Measurements of the UV solar irradiance are available from Ushuaia, Tierra del Fuego during the spring and summer seasons of 4 consecutive years beginning in 1989. In addition, column ozone amounts derived from satellite-based measurements exist for this location over the entire period from 1980 through 1991. Monthly mean column ozone over Ushuaia shows a general decline over the observing period, and a large day-to-day variability exists within a given month. Ozone amounts for the years 1980 through 1986 combined with a model of radiative transfer provide a climatological baseline against which to interpret the more recent ground-based irradiance data. We focus on monthly mean noontime irradiances integrated over 5 nm wide spectral bands near 305 nm and 340 nm, respectively. Measurements in the 340 nm band show that cloudiness has a large influence on both the absolute monthly mean irradiances and their interannual variability. For example, during December the 340 nm band irradiance varied from approximately 509/0 of the clear-sky value in 1992 to 65% in 199 1. When the influence of cloudiness is removed, most of the months show irradiances in the 305 nm band that are larger than predicted from the climatological ozone amounts. The largest percentage enhancement occurred in October 199 1 when the irradiance exceeded the baseline by 56%. The largest absolute irradiances occur in December, where the measurements range from 5.8% below the baseline in 199 1 to 3 1% above in 1990.
Abstract. To examine effects of inhomogeneous surface of the region is cold, dry and windy, due to the interaction of albedo on UV irradiance, measurements were taken near Davis katabatic winds from the continental plateau and oceanic Station (68.58 ø S, 77.97 ø E), East Antarctica during winds from the north east [Heywood, 1984]. In winter, the November/December 1997. Data were collected in transects waters near the Vestfold Hills are covered in pack ice, perpendicular to an ice/water boundary, while one instrument extending northwards to at least 59 ø S [Jacka, 1990]. As continuously measured at the ice edge. Irradiances along spring progresses, Prydz Bay becomes free of ice, although transects were normalised by coincident measurements at the coastal waters remain covered by fast ice (typically --2 m with ice edge. Systematic differences in downwelling irradiance a--20 cm snow layer) until late December each year. near the ice edge were observed over the highly reflective A series of radiation measurements were collected along surface and open water. For cloudless conditions, the relative transects perpendicular to the ice edge. The field sites were change in irradiance reached limiting values of about +5% chosen to approximate as closely as possible a straight ice from the ice edge to a point 2.5 km over snow-covered ice, edge separating a high albedo surface (ice/snow) from a low and about-5% from the ice edge to a point 2.5 km over water. albedo surface (water) (Figure 2). Because of the changing In overcast conditions, higher relative changes were observed nature of the ice edge, no one site provided an extensive (about +30% from the ice edge to a point 2.5 km over ice on straight boundary over the entire field season. As a result, the 17 November 1997) with no sign of a constant relative change field site for each day (Figure 1) was selected to best satisfy being reached. site requirements, logistic considerations and safety issues.
A nearly continuous data set of solar ultraviolet spectral irradiance exists for Ushuaia, Argentina, latitude 54°59′ S, over the period from mid‐September 1990 to mid‐March 1991. This includes a season of prolonged depletion in column ozone over Antarctica, 10° or more in latitude poleward of Ushuaia. Cloudiness provides a major source of variance in the measurements. When this influence is removed, the irradiances at wavelengths between 300 and 310 nm are enhanced relative to clear sky calculations based on a 10‐year ozone climatology. During December the average noontime irradiance at 306.5 nm, which is a good proxy for erythemal irradiance, is 45% larger than the zonal mean climatological prediction. The largest noontime radiation levels observed at Ushuaia are equivalent to moving 20° in latitude closer to the equator at the summer solstice.
[1] The enhancement of the UV global irradiance due to snow cover on the ground has been observed at the station of Briançon, in a high-altitude Alpine valley. The analysis relies on a three-dimensional (3-D) model, using an elevation map of the area. Without snow, comparison with the results of a 1-D model shows no detectable effect of topography, within the uncertainty of modeling (2-3%). The 3-D model relates the enhancement due to snow to the altitude of the snow line. The enhancement is shown to depend on the snow distribution around the site and not on the topography itself. The enhancement was measured at Briançon for nine cloudless days in winter 2002. As expected, it increases with the decrease of the snow line. In erythemal UV the enhancement reaches a maximum of about 22% in the beginning of March, in agreement with the results of the 3-D model, assuming a snow albedo of 0.3 above the snow line and below the tree line and 0.8 above the tree line. Retrieving an effective surface albedo is a very challenging problem. Very small uncertainties in enhancement (±2%) lead to large uncertainties (±0.05) in effective albedo. Using the snow distribution with a contribution function does not give good results when the snow line is high; this is explained by the low resolution of the map with the rapid variation of the contribution near the site.
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