Attenuation of global ultraviolet and visible irradiance over Greece during the total solar eclipse of 29 March 2006 A. Kazantzidis1, A. F. Bais1, C. Emde2, S. Kazadzis1, and C. S. Zerefos3 1Laboratory of Atmospheric Physics, Aristotle University of Thessaloniki, Thessaloniki, Greece 2Institut für Physik der Atmosphäre, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Oberpfaffenhofen, 82234 Wessling, Germany 3National Observatory of Athens, Athens, Greece
Abstract. The variability of ultraviolet and photosynthetically active radiation (PAR)
during the total solar eclipse of 29 March 2006 was examined in this study.
The measurements from NILU-UV multichannel radiometers at 7 stations of the
Greek UV Network were used, where the maximum eclipse percentage ranged from
73.1% to 94.8%. In addition, an extra instrument was established at a
remote Greek island, Kastelorizo, which was within the Moon's umbral shadow.
The reduction of irradiance at 305 and 312 nm relative to non-eclipse
conditions at all sites was almost 1.5 times more than the corresponding
decrease in the UVA and visible part of the spectrum and reached 98% for
eclipse percentage equal to 94%. The availability of several instruments
in close proximity to the path of the umbral shadow provided a challenging
test for the models. The measured changes in UV and visible irradiance were
compared with 1-D model calculations accounting for the limb darkening
effect. The agreement between measurements and modeled values at all sites
is within 3% for eclipse percentages of less than 30% and becomes
worse as the eclipse progresses. The 1-D model reproduced the spectral
effect of the eclipse in UVA and PAR wavelength regions within 3% for
eclipse percentages up to 50%, but only the half of the observed change
was captured as the eclipse progressed. At three sites, where the eclipse
maximum was more than 94%, the measured irradiance at 305 nm for eclipse
percentages of more than 85% decreased with slower rates than for longer
wavelengths. As a result, the total ozone values, derived from the 305/320 nm ratios, apparently decreased significantly for high eclipse percentages.
The effect is similar at all three sites, but the interpretation of this
observation remains a challenge. Comparison results with 3-D model
calculations shortly before, during and shortly after totality were
performed for the first time and revealed an agreement with measurements
within 20% in the UV-A region. However, the modeled estimates of
irradiance at 312 nm are three times lower than measured values.
Citation: Kazantzidis, A., Bais, A. F., Emde, C., Kazadzis, S., and Zerefos, C. S.: Attenuation of global ultraviolet and visible irradiance over Greece during the total solar eclipse of 29 March 2006, Atmos. Chem. Phys., 7, 5959-5969, doi:10.5194/acp-7-5959-2007, 2007.