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Arctic Research Program: Seasonal variation of the noontime UV Index for 12 Arctic and sub-Arctic sites measured by ground-based radiometers. Entry ID:
Seasonal variation of the noontime UV Index for 12 Arctic and sub-Arctic sites measured by ground-based radiometers. The upper panel for each site compares the climatological average (blue line) with the measurements in 2010 (green dots) and 2011 (red dots), and historical minima and maxima (shaded range). The latter were calculated from measurements of the periods indicated in the top-right corner of the panel. The center panel shows the anomaly in the UV Index, calculated as the percentage departure from the climatological average. The numbers indicate the maximum anomalies for March and April 2011. The bottom panel shows a similar anomaly analysis for total ozone derived from measurements of the following satellites: TOMS/Nimbus7 (1991-1992), TOMS/Meteor3 (1993-1994), TOMS/EarthProbe (1996-2004), and OMI (2005-2011). The shaded range for the ozone data set is based on data for the years 1991-2009 (1996-2009 for Trondheim and Finse). Ozone data are available at http://toms.gsfc.nasa.gov and http://ozoneaq.gsfc.nasa.gov/. Vertical broken lines indicate the times of the vernal and autumnal equinoxes, and the summer solstice. (Enlarged figure: http://www.arctic.noaa.gov/reportcard/ozone_uv_figa14.pdf)
The low levels of total ozone in March and April of 2011 led to elevated UV levels throughout the Arctic and sub-Arctic, as shown in and explained in more detail below. Note that the UV Index is a measure of the ability of UV radiation to cause erythema (sunburn) in human skin. It is calculated by weighting UV spectra with the CIE action spectrum for erythema (McKinlay and Diffey, 1987) and multiplying the result by 40 m2/W.
In addition to atmospheric ozone concentrations, UV radiation is affected by the height of the Sun above the horizon (the solar elevation), clouds, aerosols (liquid and solid particles suspended in air), the reflectivity of the surface (high, when snow or ice covered), and other factors (Weatherhead et al., 2005). The main driver of the annual cycle is the solar elevation. Sites closest to the North Pole (Alert, Eureka and Ny-Alesund have the smallest peak radiation. Clouds lead to a large variability in UV levels on time scales from minutes to days, but their effect is largely reduced when the ground is covered by fresh snow. (Bernhard et al. 2008). Measurements at Barrow, and to a lesser extent at Alert and Eureka, show a large asymmetry between spring (low variability) and fall (high variability) because the surface at these sites is covered by snow until about June and free of snow thereafter until the beginning of winter. In particular, during summer and fall the variability introduced by clouds is substantially larger than that related to ozone variations.
The abnormally low stratospheric ozone concentrations in the spring 2011 led to substantially elevated UV Indices at all sites. Changes in the UV Index anti-correlate with changes in total ozone. Noontime UV Indices of March 2011 exceeded historical measurements for this month at all sites. Enhancements of the UV Index relative to the climatological average were most pronounced at Andoya (122%), Jokioinen (120%) and Oesteraas (107%). The relative enhancement at the northernmost sites is somewhat smaller because changes in UV are less sensitive to changes in ozone when the solar elevation is small (Douglas et al., 2011). At all sites, the relative increase in the UV Index is outside the range defined by measurements of earlier years, and at many sites the anomaly is outside the envelope defined by the variability introduced by clouds. While these large relative increases are unprecedented, the absolute increases in UV levels were modest at all sites because the low-ozone event occurred early in spring when the solar elevation was still small. For example, at Andoya, the large relative increase of 122% changed the UV Index from 0.8 (average for 28-March) to 1.9. This value is less than half as large as the peak UV Index of 5.1 measured in July at this site. Although UV Indices below 2 are considered low (WHO, 2002), people involved in certain outdoor activities may receive higher-than-expected UV doses if their faces and eyes are oriented perpendicular to the low Sun or if they are exposed to UV radiation reflected off snow.
Larger absolute increases of UV Indices occurred at lower latitudes during excursions of the polar vortex in April. For example, on April 22, the clear-sky UV Index over parts of Mongolia (48°N, 98°E) estimated by TEMIS (Tropospheric Emission Monitoring Internet Service (TEMIS) at http://www.temis.nl/uvradiation/UVindex.html) was 8.6 when a lobe of the vortex extended to central Asia. The long-term average for this day at this location is 5.4 with a 1-σ standard deviation of 0.5, i.e., the anomaly was more than six standard deviations larger than the climatological mean. A similar situation occurred in central Europe on April 17 when a tongue of the vortex extended over the Alps. The noontime UV Index at Arosa, Switzerland (46.8°N, 9.7°E), was 7.4 on that day, which is four standard deviations larger than the long-term average of 5.3.
Figure also highlights measurements made in 2010. In March 2010, UV Indices were abnormally small due to larger-than-normal stratospheric ozone concentrations in that year. The variability in the second half of 2010 was within the range of previous years.
Douglas, A., and V. Fioletov (Coordinating Lead Authors), S. Godin-Beekmann, R. Müller, R.S. Stolarski, A. Webb, A. Arola, J.B. Burkholder, J.P. Burrows, M.P. Chipperfield, R. Cordero, C. et al. (2011), Stratospheric changes and climate, Chapter 4 in Scientific Assessment of Ozone Depletion: 2010, Global Ozone Research and Monitoring Project-Report No. 52, World Meteorological Organization, Geneva, Switzerland, http://ozone.unep.org/Assessment_Panels/SAP/Scientific_Assessment_2...
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