How CAESAR II considers the increasing the surface of area due to wind load?
Its not. You input the snow load as a "uniform load".
CAESAR II has no idea what this represents.
If you want to combine snow and wind, then you'll have to do some external computations so that you can use "insulation" to simulate snow. Your insulation thickness needs to be 1/2 the snow depth (because insulation will be on both the top and bottom of the pipe), and then the corresponding density should be determined to yield the correct weight/length of snow.