howl, cry loudly
咷 centers on loud vocal outbursts: howling or crying out loudly, often with strong emotion.
咷 combines 口 (mouth) with 兆, which likely contributes the sound. The mouth radical points to vocal action, fitting the howling or crying meaning.
The mouth 口 opens wide beside the 兆 shape, like a person throwing their head back to howl or cry loudly.
For ドウ, imagine a loud howl echoing through a dō (どう, temple hall): the sound dō cues ドウ while the mouth 口 cries out.