Erythemal dose in Kathmandu, Nepal based on solar UV measurements from multichannel filter radiometer, its deviation from satellite and radiative transfer simulations

Biologically active solar Ultraviolet radiation is monitored in Kathmandu valley in Nepal (27.7°N, 85.5°E, 1350 m asl) using a multichannel broadband radiometer for the period of November 2004–March 2005. Result shows noontime one-hour average CIE weighted dose rate can reach up to 0.195 W m− 2 (UV index = 7.8 at solar zenith angle, SZA = 36.8°) for a cloud free sky. Radiative transfer calculations indicate that UV index around local noon can reach as high as 14 in a clear summer sky, but will strongly depend on the content of aerosol in the atmosphere. Comparison of ground-based measurement with TOMS satellite derived data reveals an overestimation of UV indices by TOMS, within the period of measurement, on average by 31 ± 4.5%. Modeled clear sky daily dose differed between 20 and 40% from ground-based measurement in February. Assuming constant aerosol level during the campaign, 5–25% of the variation in Ultraviolet radiation is caused by clouds alone.