Recently, Shibagaki et al. (2006) found multi-scale convective interactions over western Sumatera using a ground-based x-band weather radar, which interactions drive a rainfall peak migration eastward (into inland region) during the daytime. Sakurai et al. (2005) pointed out that the westward (into offshore region) rainfall peak migration are commonly seen though the year, whereas those moving eastward (into inland region) occur only in the rainy season based on GMS/GOES cloud data analysis. However, fundamental mechanisms driving the rainfall peak migration, especially which migrate westward (into the offshore region) in the nighttime, have not been well understood because the TRMM PR has a disadvantage of coarse temporal sampling resolution, the x-band radar is located in a mountainous area and cannot cover the offshore region, and the GMS/GOES watches only the cloud top temperature.
In order to investigate physical multiple mechanisms of diurnal migrating rainfall systems over western Sumatera, we installed two x-band Doppler radars and an L-band wind profiler at the southwestern coastline of Sumatera Island (Tiku: 0.4S, 99.3E and MIA: 0.9S, 100.3E) in 2006 as a part of radar-profiler network over Indonesia deployed by the Hydrometeorological ARray for Intraseasonal variation (ISV) Monsoon Auto-monitoring (HARIMAU) project under the Japan Earth Observation System [EOS] Promotion Plan (JEPP). Although an original objective of HARIMAU/JEPP radar-profiler network is to monitor the hydrological and energy cycles of atmospheric circulation over the maritime continent, we can utilize the network for studies on diurnal to intraseasonal variability of convective activity. Wu et al. (2005) has successfully simulated the diurnal westward moving convective systems in the nighttime with a form of squall line system by using the MM5 cloud resolving model. Cold surges near the surface in front of the old convective cells are suggested to play an important role for moving squall lines. Our observation planned to investigate multiple mechanisms of diurnal convective activity in terms of not only the cold surge effect on moving squall lines but also e.g., gravity waves generated by coastal convections, radiative cooling at cloud tops in the nighttime, and cloud microphysics such as feeder-seeder process over the offshore region.
We set an intensive observation period (HARIMAU2006) from October 26 to November 27 in 2006. The dual Doppler radar observation captured three dimensional wind and reflectivity structures over the coastline to offshore region continuously, and the wind profiler monitors wind variations beneath the convections. Intensive soundings of 3-6 hours intervals at a coastal meteorological station over Sumatera Island provided fine atmospheric variations which showed well defined diurnal cycles. A network of automatic weather stations with high-sensitive barometer covered the coastal region to monitor local rainfalls, cold surges, and gravity waves. Furthermore, we collaborated with R/V Mirai Indian Ocean cruise for the Study of the MJO-convection Onset (MISMO) which consisted of Doppler radars observations and intensive rawinsonde soundings for the same period over the central Indian Ocean (EQ, 80E) and Maldives (EQ, 73E) as well as those at Sumatera/Indonesia (EQ, 100E) of our study.
Although eastward moving active cloud clusters were observed over the Indian Ocean during the period, most clusters were weaken before they reached Sumatera Island. Some active clusters passed over our sites in the beginning and the end of period. Characteristics of diurnal variations were quite different between ISV active and inactive phases, e.g., rainfall amount, echo areas, echo top height, and convective fractions. We also captured several line-shaped storm systems accompanied with strong gust winds near the coastline. Several wooden fisherman's houses were broken down by the gust wind on November 19. Furthermore, the radar data was opened for public through the internet not only for research work but also for airline safety, hydroelectric power, agriculture, and other water resource managements. Visit the MIA-XDR website http://203.88.86.149/mia_xdr/index.html for further information.
References:
Mori, S., Hamada J.-I., Y. I. Tauhid, M. D. Yamanaka, N. Okamoto, F. Murata, N. Sakurai, H. Hashiguchi, and T. Sribimawati, 2004: Diurnal land-sea rainfall peak migration over Sumatera Island, Indonesian maritime continent observed by TRMM satellite and intensive rawinsonde soundings. Mon. Wea. Rev., 132, 2021-2039.
Sakurai, N., F. Murata, M. D. Yamanaka, S. Mori, J.-I. Hamada, H. Hashiguchi, Y.-I. Tauhid, T. Sribimawati, and B. Suhardi, 2004: Diurnal cycle of cloud system migration over Sumatera Island. J. Meteor. Soc. Japan, 83, 835-850.
Sasaki, T., P. Wu, S. Mori, J.-I. Hamada, Y. I. Tauhid, M. D. Yamanaka, T. Sribimawati, T. Yoshikane, and F. Kimura, 2004: Vertical moisture transport above the mixed layer around the mountains in western Sumatra. Geophys. Res. Lett., 31, L08106.
Shibagaki, Y., T. Shimomai, T. Kozu, S. Mori, Y. Fujiyoshi, H. Hashiguchi, M. K. Yamamoto, S. Fukao, and M. D. Yamanaka, 2006: Multi-scale convective systems associated with an intraseasonal oscillation over the Indonesian maritime continent. Mon. Wea. Rev., 134, 1682-1696.
Wu, P., J.-I. Hamada, S. Mori, Y. I. Tauhid, and M. D. Yamanaka, 2003: Diurnal variation of precipitable water over a mountainous area of Sumatra Island. J. Appl. Meteor., 42, 1107-1115.
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