Observational Study on Westerly Wind Burst over Sumatra, Indonesia by the Equatorial Atmosphere Radar A Case Study During the First CPEA Campaign

Abstract

This study focuses on features of vertical wind and cloud distributions in Sumatra during the initial phase of a westerly wind burst (WWB) associated with a synoptic-scale super cloud cluster (SCC), by mainly using radar, radiosonde, and lidar data from 5 to 9 May 2004. The convective envelope of the SCC reached Sumatra from the Indian Ocean on 5 May, passing over Sumatra on 7 May. Intensification of the westerly wind occurred over Sumatra below 5.5-6.0 km as the SCC passed over it. On 7 May, the 2.5-4.0 km westerly wind at Kototabang (KT; 0.2 S, 100.32 E, 865 m MSL) was identified as a WWB. Precipitating clouds around KT were suppressed after 7 May, as drier air (lower than 60% relative humidity) was transported from the Indian Ocean over Sumatra at 2.5-6.0 km. Non-precipitation clouds were observed at 5-8 km by the lidar after 7 May.After 7 May, the vertical wind at 2.5-5.5 km showed the oscillatory motion with a timescale of about 12 hours. Similar oscillatory motion was found in the 1.5-2.5 km zonal wind. Contrary to the Corresponding author: Masayuki Yamamoto, Research Institute for Sustainable Humanosphere (RISH), Kyoto University, Uji, Kyoto 611-0011, Japan. E-mail: m-yamamo@rish.kyoto-u.ac.jp ( 2006, Meteorological Society of Japan radiosonde-derived downward wind with a horizontal scale of several hundred km, daily-averaged vertical wind at KT showed upward motion of 0.07-0.08 m s À1 on 7 and 8 May, when westerly winds larger than 10 m s À1 prevailed at 2.5-4.0 km. These facts imply that the topography around KT, which has steep mountains to the west, modulates the behaviors of the vertical wind.The vertical wind oscillation was suppressed above 3.0-5.5 km, where the Richardson number (Ri) was smaller than 0.45 and westerly wind changed to easterly wind. The small Ri was brought about by strong vertical wind shear (larger than 10 m s À1 km À1 ) and/or weak vertical gradient of potential temperature (smaller than 3 K km À1 ). Both regions appeared at the upper part of the westerly wind region. This fact implies that shear instability and horizontal wind change inhibit upward propagation of vertical wind oscillations.

Vertical wind observation in the tropical upper troposphere by VHF wind profiler: A case study

“…The results of T BB indicate that upward motions disappear as cumulus activity is suppressed over Sumatra. For details of W motions in the lower and middle troposphere at KT from 7 to 9 May, see Seto et al [2006]. They also have shown that cumulus activity at KT was suppressed after 7 May because of the dryness of the lower troposphere, and the lower tropospheric dry air was transported by westerly wind existed in the west of synoptic‐scale convectively active region.…”

“…A VHF wind profiler named the Equatorial Atmosphere Radar (EAR) was installed at Kototabang (0.20°S, 100.32°E, 865 m above sea level; hereafter KT), West Sumatra, Indonesia, in 2001. Using EAR wind data, convective features over Sumatra have been revealed [e.g., Renggono et al , 2006, Shibagaki et al , 2006a, Seto et al , 2004, 2006]. Further, EAR observations have revealed phenomena around the tropopause such as an enhancement of turbulence by Kelvin wave breaking [ Fujiwara et al , 2003] and an existence of continuous shear instability [ Yamamoto et al , 2003].…”

Vertical air motion in midlevel shallow‐layer clouds observed by 47‐MHz wind profiler and 532‐nm Mie lidar: Initial results

“…Absence of deep cumulus convection occurred on a synoptic scale. A synoptic‐scale cloud cluster, which moved from the Indian Ocean to the western Pacific Ocean and caused deep cumulus convection over Sumatra in the beginning of May 2004, already moved to the east of Sumatra during the period focused on (8–9 May 2004) [ Seto et al , 2006; Yamamoto et al , 2007].…”

“…9.4‐GHz weather radar was operated at ∼20 km southeast of KT to observe precipitation at and around KT. During the period we focused on, precipitation particles inside the clouds were not detected by 9.4‐GHz weather radar (see Figure 6 of Seto et al [2006]). Therefore the observed clouds were shallow‐layer ones, and did not have precipitation particles whose diameter is typically larger than 0.5 mm.…”

Case Study of an Intense Wind Event Associated with a Mesoscale Convective System in West Sumatera during the HARIMAU2006 Campaign

“…Murata et al (2006) showed that an intensification of westerly wind that is accompanied by a super cloud cluster can cause a dry-air intrusion from the Indian Ocean to Sumatera in the lower and middle troposphere and suggested that the dry air plays a role in suppressing convective activity over Sumatera. Seto et al (2004) and Seto et al (2006) also reported that the Sumatera region became convectively inactive after an intensification of lower-tropospheric westerly wind. These results suggest that the local enhancement of the large westerly momentum by convections seldom occurs during WWBs.…”