Study of optical scattering properties and direct radiative 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, 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 optical scattering properties and direct radiative effect 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 optical scattering properties at different wavelengths using only the extinction coefficient and cloud temperature. Second, we calculate the direct radiative effects of cirrus clouds with the discrete ordinates method, and we validate our results with SolRad-Net pyranometers and NOAA-20 measurements. Third, we present a case study analysing the direct radiative effect 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 (CLaMS-Ice). The results show that the cirrus clouds with an average ice water content of 4.97 ± 5.53 mg m−3, in the nighttime, have a positive direct radiative effect at the top of the atmosphere (TOA; +40.4 W m−2) almost twice than at the bottom of the atmosphere (BOA; +22.1 W m−2); in the daytime, they have generally a negative direct radiative effect at BOA (−11.5 W m−2, 82 % of the cases) and always a positive effect at TOA (+14.2 W m−2). In these simulations, the influence of the lower layer aerosols is negligible in the cirrus direct radiative effects, with a bias (denoted BIAS) of −1.2 %. For the case study, the net direct radiative effects produced by the cirrus cloud, are from 0 to +40 W m−2 at TOA and from −51 to +20 W m−2 at BOA. This study reveals that the complexity of the cirrus cloud direct radiative effect calculation lies in the fact that it is highly sensitive to the cirrus scene properties.