Convective Systems Developed along the Coastline of Sumatera Island, Indonesia, Observed with an X-band Doppler Radar during the HARIMAU2006 Campaign

Abstract

An overview of convective activity during the HARIMAU2006 campaign conducted from 26 October to 27 November 2006 was presented, focusing on the differences between coastal land/sea and inactive/active phases of intraseasonal variation (ISV) based on observations using an X-band Doppler radar (XDR) and intensive soundings at Sumatera Island. Diurnal variation (DV) in coastal convections and formation of the coastal heavy rainband (CHeR) along Sumatera Island were also examined in terms of diurnal land-sea migration of coastal convective systems.Convection in the ISV inactive period (PP1) contained convective rain fractions nearly twice as much as stratiform rain fractions, whereas that in the ISV active period (PP2) comprised convective and stratiform elements almost equally. Vertical profiles of radar echo coverage for stratiform rain during PP2 were greater than those during PP1, especially in the lower troposphere over the sea. The radar echo coverage for convective rain over the sea during both periods was nearly double that over land from the near surface up to 6 km high.Convection was generated in the southwestern foothills of the mountain range in the early afternoon (12-15 Local Time, LT). Part of the convective system remained over the coastal land and exhibited weak reflectivity until the next morning. The other part migrated offshore at a speed of approximately 4 m s-1 and intensified until around 21 LT while still offshore. Additional convective cells also developed offshore in the early morning hours, independent of those that formed over land. Results suggested that the CHeR along Sumatera Island is dictated by diurnal variations in coastal convective development and consists of the following phases: 1) migration of convection away from the coastal land and its redevelopment in the late evening, and 2) additional generation of convection just offshore during the early morning hours.