وَبَدَا الصَّبَاحُ كَأَنَّ غُرَّتَهُ وَجْهُ الْخَلِيفَةِ حِينَ يُمْتَدَحُ
Dawn appeared as though its bright forehead were the caliph's face when he was being praised.
An ordinary compliment would say, “The caliph's face is bright like dawn.” The poet reverses the relationship: dawn is measured against the caliph's face.
Expressed subject: dawn's luminous forehead.
Expressed model: the caliph's face.
Rhetorical effect: elevating the caliph's brilliance and dignity beyond the usual standard of comparison.