title: Two methods for retrieving UV index for all cloud conditions from sky imager products or total SW radiation measurements.
abstract: Cloud effects on UV Index (UVI) and total solar radiation (TR) as a function of cloud cover and sunny conditions (from sky images) as well as of solar zenith angle (SZA) are assessed. These analyses are undertaken for a southern-hemisphere mid-latitude site where a 10-years dataset is available. It is confirmed that clouds reduce TR more than UV, in particular for obscured Sun conditions, low cloud fraction (<60%) and large SZA (>60°). Similarly, local short-time enhancement effects are stronger for TR than for UV, mainly for visible Sun conditions, large cloud fraction and large SZA. Two methods to estimate UVI are developed: (1) from sky imaging cloud cover and sunny conditions, and (2) from TR measurements. Both methods may be used in practical applications, although Method 2 shows overall the best performance, as TR allows considering cloud optical properties. The mean absolute (relative) differences of Method 2 estimations with respect to measured values are 0.17 UVI units (6.7%, for 1 min data) and 0.79 Standard Erythemal Dose (SED) units (3.9%, for daily integrations). Method 1 shows less accurate results but it is still suitable to estimate UVI: mean absolute differences are 0.37 UVI units (15%) and 1.6 SED (8.0%).
authors.affiliation: Laboratoire de Météorologie Dynamique (LMD), Ecole Polytechnique, Palaiseau, France.
authors.forename: Jordi
authors.initials: J
authors.lastname: Badosa
authors.forename: Josep
authors.initials: J
authors.lastname: Calbó
authors.forename: Richard
authors.initials: R
authors.lastname: Mckenzie
authors.forename: Ben
authors.initials: B
authors.lastname: Liley
authors.forename: Josep-Abel
authors.initials: JA
authors.lastname: González
authors.forename: Bruce
authors.initials: B
authors.lastname: Forgan
authors.forename: Charles N
authors.initials: CN
authors.lastname: Long