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.