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.

Source: Assassins Creed 3

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.

Left: Sun + blue ambient light source. Right: Sun + Ambient + “Bounce” light source.

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.

Update 1

Thanks to gamepat & Félix we have some links to look at! Visit the comment section (also for their text) or check the links directly:

Crytek Global Illumination Paper
UDK GI Movie
Crytek GI Movie
Blender GI Movie
Mirrors Edge Light Presentation