Variability of the Daily-Mean Shortwave Cloud Radiative Forcing at the Surface at a Midlatitude Site in Southwestern Europe

Abstract

Abstract The shortwave cloud radiative forcing is calculated from surface measurements taken in Évora from 2003 to 2010 with a multifilter rotating shadowband radiometer (MFRSR) and with an Eppley black and white pyranometer. A new approach to estimate the clear-sky irradiance based on radiative transfer calculations is also proposed. The daily-mean values of the cloud radiative forcing (absolute and normalized) as well as their monthly and seasonal variabilities are analyzed. The study shows greater variability of radiative forcing during springtime with respect to the other seasons. The mean daily cloudy periods have seasonal variation proportional to the seasonal variation of the cloud radiative forcing, with maximum values also occurring during springtime. The minimum values found for the daily-mean cloud radiative forcing are −139.5 and −198.4 W m−2 for MFRSR and Eppley data, respectively; the normalized values present about 40% of sample amplitude, both for MFRSR and Eppley. In addition, a quantitative relationship between the MFRSR and Eppley cloud radiative forcings applicable to other locations is proposed.