A basic outdoor light setup would contain a reddish sun and a blueish ambient light to fake some global illumination of the sky. But in AC3 i noticed a third source of light: bouncing light. It’s just the sun light coming back from the ground.
I don’t know how they did it, but i would guess they just created a third light on the opposite site of the sun & below the ground. Because you don’t want to have the ground shadows affected by this light.
I tried this in a small render and i think the third light gives a very nice contrast and more volume. I used the same color as the sun had, but with less intensity but more saturation instead.
I tried to find out if UDK has something like this in their light settings so you wouldn’t need to create a 3rd light source but i couldn’t find it on the first look. Feel free to drop a comment if you know how this is done on modern engines.