“Comparison of measured and modelled spectral UV irradiance at Izaña high mountain station: estimation of the underlying effective albedo” paper published in International Journal of Climatology.

In this work, we have carried out a comparison study between measured and modelled direct and global ultraviolet (UV) (300–400 nm) spectral irradiances and the determination of the effective surface albedo at the high altitude Izaña Observatory [IZO; 2400 m above sea level (a.s.l.)] located in Tenerife (Canary Islands, Spain). The spectral measurements were performed with a Bentham spectroradiometer during the Quality Assurance of Solar Ultraviolet Spectral Irradiance Measurements (QASUME) comparison campaign in June 2005. The simulations were obtained with the LibRadtran radiative transfer model. The model input parameters, such as total ozone and aerosol optical depth (AOD), were measured at IZO. The comparison between measured and modelled direct solar radiation component was made at 0.5 nm spectral resolution, showing excellent agreement, with differences below 5% for solar zenith angle (SZA)<60° (see Figure 1) and 10% for global radiation. These differences are mainly attributed to surface albedo due to the presence of sea of clouds, in particular, its extent and location in relation to the altitude of the Izaña station (Figure 2). The effective surface albedo was determined by using two methods: 1) Kylling’s method and 2) a Lookup-Table (LUT) performed with the LibRadtran model. The results show that the surface albedo depends on the sea of clouds coverage around IZO (see Figure 2 and Table 1).


Figure 1.- Comparison between UV spectral direct irradiance measurements (the black line) and the simulations performed with LibRadtran model (the grey line) on 5 June 2005 at (a) SZA 5.8° [13 GMT, AOD (340 nm) 0.11] (b) SZA 26.7° [15 GMT, AOD (340 nm) 0.12] and (c) SZA 65.9° [18 GMT, AOD (340 nm) 0.07].


Figure 2.-  High resolution of visible images from MSG (Meteosat Second Generation) over Tenerife on 9 June 2005 at 08:00, 10:00, 12:30, 14:00, 16:00 and 18:00 UTC. The white line marks the Tenerife Island contour and the white point indicates the IZO location.

Table 1.- Summary of surface albedo obtained from the Kylling’s method and Lookup-Table (LUT) based on radiative transfer models (RTM) on 9 June 2005 at 08:00, 10:00, 12:30, 14:00, 16:00 and 18:00 UTC (±SEM as standard error of the mean).

The article reference is:

García, R.D., Cachorro, V.E., Cuevas, E., Toledano, C., Redondas, A., Blumthaler, M., and Benounna, Y.: Comparison of measured and modelled spectral UV irradiance at Izaña high mountain station. Estimation of the underlying effective albedo, International Journal Climatology, DOI: 10.1002/joc.4355,2015.

It can be downloaded at:  http://onlinelibrary.wiley.com/doi/10.1002/joc.4355/pdf

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