PREHEAT broiler. Cut grapefruit in half; use a sharp knife to separate sections of grapefruit from the peel.
COMBINE sugar and cinnamon. Remove 1/4 tsp. of the cinnamon mixture; sprinkle remaining cinnamon mixture over cut sides of grapefruit. Place, cut sides up, in broiler pan.
BROIL, 3 to 4 inches from heat, 3 to 4 minutes or until hot and bubbly. Top each with 2 Tbsp. of the whipped topping; sprinkle with the reserved cinnamon mixture.