There is no terrain in this case, so HD will not matter. HD only affects terrain geometry detail generation.
It looks like there are some areas of your cloud that are very hard for the RTP to sample and get to a low noise level. I would guess that what you're seeing is undersampling; a lot of noise that is giving an inaccurate result. The first thing I would try is enabling the crop region and setting it to focus on one of the problem areas. The idea here is to let it resolve a lot more to see if it gets more accurate, and doing it in a crop will happen a lot faster. If it eventually resolves to something that looks more accurate then it's basically a sampling problem. One that we might be able to improve on, but at the same time there will always be situations with hard-to-sample shaders which will just need to resolve more to be accurate.
So, assuming that letting it resolve further gives you a more accurate result, then I would say the bottom line is where you see a lot of noise like this, use a crop region and let it resolve further to get a more accurate view.
- Oshyan