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FMI's researchers publish about 350 peer-reviewed articles annually.

In Science News we publish current information about FMI's studies on the weather, the sea and the climate.

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,

Räisänen, P. and A. V. Lindfors, 2019: On the computation of apparent direct solar radiation. J. Atmos. Sci., 76, 2761–2780,

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,, 2019.

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