Effect of water vapor in the SW and LW downward irradiance at the surface during a day with low aerosol load

Abstract

The aim of this study is to analyze the effect of the water vapor content on the downward irradiance measured at the Earth's surface. For that purpose, downward irradiance values have been estimated with the radiative transfer model libRadtran in different spectral ranges: shortwave (SW: 285-2800 nm) and longwave (LW: 3500-50000 nm), and with different water vapor content in the column. These simulations have been made for Évora, Portugal, the August 4, 2012, a cloud-free day and with low aerosol optical depth.The comparison between the simulated irradiance with different water vapor contents shows differences in both spectral ranges. For SW, the irradiance reaching the surface increases when the water vapor content decreases, obtaining an increase of up to 4%, 2% and 1%, corresponding the largest increases to the smallest values of water vapor. For LW, the behaviour is the opposite, the irradiance decreases when the water vapor content decreases, obtaining a decrease of up to 10%, 4% and 2%, corresponding the largest decreases to the smallest values of water vapor. The effect of water vapor in the aerosol radiative forcing (ARF) has also been analyzed, obtaining relative difference values of up to 2.5% for SW and 35% for LW.