Assessment of Three Satellite-Estimated Land Surface Downwelling Shortwave Irradiance Data Sets

Abstract

This letter assesses three satellite-estimated surface downwelling shortwave irradiance data sets: 1) GEWEX-SRB; 2) ISCCP-FD; and 3) CERES-FSW, using ground measurements collected at 36 globally distributed sites from 2000–2002. SRB and FD solar irradiance are available at three hourly intervals during daytime and FSW hourly solar irradiance is available at late morning. The data are compared to ground measurements at the temporal resolutions of the satellite measurements. Results indicate that the downwelling solar irradiances of the three products show good overall agreement with ground measurements. FSW has an overall coefficient of determination of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$R^{2} = \hbox{0.69}$</tex></formula> , a bias of 29.7 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{Wm}^{-2}$</tex></formula> (6.0% in relative value), and a standard deviation (STD) of 123.2 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{Wm}^{-2}$</tex></formula> (25.1% in relative value). The values are 0.83, <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$-5.5\ \hbox{Wm}^{-2}$</tex></formula> ( <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$-$</tex></formula> 1.9%), 101.3 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$ \hbox{Wm}^{-2}$</tex></formula> (35.0%) for SRB, and 0.83, 2.8 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{Wm}^{-2}$ </tex></formula> (0.3%), 101.7 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{W m}^{-2}$</tex></formula> (35.0%) for FD. However, there are substantial uncertainties in these products in some regions. For example, large biases ranging from <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$-90.2\ \hbox{Wm}^{-2}$</tex></formula> to 45.8 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{Wm}^{-2}$</tex></formula> are found in SRB in Southeast Asia. FD has large biases in Southeast Asia (58.9 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{Wm}^{-2}$</tex></formula> ) and Greenland <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{(-33.2}\ \hbox{Wm}^{-2}\hbox{)}$</tex></formula> . FSW has substantial biases in Southeast Asia (72.6 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{Wm}^{-2}$</tex></formula> ) and Japan (66.5 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{Wm}^{-2}$</tex></formula> ); while <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX"> $R^{2}$</tex></formula> is 0.35 in Tibetan Plateau and 0.47 in Southeast Asia.