5 - 6 cups (about 3 pints) lime sherbet or sorbet
2 cups vanilla ice cream
5 - 6 cups (about 3 pints) raspberry sorbet or sherbet
2-3 tablespoons miniature chocolate chips
Take a 12 cup metal or plastic mixing bowl and line it with plastic wrap, making sure there is enough plastic wrap to overhang the edges of the bowl (this makes it easy to unmold). Alternatively, spray the bowl with a nonstick vegetable spray. Place the lined bowl in the freezer for about 15 - 30 minutes so the bowl will be cold.
Place the lime sherbet in the bowl of your electric mixer and beat on low speed until the sherbet is soft enough to spread easily, about 20-30 seconds. Remove the plastic wrap lined metal bowl from the freezer, and with a rubber spatula or spoon, coat the whole inside of the bowl with an even layer of the softened lime sherbet. Return the bowl with the sherbet to the freezer to refreeze the sherbet. This will take about 30 minutes to an hour.
Once the lime sherbet is frozen, take the vanilla ice cream and place it in a clean mixing bowl. As with the lime sherbet, beat the ice cream until it is spreadable (20-30 seconds). Remove the bowl from the freezer and again, using a rubber spatula or spoon, evenly coat the lime sherbet layer with the vanilla ice cream. Return the bowl to the freezer so the vanilla ice cream can refreeze. This will take about 30 minutes to an hour.
Once that layer has frozen, take the raspberry sorbet and place it in a clean mixing bowl. Again, beat the sorbet until it is spreadable, 20-30 seconds. Fold in the chocolate chips. Remove the bowl from the freezer and fill the center of the bowl with the raspberry sorbet. Return the bowl to the freezer and freeze until the sherbet/sorbet and ice cream is completely hardened (this will take several hours or overnight).
To serve: Place a cold serving plate upside down on top of the bowl. Holding the plate against the bowl, turn it upside down. Use a hair dryer on low, or place a hot wet towel over the metal bowl for a few seconds to loosen the bowl from the ice cream. Remove the metal bowl and place the watermelon bombe back in the freezer until serving time (the lime sherbet may have soften a bit from the hot towel).