Trends of solar ultraviolet irradiance at Barrow, Alaska, and the effect of measurement uncertainties on trend detection

Abstract

Abstract. Spectral ultraviolet (UV) irradiance has been observed near Barrow, Alaska (71 • N, 157 • W) between 1991 and 2011 with an SUV-100 spectroradiometer. The instrument was historically part of the US National Science Foundation's UV Monitoring Network and is now a component of NSF's Arctic Observing Network. From these measurements, trends in monthly average irradiance and their uncertainties were calculated. The analysis focuses on two quantities, the UV Index (which is affected by atmospheric ozone concentrations) and irradiance at 345 nm (which is virtually insensitive to ozone). Uncertainties of trend estimates depend on variations in the data due to (1) natural variability, (2) systematic and random errors of the measurements, and (3) uncertainties caused by gaps in the time series. Using radiative transfer model calculations, systematic errors of the measurements were detected and corrected. Different correction schemes were tested to quantify the sensitivity of the trend estimates on the treatment of systematic errors. Depending on the correction method, estimates of decadal trends changed between 1.5 % and 2.9 %. Uncertainties in the trend estimates caused by error sources (2) and (3) were set into relation with the overall uncertainty of the trend determinations. Results show that these error sources are only relevant for February, March, and April when natural variability is low due to high surface albedo. This method of addressing measurement uncertainties in time series analysis is also applicable to other geophysical parameters. Trend estimates varied between −14 % and +5 % per decade and were significant (95.45 % confidence level) only for the month of October. Depending on the correction method, October trends varied between −11.4 % and −13.7 % for irradiance at 345 nm and between −11.7 % and −14.1 % for the UV Index. These large trends are consistent with trends Correspondence to: G. Bernhard (bernhard@biospherical.com) in short-wave (0.3-3.0 µm) solar irradiance measured with pyranometers at NOAA's Barrow Observatory and can be explained by a change in snow cover over the observation period: analysis of pyranometer data indicates that the first day of fall when albedo becomes larger than 0.6 after snow fall, and remains above 0.6 for the rest of the winter, has advanced with a statistically significant trend of 13.6 ± 9.7 days per decade.

Ozone—climate interactions and effects on solar ultraviolet radiation

“…This decrease cannot be attributed to changes in ozone and is largely the result of a longer snow-free period in October. 131 For the period 1995-2014, there is a significant anti-correlation (R 2 = 0.56) between the onset of persistent snow cover and monthly mean UV-A irradiance. Based on data from different satellites, statistically significant (at the 95% level) negative trends in UV reflectivity were found for areas in the Bellingshausen/Amundsen Seas near Antarctica, with sea-ice coverage greater than 30%.…”

“…The dotted horizontal line indicates 1 October. The figure is adapted from Bernhard131 and updated with data from 2010−2016.Surface albedo was measured with pyranometers that are part of the Baseline Surface Radiation Network (BSRN). UV measurements are not available for 2009, 2015, and 2016.…”

Trends in erythemal doses at the Polish Polar Station, Hornsund, Svalbard based on the homogenized measurements (1996–2016) and reconstructed data (1983–1995)

“…Sunshine duration measurements by a Campbell-Stokes instrument seem to be less influenced by deterioration of the instrument's sensitivity, and its calibration is very simple, as during cloudless conditions the sunburn track on a recorded cart should appear throughout the whole day. Bernhard et al (2011) analysis of the monthly trends at Barrow (Alaska) for the period 1990-2010 revealed a statistically significant trend only in October (decline of about −1 % per year) when the UV intensity was rather weak without the erythemal risk.…”

Comparison of OMI UV observations with ground-based measurements at high northern latitudes

“…fAlbedo starts to increase again at the beginning of September, while a eff does not increase before October. Reliable snow coverage at Barrow was typically observed only after mid-October during the last decade (Bernhard, 2011). Su-fAlbedo in September and October is therefore likely too large by up to 0.3.…”