UV radiation in the Netherlands: Assessing long‐term variability and trends in relation to ozone and clouds

[1] The variability and long-term changes in the ultraviolet (UV) climate in the Netherlands have been studied in relation to ozone and clouds, by analyzing modeled and measured values for daily, monthly, and yearly integrated erythemally weighted UV doses. At Bilthoven, Netherlands (longitude 5.19°E, latitude 52.12°N), UV irradiance measurements for the 1994–2003 period yielded a mean annual dose of 447 ± 29 kJ/m2 and a mean daily dose of 2.5 ± 0.5 kJ/m2 for June and July. On average, the maximum UV index exceeded 6.5 (i.e., 0.1625 W/m2 erythemally weighted) on 10 days per year (21 days in 2003). The mean value of measured-to-modeled ratios of erythemal UV irradiances was 1.00 with a standard deviation of 0.06 for days when the measured global solar radiation agrees within 5% with the cloudless sky value. Three previously introduced approaches to model cloud effects on UV doses were shown to have limitations when applied for low Sun and/or optically thick clouds, while a new approach provided the most consistent results with an average ratio of the measured-to-modeled daily doses of 1.02 and a standard deviation of 0.09, for all seasons and weather conditions for the period 1994–2002. Further analysis also revealed a wavelength dependency of the correlation between global solar radiation and UV radiation. Clouds, on average, reduced the daily dose of erythemal UV to 68% of the clear-sky value, whereas for global solar radiation this was 57%. The modeled annual erythemal UV dose was 622 kJ/m2 (402 kJ/m2) averaged over the years 1979–1982, while the years 2000–2003 yield 662 kJ/m2 (448 kJ/m2) for cloudless (cloudy) conditions. In the past 25 years the highest annual doses were received in 1995 (485 kJ/m2) and 2003 (488 kJ/m2): in 1995 as a result of extremely low ozone values and moderate cloud reduction and in 2003 as a result of extremely low cloud reduction combined with moderately low ozone values. As an indication of the changes over time, a linear regression is performed showing that the annual UV dose received at the ground for all weather conditions increased with 5.5 ± 2% per decade for erythemal UV over the 1979–2003 period.