Study of radiative properties and effects of high-altitude cirrus clouds in Barcelona, Spain with 4 years of lidar measurements

Abstract. Cloud-radiation interaction still drives large uncertainties in climate models and its estimation is key to make more accurate predictions. In this context, the high-altitude cirrus clouds play a fundamental role, because 1) they have a high occurrence frequency globally and 2) they are the only cloud that can readily cool or warm the atmosphere during daytime, depending on their properties. This study presents a comprehensive analysis of radiative properties and effects of cirrus clouds based on 4 years of continuous ground-based lidar measurements with the Barcelona (Spain) Micro Pulse Lidar. First, we introduce a novel approach of a self-consistent scattering model for cirrus clouds to determine their radiative properties at different wavelengths using only the effective extinction coefficient and mid-cloud temperature. Second, we calculate the radiative effects of cirrus clouds with the Discrete Ordinates Method and we validate our results with SolRad-Net pyranometers and CERES measurements. Third, we present a case study analyzing the radiative effects of a cirrus cloud along its back-trajectory using data from the Chemical LAgrangian Model of the Stratosphere with microphysics scheme for Ice clouds formation. The results show that the cirrus clouds with an average ice water content of 4.97±5.53 mg/m³, at nighttime, warm the atmosphere at top-of-the-atmosphere (TOA; +50.1 Wm⁻²) almost twice than at bottom-of-the-atmosphere (BOA; +23.0 Wm⁻²); at daytime, they generally cool the BOA (-8.57 Wm⁻², 80 % of the cases) and always warm the TOA (+18.9 Wm⁻²).In these simulations, the influence of the lower layer aerosols is negligible in the cirrus radiative effects, with a BIAS of -0.71 %. For the case study, the net radiative effects produced by the cirrus cloud, going at TOA from 0 to +42 Wm⁻² and at BOA from -51 to +20 Wm⁻². This study reveals that the complexity of the cirrus cloud radiative effect calculation lies in the fact that it is highly sensitive to the cirrus scene properties.