Migration Process and 3D Wind Field of Precipitation Systems Associated with a Diurnal Cycle in West Sumatera: Dual Doppler Radar Analysis during the HARIMAU2006 Campaign

Abstract

This study describes the three-dimensional structure and migration process of a westward-migrating precipitation system with a diurnal cycle observed on 10 November 2006 in west Sumatera as based on dual-Doppler radar analysis, rawinsonde data, and surface data. The location of convective cell generation over land in the daytime and the timing of an evening change in the migration direction of precipitation systems from landward to seaward were influenced by local circulation: in the morning, an isolated convective cell generated near the west coast propagated inland (toward the mountains) via the successive generation of new convective cells over the sea breeze. In the afternoon, convective cells in a precipitation system were generated over the western slopes of the mountains surrounding Lake Maninjau by thermally induced local circulation and the slope effect. In the evening, precipitation systems located over the mountains started to propagate toward the sea (westward) in response to a change in local circulation from landward to seaward winds. Subsequently, precipitation systems were newly generated over the sea near the west coast of Sumatera Island, merging with the systems that originated over land. These two sets of systems formed a larger system (long axis > 100 km) than that over land (long axis of several tens of kilometers). The expanded precipitation system had a convective region with a long axis oriented parallel to the west coast of Sumatera Island and a short axis (∼7 km) oriented perpendicular to the west coast at the leading edge of the system. The echo top height of the system was located at approximately 13 km and an anvil moved faster than the convective region above 6 km in height. The convergence was formed by an easterly wind component in the system and a southerly wind component over the sea around the leading edge of the system at the lowest layer. The expanded system migrated farther offshore at a speed of about 5 m s^-1 via the self-replication of convective cells over the convergence at the leading edge of the system and via advection by ambient wind in the lower troposphere. The convergence regions at the leading edge of the system were continuously strengthened by downward transportation of horizontal momentum below 4 km, meaning that the system could be maintained for a long time and migrate offshore for a long distance.