  Using only lidar or radar an accurate cloud boundary height estimate is often
not possible. The combination of lidar and radar can give a reliable cloud
boundary estimate in a much broader range of cases. However, also this
combination with standard methods still can not measure the cloud boundaries in
all cases. This will be illustrated with data from the Clouds and Radiation
measurement campaigns, CLARA. Rain is a problem: the radar has problems to
measure the small cloud droplets in the presence of raindrops. Similarly, few
large particles below cloud base can obscure the cloud base in radar
measurements. And the radar reflectivity can be very low at the cloud base of
water clouds or in large regions of ice clouds, due to small particles.
Multiple cloud layers and clouds with specular reflections can pose problems
for lidar. More advanced measurement techniques are suggested to solve these
problems. An angle scanning lidar can, for example, detect specular
reflections, while using information from the radars Doppler velocity spectrum
may help to detect clouds during rain.
