PHASE VARIATION OF PRECIPITATION DIURNAL CYCLE OVER THE NORTHERN COAST OF THE WESTERN PART OF JAVA AND ITS RELATIONSHIP TO CROSS EQUATORIAL NORTHERLY SURGE-COLD TONGUE PHENOMENA

• Main Author: Yulihastin, Erma
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/69886
• Institution: Institut Teknologi Bandung

Flood events over some regions on the northern coast of the western part of Java were mostly related to the persistent rainfall that spread between morning and afternoon. This research aims to determine the phase variation of the diurnal rainfall cycle over this area and analyze two possible causes, i.e., the Cross- Equatorial Northerly Surge (CENS) and Cold Tongue (CT). In these terms, CENS is a northerly flow (925 hPa) cross over the equatorial region and CT is a southward transports of colder seawater along Vietnam coast and form an extension of cool sea surface temperature (SST) in South China Sea (SCS). We investigate this problem using multi-timescale variables spanned from November to March (NDJFM) for 17 years (2000-2016). The variables used here are hourly data of rainfall from the Tropical Rainfall Measuring Mission (TRMM) Multi- Satellite Precipitation Analysis (TMPA-RT) 3B41RT with the spatial resolution of 0.25o????0.25o and daily data of surface wind (925 hPa) from the National Centre for Environmental Prediction/National Centre for Atmospheric Research (NCEP/NCAR) with 2.5o ???? 2.5o horizontal grid spacing. Furthermore, we also utilize the daily data of sea surface temperature (SST) from the Optimal Interpolated Sea Surface Temperature Advanced Very High-resolution Radiometer (OISST AVHRR) with a quarter degree spatial resolution, and hourly data based on a numerical study using the Weather Research and Forecasting (WRF) model with three real cases. The phase variation determined by climatological analysis of the month-to-month diurnal rainfall variation and harmonic method. The result shows that the highest variation in phase, amplitude, and frequency happened particularly in January and February. It also found that both of the Late Afternoon (LA) and Morning (M) rainfall have significant frequencies with 55% and 45% contribution, respectively. The M rainfall further classified into the Early Morning (EM) and Late Morning (LM) rainfall. The LA rainfall has a normal distribution, and its maximum frequencies appeared during 18:00-20:00 local time (LT) with 0.5-0.8 mm/hr. Meanwhile, the EM rainfall has a flat frequency from 01:00-04:00 LT (0.6-0.9 mm/hr). Additionally, the LM rainfall has gamma distribution and attain the peaks during 05:00-06:00 LT (0.5-0.7 mm/hr), thus continues to decrease until it ceased at noon. The time-latitude cross-section (Hovmöller) of the LA and EM rainfall iv depicts propagation characteristics and strong connections between land-sea convection systems. In particular, the EM rainfall over coastal areas characterized by large amplitude, high randomness, propagate and associated with extreme rainfall. To recognize the effect of CENS and CT on the diurnal phase variations, we then select the LA, EM, and LM rainfall of random samples based on the four possible combinations of CENS and CT events. The combinations are CENS–CT, CENS– nCT, nCENS–CT, and nCENS–nCT, with preceding n indicate no effect of either CENS or CT to the time-specific rainfall. The results show that the frequency of CENS-nCT, nCENS-CT, and CENS-CT, are more prominent during the EM and the LM rainfall, compared to the LA rainfall. Also, during the EM rainfall, CENS seems significant (more than 8 m/s) and extending to the Java Sea compared to the LM (6-8 m/s) and LA (4-6 m/s). Furthermore, during the EM rainfall, the CENS strengthen the westerly monsoon over Java island, coincide with the CT that appears cooler and extends to the Karimata Strait. The results also conclude that the CENS-CT events more frequently coincide with the EM rainfall. To further scrutinize the relationship between CENS-CT events and the EM rainfall propagation, a numerical study was conducted using the WRF model by simulating three real cases through classifications: CENS-CT, nCENS-CT, and nCENS-nCT. The CENS-nCT was not simulated because it represented by the CENS-CT since the CT does not have a significant effect on the diurnal cycle phase variation, so if CT coincides with CENS, as a consequence, CT can be ignored. Furthermore, the results clarify that during the nCENS-nCT, the EM rainfall has an offshore propagation. Concurrently, during the nCENS-CT event, the EM rainfall is triggered by the initial convection over the sea that produces a landward propagation. The convection eventually concentrated over the coastal area that influenced by the spread of the convergence region over the mainland. The role of a gravity wave with phase velocity around 10-20 m/s influences offshore (nCENSnCT) as well as landward propagation (CENS-CT, nCENS-CT) through modulating the EM rainfall. Additionally, the internal processes in convective clouds can also increase propagation at a lower speed (~5 m/s). The results also clarify that the direction of propagation over the region can randomly spread in all directions. The novelties in this dissertation are the documentation and classification of the phases, amplitudes, and frequencies of the diurnal rainfall variation with the statistical examination in its correlation and significance for the extreme events over the northern coast of the western part of Java. Furthermore, this study proposes a new comprehension regarding the source for the diurnal rainfall phase variation over this region.