UV Raman lidar measurements of relative humidity for the characterization of aerosol hygroscopicity and cloud microphysical properties

A UV Raman lidar system (BASIL) is operational at DIFA-Univ. of Basilicata (Potenza-Italy). The major feature of BASIL is its capability to perform high-resolution and accurate measurements of atmospheric temperature, both in daytime and night-time, based on the application of the rotational Raman lidar technique in the UV [1]. Besides temperature, BASIL is capable to provide measurements of particle backscatter at 355 and 532 nm, particle extinction at 355 nm, particle depolarization at 355 and water vapour mixing ratio. Relative humidity (RH) measurements are obtained from simultaneous water vapour mixing ratio and temperature measurements. These parameters represents a suitable ensemble of measurements for the study of meteorological processes. Specific case studies are considered and discussed to assess RH lidar measurement capability in presence of aerosols and clouds. Measurements of aerosol backscatter as a function of relative humidity are illustrated, highlighting the swelling tendency of hygroscopic aerosol for large relative humidity values (in excess of 80 %). Relative humidity measurements in the proximity and within cirrus clouds are also illustrated and discussed. Both ice super-saturation and undersaturation conditions are found inside these clouds, but air is observed to be always under-saturated with respect to water. The sublimation of the precipitating ice crystals in cirrus clouds is found to cause upper tropospheric humidification, and lead to increased relative humidity values beneath the clouds.