Study Cases of Cirrus Cloud Radiative Effect using Lidar Backscatter Data, Radiative Transfer Code and Solar Radiation Measurements in Manaus Region

Cirrus clouds radiative effects on shortwave radiation (CRE) have been studied combining lidar measurements with a state of the art radiative transfer code and solar radiation measurements at ground. For this purpose, four days with persistent cirrus clouds over the Lidar site, near Manaus - Brazil, were selected for different seasons during 2011 - 2014. The UV Raman Lidar operates at 355 nm with 10 Hz and 95 mJ per pulse and the detection efficiency is high enough to give good signal to noise ratio at the cirrus altitudes even during daytime. The radiative transfer code was adapted to the local conditions at the site, using water vapor and temperature profiles from the operational radiosondes (~30 km) as well as locally measured surface albedo (0.16). Runs of the atmospheric radiative transfer code were conducted both under the presence of cirrus clouds and in clear sky conditions. The calculated shortwave broadband cloud radiative effect values have negative sign, the mean daily cloud radiative effect ranging from -5 W/m² to -100 W/m², at the top of atmosphere and lower modular values in the surface. The CRE vary according to the different optical depths of thin cirrus clouds. A close correlation between the negative cirrus radiative effect and optical depth (anticorrelation) was found at the top of the atmosphere and at the surface when broadband solar irradiances calculations are analyzed. This was found also for different intervals of wavelength in the solar spectrum (near infrared, visible, and ultraviolet).