Sforno on Genesis 9:17/ספורנו על בראשית ט׳:י״ז

1ויאמר אלוקים לנח זאת אות הברית, the word זאת refers to the “secondary” rainbow, [I mentioned earlier. Ed.] which acts as the sign of the warning aspect of the covenant. When this rainbow appears it is high time to call people to order and to warn them of impending natural calamities unless they change their ways.