title: Investigation of the 3-D actinic flux field in mountainous terrain.
abstract: During three field campaigns spectral actinic flux was measured from 290-500 nm under clear sky conditions in Alpine terrain and the associated O3- and NO2-photolysis frequencies were calculated and the measurement products were then compared with 1-D- and 3-D-model calculations. To do this 3-D-radiative transfer model was adapted for actinic flux calculations in mountainous terrain and the maps of the actinic flux field at the surface, calculated with the 3-D-radiative transfer model, are given. The differences between the 3-D- and 1-D-model results for selected days during the campaigns are shown, together with the ratios of the modeled actinic flux values to the measurements. In many cases the 1-D-model overestimates actinic flux by more than the measurement uncertainty of 10%. The results of using a 3-D-model generally show significantly lower values, and can underestimate the actinic flux by up to 30%. This case study attempts to quantify the impact of snow cover in combination with topography on spectral actinic flux. The impact of snow cover on the actinic flux was ~ 25% in narrow snow covered valleys, but for snow free areas there were no significant changes due snow cover in the surrounding area and it is found that the effect snow-cover at distances over 5 km from the point of interest was below 5%. Overall the 3-D-model can calculate actinic flux to the same accuracy as the 1-D-model for single points, but gives a much more realistic view of the surface actinic flux field in mountains as topography and obstruction of the horizon are taken into account.
authors.affiliation: Institute for Meteorology, University of Natural Resources and Life Sciences, Peter-Jordan-Straße 82, A-1190 Vienna, Austria.
authors.forename: J E
authors.initials: JE
authors.lastname: Wagner
authors.affiliation: Institute of Atmospheric Sciences and Climate, Rome, Italy.
authors.forename: F
authors.initials: F
authors.lastname: Angelini
authors.affiliation: Division for Biomedical Physics, Innsbruck Medical University, Innsbruck, Austria.
authors.forename: M
authors.initials: M
authors.lastname: Blumthaler
authors.affiliation: Institute for Meteorology, University of Natural Resources and Life Sciences, Peter-Jordan-Straße 82, A-1190 Vienna, Austria.
authors.forename: M
authors.initials: M
authors.lastname: Fitzka
authors.affiliation: Institute of Atmospheric Sciences and Climate, Rome, Italy.
authors.forename: G P
authors.initials: GP
authors.lastname: Gobbi
authors.affiliation: School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, United Kingdom.
authors.forename: R
authors.initials: R
authors.lastname: Kift
authors.affiliation: Division for Biomedical Physics, Innsbruck Medical University, Innsbruck, Austria.
authors.forename: A
authors.initials: A
authors.lastname: Kreuter
authors.affiliation: Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland.
authors.forename: H E
authors.initials: HE
authors.lastname: Rieder
authors.affiliation: Institute for Meteorology, University of Natural Resources and Life Sciences, Peter-Jordan-Straße 82, A-1190 Vienna, Austria.
authors.forename: S
authors.initials: S
authors.lastname: Simic
authors.affiliation: School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, United Kingdom.
authors.forename: A
authors.initials: A
authors.lastname: Webb
authors.affiliation: Institute for Meteorology, University of Natural Resources and Life Sciences, Peter-Jordan-Straße 82, A-1190 Vienna, Austria.
authors.forename: P
authors.initials: P
authors.lastname: Weihs
keywords: Actinic flux; Monte Carlo Model; Photolysis frequencies; Radiative transfer; Spectroradiometry; Surface albedo; UV radiation