Torrential rains that repeatedly occurred over Java Island causing widespread floods in late January and early February 2007 coincided with a strong and persistent trans-equatorial monsoon flow from the Northern Hemisphere. While convections develop frequently over the island's mountainous areas in the afternoon, convections over the northern plains are active during the night and morning hours. The strong trans-equatorial monsoon flow with an upper southeasterly wind produces a strong low-level vertical shear of wind and dry mid-level environment over the island. These conditions allow the severe convections to occur repeatedly for days and to sustain for an extended period of time. The results suggest that the trans-equatorial monsoon flow plays a principal role in the formation of the repeated torrential rains. The probability of occurrence of a strong and persistent trans-equatorial monsoon flow that causes torrential rains and widespread floods over Java Island is estimated to be once every 5 10 years.

No abstract

This paper presents an overview of the HARIMAU2010 campaign focusing on convective activity with the diurnal rainfall meridional march (DRMM) over Jakarta, which is located on the northern coast of Jawa Island of the Indonesian maritime continent (IMC), based on 1-month intensive observations by a C-band Doppler radar and multi-point atmospheric sounding array conducted during 16 January-14 February 2010. The campaign period corresponded to a phase after large-scale Madden-Julian oscillation (MJO) active convections passed over Jakarta (MJO inactive phase). The cross-equatorial northerly surge (CENS) intruded into the Jawa Sea with a cold tongue (CT) of sea surface temperature (SST) in the beginning of the period (CENS active period: 16-26 January), and then, it started to retreat (transition period: 27 January-05 February); afterward, only a few signs of it were apparent (CENS inactive period: 06-14 February). The observational results showed that (1) rainfall over Jakarta has the nature of DRMM during the MJO inactive phase at least, (2) the DRMM is likely driven primarily by "land-breeze"-like local meridional circulation, and (3) the meridional spatiotemporal variation of rainfall over Jakarta is thus controlled by activities of both the CENS and CT over the Jawa Sea.

An overview of convective activity during the HARIMAU2006 campaign conducted from 26 October to 27 November 2006 was presented, focusing on the di¤erences 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 o¤shore at a speed of approximately 4 m s À1 and intensified until around 21 LT while still o¤shore. Additional convective cells also developed o¤shore 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 o¤shore during the early morning hours.

A 1-month-long observation of the Ciliwung River, which flows through Jakarta in Indonesia, has revealed evidence of the persistent existence of a diurnal cycle in the water level of a tropical river. This was consistent with the diurnal cycle in rainfall observed by meteorological radar and five rain-gauge stations. The river's diurnal cycle was distinguishable from the effects of oceanic and atmospheric tides and has a locally time-locked 1-day periodicity and an amplitude of 0.05 m. The day-to-day variation in the amplitude of the river's diurnal cycle was smaller than the diurnal cycle of the rainfall.