Abstract
Abstract. Broadband short-wave (SW) surface direct and diffuse irradiances are not typically within the set of output variables produced by numerical weather prediction (NWP) models. However, they are frequently requested for solar energy applications. In order to compute them, a detailed representation of the aerosol optical properties is important. Nonetheless, NWP models typically oversimplify aerosol representation or even neglect their effect. In this work, a flexible method to account for the SW aerosol optical properties in the computation of broadband SW surface direct and diffuse irradiances is presented. It only requires aerosol optical depth at 0.55 μm and knowledge of the type of predominant aerosol. Other parameters needed to consider spectral aerosol extinction, namely, Angström exponent, aerosol single-scattering albedo and aerosol asymmetry factor, are parameterized. The parameterization has been tested using the Rapid Radiative Transfer Model for climate and weather models (RRTMG) SW scheme of the Weather Research and Forecasting (WRF) NWP model for data over the continental US. In principle, it can be adapted to any other SW radiative transfer band model. It has been verified against a control experiment and using data from five radiometric stations in the contiguous US. The control experiment consisted of a clear-sky evaluation of the RRTMG solar radiation estimates obtained in WRF when RRTMG is driven with ground-observed aerosol optical properties. Overall, the verification has shown satisfactory results for both broadband SW surface direct and diffuse irradiances. The parameterization has proven effective in significantly reducing the prediction error and constraining the seasonal bias in clear-sky conditions to within the typical observational error expected in well maintained radiometers.
Highlights
Broadband short-wave (SW) surface downward total solar irradiance is the sum of broadband SW surface downward direct normal irradiance (DNI, received from the sun’s direction) projected onto a horizontal plane and broadband SW surface downward diffuse irradiance (DIF, received from other directions)
Surface irradiance is referred to as downward SW flux in other disciplines. Both DNI and DIF are rarely included in predictions made with numerical weather prediction (NWP) models
They are necessary in multiple applications such as those requiring a precise representation of surface solar radiation or solar energy applications (Geiger, 1965; Hay, 1993; Whiteman, 2000; Gu et al, 2002; Oliphant et al, 2003; Pierce et al, 2005; Stoffel et al, 2010; Kleissl, 2013)
Summary
Broadband short-wave (SW) surface downward total solar irradiance ( known as global horizontal irradiance, GHI) is the sum of broadband SW surface downward direct normal irradiance (DNI, received from the sun’s direction) projected onto a horizontal plane and broadband SW surface downward diffuse irradiance (DIF, received from other directions). Surface irradiance is referred to as downward (or downwelling) SW flux in other disciplines Both DNI and DIF are rarely included in predictions made with numerical weather prediction (NWP) models. Under scattered clouds or steep terrain, the isotropy assumption fails In such cases, a 3-D solar radiation model would provide the best predictions (Cahalan et al, 2005; Iwabuchi, 2006; Pincus and Evans, 2009). Traditional flat-photovoltaic (PV) systems – the more mature and widely utilized solar energy technology – are driven primarily by the incoming global irradiance onto the PV plane of array As this plane very rarely coincides with the horizontal plane (the common irradiance output in most of the NWP models), a transposition model from the horizontal to the PV plane is required; accurate transposition models need DNI and DIF irradiances. When AOP observations were used, the mean and root-mean square errors substantially decreased to 2 W m−2 (3 %) and 5 W m−2 (6 %), respectively
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