Barrow is located at the Arctic coast. Land and ocean adjacent to the instrument are typically covered by snow between October and June. The UV albedo is 0.83 ± 0.08 (± 1a) between November and May and smaller than 0.1 during the summer months (Bernhard et al., 2007). Clouds are more frequent in summer than in spring. The small differences between the 10th and 90th percentiles in March and April (Fig. 3.8(b)) are attributable to high albedo and low cloudiness during these months. Barrow is affected by ozone depletion between February and April, but the magnitude is much smaller than at Antarctic sites. Depletion events are typically short and lead to spikes in the UV Index of up to one UV Index unit only. The largest daily maximum UV Indices of 4.5 - 5 are observed in May and June when low ozone episodes coincide with high albedo conditions.

Figure 3.8 Same as Fig. 3.4(a), but for San Diego (Panel (a)) and Barrow (Panel (b)). Historical data were not calculated for San Diego

Total ozone at Barrow has been measured by GMD/ESRL with Dobson photometers since 1973. Historical UV intensities for Barrow were calculated in a similar way as UV levels were at South Pole using Dobson measurements from the years 1973 -1980. Dutton et al. (2004) have reported a statistically significant decrease of effective cloud transmission from 0.64 in 1976 to 0.61 in 2001 based on their analysis of pyranometer data. When estimating past UV Indices, we did not consider this change. First, it is unknown whether the downward trend was already present during the 1960s, and second, changes in cloud transmission were likely smaller in the UV, particularly in spring when attenuation by clouds is reduced by high albedo.

To estimate the range of historical UV, we also considered year-to-year changes in surface albedo. The grey area in Fig. 3.8(b) includes changes in total ozone estimated from the years 1973 -1980, as well as variations due to clouds and albedo estimated from the years 1991 - 2006. UV Indices measured since 1991 are very similar to the historical estimate. For the months February through April, the average increase is 4% - 7% only, and the increase for the 90th percentile is 7% -13%. Differences for summer months are smaller and may even be negative: recent measurements for August are about 5% below the historical estimate. Differences of this magnitude are within the uncertainty of the data. This shows that there was little change in UV levels during the last 30 years at Barrow, with the exception of several spikes observed during recent low-ozone episodes.

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