That largely agrees with what I've read. There were no large air raids on the day of the Hiroshima bombing, and very few would have considered a bomb of that p

That largely agrees with what I've read. There were no large air raids on the day of the Hiroshima bombing, and very few would have considered a bomb of that power. So in the first hours at least and into the next day, outside of the city there wasn't an understanding the whole city was basically gone. From their perspective immediately after, there were communication problems that had cut all the telegraph and telephone lines, presumably due to some sort of large (but more normal) disaster in the city centre. Only once the smoke started to clear and an aerial survey could be performed, along with large numbers of survivors pouring into neighbouring cities and towns, would the true scale of it have been clear to the Japanese government (around the same time the Americans were surveying the results, I suppose, a day or two later).