Equatorial Kelvin wave propagation past Sumatra: June 2006 case analysis and forecast sensitivity experiments with COAMPS®

Two closely-spaced equatorial Kelvin waves are examined in Indian Ocean regional atmospheric analyses and Tropical Rainfall Measuring Mission rainfall data, focusing on their passage past Sumatra. Wave propagation past the island in these cases involves the looping of low-level clock-wise turning winds either through a gap in the Bukit Barisan mountain range near the equator, or around the northern tip of the island. The first wave examined was substantially weakened as a portion of it passed through the gap, and the wave subsequently dissipated along the coast of Borneo. The second wave exhibited a considerable north-south tilt as its low-level flow passed around the northern end of the island, and then strengthened as it continued moving eastward. To shed light on the impact of the Sumatran terrain on such waves, and on reported difficulties it poses for numerical models, the passage of the first wave is examined in data-assimilation experiments with COAMPS^®. A nested model configuration is employed, with convection treated explicitly on the 4-km inner mesh.