Warm rain collision coalescence has been persistently difficult to
parameterize in bulk microphysics schemes. Here we use a flexible bulk
microphysics scheme with bin scheme process parameterizations, called
AMP, to investigate reasons for the difficulty. AMP is configured in a
variety of ways to mimic bulk schemes and is compared to simulations
with the bin scheme upon which AMP is built. We find that the biggest
limitation in traditional bulk schemes is the use of separate cloud and
rain categories. When the drop size distribution is instead represented
by a continuous distribution with or without an explicit functional
form, the simulation of cloud-to-rain conversion is substantially
improved. We find that the use of an assumed double-mode gamma
distribution and the choice of predicted distribution moments do
somewhat influence the ability of AMP to simulate rain production, but
much less than using a single liquid category compared to separate cloud
and rain categories. Traditional two category configurations of AMP are
always too slow in producing rain due to their struggle to capture the
emergence of the rain mode. Single category configurations may produce
rain either too slowly or too quickly, with too slow production more
likely for initially narrow droplet size distributions. However, the
average error magnitude is much smaller using a single category than two
categories. Optimal moment combinations for the single category approach
appear to be linked more to the information content they provide for
constraining the size distributions than to their correlation with
collision-coalescence rates.