Low ozone level episodes (also known as mini-holes) have been analysed over Valencia, Spain, during the period from 1 January 2000 to 31 December 2007. In this study, 24 episodes, lasting 1 or 2 days each, were detected. Most of them can be explained dynamically by the existence of an anticyclone over the Azores, which sends air masses coming from the Caribbean over the Iberian Peninsula. A positive anomaly in the vertical temperature profile in the high troposphere and one negative anomaly in the medium and low stratosphere are also observed in most of the analysed episodes. The total ozone column decrease is accompanied by a rise in the UV erythemal radiation levels. This increase is quantified at 1.29 times the decrease in the total ozone column. This is known as the radiation amplification factor (RAF) of UV erythemal radiation.
ABSTRACT). These measurements, and its statistical indices, have been analyzed. For the UVB irradiance, the values corresponding to July (maximum) and December (minimum) have been analyzed as representative of the year during the whole period for all locations. For the UVB radiant exposure, the temporal evolution of daily values has been evaluated for all locations to estimate an average yearly behavior. The accumulated radiant exposure for an average year has also been studied for each location. Finally, to determine possible trends in the evolution of the UVB levels, the linear regressions for the mean daily values for all locations have been determined.
The daily values of UVB irradiation (290–315 nm), IUVB, and the broadband total irradiation (300–2800 nm), IT, measured on a horizontal plane have been correlated for the period 2000–2008 at 16 measurement sites in Spain. The results have been compared with the daily experimental values registered at the same sites during the period 2009–2011. The coefficients of determination R2 obtained by applying a linear regression are higher than 0.88 for all sites and increase to 0.94 when using a quadratic regression. When all data are considered together, the values of R2 are 0.91 and 0.97 for the linear and quadratic regressions, respectively. Three different clearness indices, which are dimensionless parameters, have been also used in order to reduce the uncertainties of the estimations and to reduce the local nature of the coefficients: (1) the UVB clearness index, kTUVB = IUVB/I0UVB; (2) the modified UVB clearness index, k′TUVB and (3) the estimated UVB clearness index, k*TUVB. The results show a strong dependence on the solar zenith angle. The bi‐dimensional regression of kTUVB as a function of the solar zenith angle and the clearness index, kT, substantially improves the correlation, with R2 values higher than 0.81 for all sites. Finally, a formula relating kTUVB and kT, valid for latitudes between 36°N and 44°N, has been obtained: kTUVB = 0.064 kT – 0.0011 θz – 1.24 E‐05 ϕ + 7.6 E‐05 λ + 0.063, where θz represents the solar zenith angle (in degrees), ϕ is the latitude (in degrees) of the site and λ is the distance to the sea (in km). We have compared the experimental values for the period 2009–2011 with those estimated using this expression. For this, the coefficient of determination (R2) is 0.87, with a MBD of –3.4%, a RMSE of 21% and a NRMSE of 8.7%.
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