Ryu had three birdies and a bogey on each side in getting to 6-under 140 after 36 holes. Seo Hee-kyung had six birdies in an eight-hole span en route to a 7-under 66 and is in second place at 5-under for the tournament.
They were the only players to break 70 and only 14 others shot better than par as weather conditions soured during the day.
Four players, including first-round co-leader Stacy Lewis, are tied for third at 4-under and seventh-place belongs to three players at 1-under.
Lewis shot even-par 73 but that included a stretch over Nos. 11-17 in which she had two birdies, three bogeys, a triple-bogey at 14 and an eagle at 17. Sarah Kemp, who shared the first-round lead, sank further with a 79 that included two double bogeys and four bogeys. She's tied for 19th at 2-over
Two-time defending champion and world No. 1 Yani Tseng is tied for 10th at even-par after a 3-over 76. Her round included a quadruple-bogey 8 at the seventh hole.
Ryu played the second nine first and had a bogey on her second hole. But she had a birdie at 13 and two more at 16 and 14. She got to 5-under for the day with birdies at 2 and then 5 and 6 before a bogey at 7.
Swim Week Miami Beach 2014 [PHOTOS]