Ground-based remote sensing of stratocumulus properties during CLARA, 1996

A method is presented to obtain droplet concentration for water clouds from ground-based remote sensing observations. It relies on observations of clo ud thickness, liquid water path, and optical extinction near the cloud base . The method was tested for two case studies (19 April 1996 and 3 September 1996) during the Clouds And Radiation experiment (CLARA). The CLARA experi ment was designed to observe clouds using a variety of remote sensing instr uments near the city of Delft in the western part of the Netherlands. The m easurement of cloud thickness is dependent on the detection of cloud base b y Lidar and cloud top by radar. It is shown that during CLARA it was possib le to detect cloud base with an uncertainty of less than 30 m using current Lidar techniques. The agreement between in situ and remote sensing observa tions of droplet concentration was reasonable. An error analysis indicates that this method is most sensitive to uncertainties in liquid water path an d the unknown effects of multiple scattering on lidar signal returns. When the liquid water path is very small the relative error of the liquid water path increases to unacceptable levels, so that the retrieval of droplet con centration becomes very difficult. The estimated uncertainty in the strengt h of multiple scattering can explain differences between observations and r etrievals of droplet concentration on one day, but not the other.