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Forward scattering of light interferes with measurements of direct solar radiation

Forward scattering of light interferes with measurements of direct solar radiation

A new method was developed for accounting for the near-forward scattered radiation. Also, the impact of near-forward scattering on the retrieval of aerosol optical depth was evaluated.

Ice crystals, cloud droplets and large aerosol particles scatter considerable amounts of solar radiation in near-forward directions. In measurements, near-forward scattered radiation cannot be distinguished from direct (i.e., non-scattered) radiation. This is a complication for comparisons of modelled solar radiation with measurements, and for retrievals of aerosol optical depth. The issue is also relevant for concentrating solar energy applications.

A study conducted at the Finnish Meteorological Institute (FMI) addressed how the amount of scattered radiation depends on the properties of clouds and aerosols and the field-of-view of the instrument used for measurements. A simple method was developed for computing an estimate of the "direct" radiation compatible with measurements, e.g. for use in numerical weather prediction models.

FMI also participated in a study, in which two instruments widely used in retrievals of aerosol optical depth (GAW-PFR and AERONET-Cimel) were compared at the Izana Observatory in Spain. Radiative transfer simulations made at FMI showed that the difference in field-of-view between the two instruments can explain a large part of the differences between optical depths retrieved with the instruments, in cases in which Saharan dust was transported over the measurement site.

More information:

Senior research scientist Petri Räisänen, Finnish Meteorological Institute, tel. +358 29 539 2224, petri.raisanen@fmi.fi

Räisänen, P. and A. V. Lindfors, 2019: On the computation of apparent direct solar radiation. J. Atmos. Sci., 76, 2761–2780, https://doi.org/10.1175/JAS-D-19-0030.1

Cuevas, E., Romero-Campos, P. M., Kouremeti, N., Kazadzis, S., Räisänen, P., García, R. D., Barreto, A., Guirado-Fuentes, C., Ramos, R., Toledano, C., Almansa, F., and Gröbner, J.: Aerosol optical depth comparison between GAW-PFR and AERONET-Cimel radiometers from long-term (2005–2015) 1 min synchronous measurements, Atmos. Meas. Tech., 12, 4309–4337, https://doi.org/10.5194/amt-12-4309-2019, 2019.


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