Bachus, I feel you are confused. The ultimatum to surrender was actually given by US and the UK much earlier on 26th of July 1945 in Potsdam Declaration and they warned of quick and utter destruction. Its true Japan ignored the ultimatum as they probably couldn't gauge the potential of the bomb.

Now, the question is, why did US drop the Fat Man knowing very well they had already done immense damage to the civilian life with the first bomb.

Bachus, the answer to this is what I have already told earlier..