Abstract
Accurate solar surface irradiance data (SSI) is a prerequisite for efficient planning and operation of solar energy sys- tems. Respective data are also essential for climate monitoring and analysis. Satellite-based SSI has grown in importance over the last few decades. However, a retrieval method is needed to relate the measured radiances at the satellite to the solar surface irradiance. In a widespread classical approach, these radiances are used directly to derive the effective cloud albedo (CAL) as basis for the estimation of the solar surface irradiance. This approach has been already introduced and discussed in the early 1980s. Various approaches are briefly discussed and analyzed, including an overview of open questions and opportunities for improvement. Special emphasis is placed on the reflection of fundamental physical laws and atmospheric measurement tech- niques. In addition, atmospheric input data and key applications are briefly discussed. It is concluded that the well established observational-based CAL approach is still an excellent choice for the retrieval of the cloud transmission. The coupling with Look-Up-Table based clear sky models enables the estimation of solar surface irradiance with high accuracy and homogeneity. This could explain why, despite its age, the direct CAL approach is still used by key players in energy meteorology and the climate community. For the clear sky input data it is recommended to use ECMWF forecast and reanalysis data.
Highlights
The surface solar irradiance (SSI) is defined as the incoming solar radiation at the surface in the 0.2–4.0 μm wavelength region 15 on a given surface area, and is usually expressed in W/m2 (Watts per square meter)
The effective cloud albedo CAL, reffered to as cloud index ore effective cloud fraction, can be retrieved directly from 745 the radiances observed by the weather satellites without the need of any modelling or external informaion (Mueller et al, 2011)
The respective CALSAT approach was initially developed by Möser and Raschke (1984); Cano et al (1986) and Diekmann et al (1988)
Summary
The surface solar irradiance (SSI) is defined as the incoming solar radiation at the surface in the 0.2–4.0 μm wavelength region 15 on a given surface area, and is usually expressed in W/m2 (Watts per square meter). For any application requiring direct irradiance, satellite data are the main observational source, in regions with a dense network of SSI measurements. Satellite based solar surface radiation data sets are available from different sources all over the world. 95 SAF develops, processes and distributes near real-time products related to the ocean-atmosphere interaction This includes, scatterometer winds, sea ice concentration, sea surface temperature, as well as radiation products, which are generated over land. A main goal of ISCCP is the generation of satellite based global cloud data for the monitoring and analysis of the global distribution of clouds, their properties, and their diurnal, seasonal and inter-annual variations. The main data source in the US is NREL (Cox et al, 2018; NREL1)
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