Air conditioners are designed to be outside all year long in all types of weather. You can cover the top only (with plywood and brick to keep it in place) during the winter to prevent snow from piling on top of the unit that might have slid off of your roof. Be sure to leave the sides of the unit open, this allows the air to get at it and keeps your A/C unit dry.
Our most popular air conditioner unit by Carrier has a Weather Armor™ protection to ensure protection against the sun and winter elements, which means it needs no special covering for the winter.