Estimation of cloud optical depth for low clouds from UV erythemal irradiance

A methodology to calculate the cloud optical depth for overcast and partly covered skies is proposed. It is based in a combination of measurements of surface diffuse UV erythemal irradiance (UVER), a sky camera for estimation of the cloud cover and radiative transfer simulations by means of SBDART. Data refer to the year 2011 in the city of Valencia (Spain). The results show that in most cases cloud optical depth values are in the interval between 10-20, being the cloud optical depth less than 70 in more than 80% of cases. The monthly cloud optical depth over the year was also studied, as a result, values are found to be higher during the rainy seasons in Valencia (April and September).