VARIASI INTRA-MUSIMAN DARI DISTRIBUSI VERTIKAL BUTIRAN HUJAN DI KOTO TABANG, SUMATERA BARAT

Marzuki, M.Eng.MSc (2010) VARIASI INTRA-MUSIMAN DARI DISTRIBUSI VERTIKAL BUTIRAN HUJAN DI KOTO TABANG, SUMATERA BARAT. Project Report. lp unand. (Unpublished)

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Abstract

2D video disdrometer (2DVD), Boundary Layer Radar (BLR) and Equatorial Atmosphere Radar (EAR) were used to elucidate the influence of intraseasonal variation (ISV) in response to Madden-Julian oscillation (MJO) on raindrop size distribution (DSD) at Koto Tabang (KT), West Sumatra, Indonesia, during the first Coupling Process of Equatorial Atmosphere (CPEA) campaign (10 April – 09 May 2004). During the observation period, a clear transition of the MJO phase, from convectively inactive to active has been reported by some investigators. The precipitation data are partitioned into four categories (i.e., stratifrom, mixed stratiform/convective, deep convective and shallow convective) based on a modified version of Williams method. From this study it is found that the total accumulation rainfall at KT is strongly modulated by the ISV. The analysis also revealed that during the inactive phase of ISV, daily precipitating clouds are dominated by deep convective, while stratiform and shallow convective events are dominant during the active phase. Analysis of gamma raindrop size distribution (DSD) at the ground shows that deep convective spectra are significantly influenced by ISV. Regarding the 2DVD derived median volume diameter Do, total number of raindrops NT, and Liquid water content LWC plotted versus the rain rate (R), the Do is larger, NT and LWC are smaller in the inactive than in the active phase of ISV. These suggest the raindrop spectra during the inactive phase are broader and contain less small drops than during the active phase of ISV. Vertical structure of the DSD derived from the EAR is consistent with the ground based-DSD analysis. Broad drop size spectra are observed in rain column throughout (2.89 – 3.94 km) marked by larger values of Do during the inactive than active phase of ISV. Furthermore, the vertical profiles of Do during the inactive phase showed a more heterogeneous structure compared to active one, probably as a consequence of the complex microphysical processes affecting the DSD during their fall. Analysis of individual spectra shows that during the inactive phase, breakup and coalescence seem to be occurred by turns, contrary to the active phase where coalescence seem dominant during this period. Large drops generally decreased (increased) downward with height consistent with downward decreasing (increasing) of reflectivity gradient previously reported by other investigators.

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