Cloud impacts on UV spectral actinic flux observed during the International Photolysis Frequency Measurement and Model Intercomparison (IPMMI)

Abstract

Observations collected during the International Photolysis Frequency Measurement and Model Intercomparison (IPMMI) have enabled a semiquantitative assessment of cloud impacts on UV spectral actinic flux. Critical to this analysis is the availability of spectroradiometer data accompanied by sky camera images. Data reveal several interesting aspects of cloud influences. Enhancements in UV actinic flux of up to 40% over clear‐sky values are observed for cloudy conditions when the solar disk is unoccluded. When the solar disk is occluded, reductions in actinic flux appear to vary inversely with cloud fraction in some instances. A wavelength dependence is also observed with shorter wavelengths generally exhibiting lower variability for both enhancements and reductions (e.g., 20–30% less variability at 320 versus 420 nm). Enhancements in actinic flux show a stronger wavelength dependence with increasing cloud fraction. By contrast, wavelength dependence appears to weaken with increasing cloud fraction for actinic flux reduction. At low Sun, however, a wavelength dependence appears to occur even for overcast conditions. Despite these differences, integration over longer periods shows cloud impacts on actinic flux to be similar for all UV wavelengths, suggesting that differences in wavelength response are localized phenomena with impacts that are minimized when integrated over the surrounding area. Further studies at other latitudes and in other seasons are needed to verify this behavior.