Dual Wind Profiler Observations of a Line-Shaped Convective System in Southern Kyushu, Japan

Abstract

A line-shaped convective system (LCS) was observed on 19^th June 2001 in southern Kyushu, Japan in a field experiment called X-BAIU-01. Operational meteorological radar observed a mesoscale line of precipitation in this experiment. A detailed analysis of the radar observation indicated that this line of precipitation had a structure of the LCS, which is a type of mesoscale convective system (MCS). The convective cells in the LCS propagated east-northeastward at the speed of 25 m s^-1.
The LCS formed between two wind profilers. “Dual wind profiler” analysis illustrates mesoscale-flow patterns upstream of the LCS, such as horizontal convergence with upward airflow above 500 m in altitude, and divergence with downward airflow underneath. Moreover, an integrated analysis of the dual wind profiler, operational weather radar and radiosonde measurements revealed the formation of the LCS as follows. The surface atmosphere from the ocean climbs a mountain slope in southern Kyushu up to the lifting condensation level (LCL) by environmental wind, and forms a stratus cloud. The atmosphere continues climbing the mountain slope up to the height of a convergence layer aloft. The strong updraft in the convergence layer changes the type of cloud from stratus to cumulus at this height, and increases precipitation intensity. The atmosphere continues ascending up to the level of free convection (LFC) in the updraft region. After reaching the LFC, the atmosphere increases the ascending speed by buoyancy inside, and develops convective cells. A number of cells are advected by the environmental wind and form the LCS. This process indicates that this type of the LCS does not have a back-building structure, which is one type of the LCS, but has a terrain-induced structure. A refined conceptual model of this type of the LCS is presented based on previous studies and observations reported in this paper. The analysis also indicates that the isolated mountains or islands upstream are not essential for the formation of this type of the LCS, because the profile of the atmosphere does not satisfy the conditions necessary to trigger convections or to excite lee waves by the mountains or islands.