According to the flight and cabin crew statements, Atlanta Approach Control advised the crew that radar was showing an area of rain on the arrival route although the flight crew did not see any areas of precipitation on the airborne weather radar. During the descent to 11,000 feet the seatbelt sign was illuminated and the captain, who was the pilot monitoring, cycled the seatbelt sign and called the flight attendant (FA) at the 1L door via the interphone to notify the cabin crew to be seated. Before the 1L flight attendant was able to relay the message to all of the FAs to be seated, as they were all preparing the cabin for arrival, the airplane encountered several distinct jolts of turbulence. According to the flight data recorder data, as the airplane was descending through about 13,200 feet, the vertical acceleration fluctuated between +1.56 g's and +0.024 g's over a period of about 10 seconds. During the turbulence the 4Rc FA who was in the aisle near the 3R door was lifted into the air and fell back down to the floor on her feet and knees. The FA was assisted to the 3R jumpseat, the flight crew was notified of the injury, and they requested paramedics meet the airplane upon arrival The 4Rc FA was transported to the hospital where she was diagnosed with two broken ankles.