Abstract
High quality data of surface radiation is a prerequisite for climate monitoring (Earth radiation budget) and solar energy applications. A very common method to derive solar surface irradiance is the Heliosat method, a one channel approach for the retrieval of the effective cloud albedo (CAL). This information is then used to derive the solar surface irradiance by application of a clear sky model. The results of this study are based on radiative transfer modelling, visual inspection of satellite images and evaluation of satellite based solar surface radiation with ground measurements. The respective results provide evidence that variations in Aerosol Optical depth induced by desert storms and biomass burning events lead to a significant increase of the effective cloud albedo, thus, that certain aerosol events are interpreted as clouds by the method. For the estimation of the solar surface radiation aerosol information is needed as input for the clear sky model. As the aerosol effect is partly considered by CAL, there is a need to modify external aerosol information for the use within the clear sky model, e.g., by truncation of high aerosol loads. Indeed, it has been shown that a modified version of the Monitoring Atmospheric Composition and Climate (MACC) aerosol information leads to better accuracy of the retrieved solar surface radiation than the original MACC data for the investigated 9 sites and time period (2006–2009). Further, the assumption of a constant aerosol optical depth of 0.18 provides also better accuracies of the estimated solar surface radiation than the original MACC data for the investigated sites and period. It is concluded that this is partly due to the consideration of scattering aerosols by the effective cloud albedo.
Highlights
The solar surface irradiance (I) is an important variable for climate monitoring and analysis (e.g., [1]), and is widely used, beside others, in agrarmeteorology and solar energy applications (e.g., [2]).Satellite based solar surface radiation is the primary observational source of information in many regions of the world due to the low spatial coverage of well maintained ground measurements.Nowadays, the accuracy of satellite based solar surface radiation data is close to that of ground measurements (e.g., [3]).Aerosols affect the solar surface radiation significantly (e.g., [4])
The results show that aerosol information used within the clear sky model has to be modified in order to account for the aerosol effect on the cloud albedo (CAL) retrieval
This study provides evidence that the effect of temporal and spatial aerosol variability on solar surface irradiance is partly considered by the retrieval of the effective cloud albedo performed within the Heliosat method [2,3,9], e.g., dust storms and biomass burning events are interpreted as clouds, leading to an increase of the effective cloud albedo
Summary
The solar surface irradiance (I) is an important variable for climate monitoring and analysis (e.g., [1]), and is widely used, beside others, in agrarmeteorology and solar energy applications (e.g., [2]). It has been briefly discussed that high aerosol loads (e.g., desert storms, biomass burning) might be considered as clouds by the Heliosat method [2,3,9]. This would have significant consequences for the interpretation of the effective cloud albedo and the optimal aerosol information. Several key producer use either the Heliosat approach or the satellite derived cloud albedo as input to estimate the solar surface irradiance, e.g., the Climate Monitoring Satellite Application Facility (CM SAF) [10], Satellight [11], Solar Radiation Data (SODA) [12], PVGIS [13], Land Surface Analysis. SAF (LSA SAF) [14] and the Ocean and Sea Ice SAF (OSI SAF) [15] to mention some examples
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