input parameters. The models considered ranged from simple clear sky models (Berger-Duffie, ABCG, and Haurwitz) that require few input parameters (such as the sun's zenith angle \((\theta_{z})\) and the top of the atmosphere's direct normal irradiance (\(DNI_{TOA}\))) to complex clear sky models ([PERSON] and Perez, Bird, Simplified Solis) that require many input parameters in addition to \(\theta_{z}\) and \(DNI_{TOA}\), as summarized in Table 3. The SOLCAST satellite-based dataset was also included in the study because it performed best when validated against GHI observation data such as [PERSON] et al. [39] in South Africa, Yang and Bright [40] globally using Baseline Surface Radiation Network (BSRN) [41] stations, and Bright [42] in all climatic regions.