The affix ghañ comes after the verb rū 'to roar' when it has an upasarga in composition with it.,
This debars अप् 3.3.57. As संरावः 'clamour', उपरावः &c.
Why do we say 'compounded with an upasarga'? Observe रवः 'sound'.,
उपसर्गे रुवः
Adhyāya 3 · Pāda 3 · Rule 22