  Simulating the collision behaviour of cloud drops in a turbulent environment
is numerically expensive. Because of the typical sizes of cloud drops, their
motion is predominantly influenced by the smallest turbulent scales in the
flow. We can exploit this property by using an Arnold--Beltrami--Childress
(ABC) flow instead of a full direct numerical simulation (DNS) to simulate the
turbulent effect on cloud drop collisions. This allows simulation of drop
motion using much less computational resources than needed by DNS and
therefore, allows for simulations with many more drops and larger model domains
or more complex cloud physics. This is useful in cases where a specific process
needs to be studied and the complexity and details of a realistic turbulent
flow are of secondary interest.
  It is shown that the simulations using ABC flow can faithfully reproduce
collision statistics from much more comprehensive DNS calculations when using a
similar Taylor microscale and root mean square velocity in the ABC and DNS
flows. The results from the ABC flow are robust to changes in these two
parameters within a reasonable range, even though adjustments to the parameters
can be used to further tune the results.
