Effects of stratocumulus clouds and a low-ozone event on ultraviolet erythemal radiation exposure.

Measurements of the basic meteorological parameters and ultraviolet (UV) erythemalradiation (UVER) were performed in Didcot, England, on 6 and 7 April 2017. The UVERdata was collected by dosimeters, which consist of two sensors (named UVA and UVBC)collecting the erythemally weighted UV-A (320 - 400 nm) and UVBC (UV-B and UV-C, 250- 320 nm) radiation parts, respectively. Both measurement days were characterized by clearconditions during the morning and afternoon hours. Around noon of both days,however, development of shallow stratocumulus clouds (SSC) occurred. In addition, alow-ozone event (ozone mini-hole) took place on 7 April being characterized bya 34 DU (Dobson Unit) drop in total stratospheric ozone content. Compared to6 April, the UVER increase as a result of the low-ozone event amounts to 2.67standard erythema dose (SED) for diffuse and 4.32 SED for global radiation. Thecorresponding radiation amplification factors (RAF) amount to 1.62 and 1.52 fordiffuse and global UVER, respectively. The SSC coverage caused reductions forthe total global UVER doses amounting to 2.33 SED (6 April) and 2.81 SED(7 April). As innovation, we present a decomposition of the RAF into two parts, called cloud ozonefactor (COF) and radiation amplification factor based on measured data (RAFm). The goalof this decomposition is to quantify the low-ozone event’s effect and the SSC influence inindependently modifying the UVER doses. Hereby the newly introduced COFexpresses the weight of each of these two effects acting during the same low-ozoneevent. For the case presented, the COF values range between -0.13 and -0.11 fordiffuse UVER and -0.03 to -0.07 for the global UV and UV-B parts. A positive COFvalue (0.18) results for the global UV-A range. Negative (positive) COF valuescorrespond to a decreased (increased) cloud influence on the second of both referencedays. [ABSTRACT FROM AUTHOR]