A slight tilting of the ceilometer gives  better performance in rain.

Rain drops tend to flatten as they fall.  See the NASA explanation for this 

Consequently, when aligned vertically the backscatter from raindrops may be sufficiently high to cause difficult in resolving the cloud base above the rain.

However when tilted, the backscatter of laser pulses by the raindrops is reduced .

In heavy rain even a tilted ceilometer cannot resolve the cloud base since the integrated backscatter quickly dominates and prevents further penetration up to the cloudbase,  while extinction in the return path also starts to extinguish the return signal..

Please refer to the screen below.

These tests were carried out during light rain.  The tilted unit shows resolvable cloud base while the untilted unit reverts to vertical visibility.