Hmm, lots of feedback to give here.
The first thing you want to do in a situation like this is determine where the problem you want to address is coming from. As Klaus pointed out, those are cloud shadows. An easy way to determine this kind of thing is just disable each element of the scene one by one and see when the area you're looking at changes. Then you can find out what contributes to the area in question and have a better understanding of how to fix it.
So, given that it's cloud shadows, Antialiasing and Micropoly detail with both have very little effect on this by themselves. This is an issue of the sampling of the cloud shadows themselves. There are two main solutions, both of which increase render time. Since this is noise you're dealing with, in general any solution is going to involve more sampling, which involves higher render time.
The easiest and most direct option is you can increase cloud Quality. This will increase render time for your clouds overall (but also reduce any noise in them), and will clean up the shadows nicely (I used a value of 5 just as a test, looks great). However this may increase render time too much if your clouds do not need any more quality and it's just the shadows that are a problem.
The second option also increases render time but primarily acts on the terrain, which is to use Defer All as your render method. This effectively super samples shading calculations on the terrain and can smooth out shadow noise very well. When using Defer All your render time and quality are primarily determined by antialiasing level.
I found that enabling Soft Shadows mostly just made the shadows quite a bit lighter. They are a bit less noisy probably due to the default 9 samples for soft shadows, but still not as low noise as just increasing cloud quality or using Defer All above.
Also, did you turn off Ray Traced Shadows for a reason? Generally I wouldn't recommend you disable it.
Finally, we don't recommend you setting core limits for rendering in the render node *unless* you need to change it frequently. If you want it to be limited to 7 for *all* (or even most) renders, just set it in Preferences. Otherwise it needlessly limits the number of cores used when other people or other computers use your file (unless they notice it). And also saves you having to change it every time. A minor issue, but something to be aware of. Also the CPU you have listed in your signature only has 4 threads, 2 cores, so setting it to 7 threads seems non-ideal to me, though maybe you have tested and found otherwise.
- Oshyan