How does the poet in "It is raining on the house of Anne frank" use rain to develop the idea of suffering in the poem?

It could be that rain has many cultural and historical ties to suffering. It's associated with flooding, drowning, dark skies, crops not getting sunlight, etc. Especially (as Anne Frank is Jewish) the holiday of Passover has a relationship to rain. This is a subtle way to foreshadow and develop a tone of future and present suffering.