論文の公開元へA Study on the Influence of Cross-Equatorial Northerly Surge on the Diurnal Cycle of Rainfall over Java Island
概要
The diurnal cycle of rainfall (DCR) is the pronounced rainfall characteristic over the Indonesian Maritime Continent (IMC). The pattern of the DCR is affected by the interaction between the local wind circulation and the prevailing wind. The local wind circulation over the IMC depends on the land-sea thermal gradient. At the same time, the prevailing is varied seasonally affected by the monsoon flow, namely Austral monsoon in June-July-August (JJA) and Asian monsoon in December- January-February (DJF). In the boreal winter, when the Asian monsoon flow equatorward, an intermittent strong northerly wind is observed below the equator line that significantly enhances the precipitation over Java Island in the IMC. This phenomenon is identified as a cross-equatorial northerly surge (CENS).
CENS is reported to cause torrential rainfall that causes severe flood events in northwestern Java. The flood disaster in Java Island has greatly impacted many lives and socio-economic activity, considering more than half of the Indonesian population lives there. In this situation, Java Island becomes more vulnerable to flood disasters since boreal winter is the peak of the rainfall season.
Some studies indicated that the rainfall increase is attributed to changes in the DCR in northwestern Java. It is not well understood whether the DCR changes are present in the other parts of Java Island and whether the changes are similar to that in northwestern Java. Furthermore, the mechanism of the DCR modification by CENS over northwestern Java and the other regions is still unclear. Hence, this study aims to clarify the influence of CENS on the DCR over different regions of Java Island and to clarify the mechanism of the DCR modification due to CENS. To achieve the study objectives, we perform a climatological analysis of twenty years of boreal winter and long-term numerical weather simulation to achieve study aims.
In chapter 2, we first evaluate the rainfall estimate from the integrated multi- satellite retrievals for GPM (IMERG ) that is mainly used to study the DCR. Then, we investigate how the IMERG represents the general feature of the DCR in the boreal winter.
Chapter 3 discusses the regional variation of the influence of the CENS on the DCR over Java Island using IMERG and the fifth-generation ECMWF atmospheric reanalysis of the global climate (ERA5 ). We analyzed the DCR on the days without CENS (nonCENS) and the days with CENS to understand the impact of the strong northerly wind during CENS. The analysis is focused on three different regions, namely western Java, central Java, and eastern Java.
In chapter 4, we conduct a numerical simulation using Weather Research and Forecasting (WRF) model to explain the mechanism of DCR modification, especially in the rainfall enhancement process, since the resolution of observational data is too rough to cover the localized process over Java Island. In addition, since a numerical study on the DCR is challenging due to the model deficiency in representing the convective system over IMC, we evaluate the model result in representing the influence of CENS on the DCR. Therefore, the scope of the numerical model simulation is limited to the WRF model evaluation and focuses to clarify the mechanism of the rainfall enhancement initiation by CENS.
The IMERG data evaluation shows that IMERG could well depict the DCR pattern over Java Island. A comparison of IMERG with the automatic weather station (AWS) of Bandung Institute of Technology-Durham University in the coastal and mountainous locations shows that IMERG has a better estimate in the coastal location. Even IMERG could represent the secondary peak that appears in December and January 2019 at the coastal station. The climatological analysis reveals the general feature of the DCR phase in the boreal winter. A distinct pattern shows in the inland and sea areas where the inland area has an afternoon peak, while the ocean has an early morning to morning peak. In some coastal land areas close to the shoreline, the early morning peak is dominant. In the sea area, the coastal sea is predominated by the early morning peak (01-04 LT) while the offshore is predominate by the morning peak (07-10 LT).
We find that there are differences in intensity, timing, and coverage of DCR responses over western, central, and eastern Java. CENS modifies the timing of coastal rainfall on the northern coast, resulting in early morning rainfall enhancement owing to the convergence between the incoming northerlies and land breeze. A small increase in early morning rainfall is observed in eastern Java, possibly due to the northerly flow gaining more zonal components on the eastern coast, which then results in relatively weaker convergence than the western coast. The timing of early morning rainfall appears to be slightly different between the three areas. CENS is also found to suppress afternoon rainfall inland because of increasing static stability due to cold air advection from the north. The suppression varies from western to eastern Java, with the largest suppression seen in western Java. Relatively narrower and wider suppression areas are found in central and eastern Java, respectively. These differences are attributed to the topography characteristics of Java.
The WRF model results show good performance in representing torrential early morning rainfall events in the coastal area, which is the biggest concern in the area. The model could also reproduce regional variabilities of the DCR in western, central, and eastern Java, which becomes important since the CENS generates different influences in each region, as suggested in Chapter 3. The investigation of the mechanism of heavy rainfall initiation that triggered propagating rainfall indicates that more complex processes are involved. Convergence between local circulation with the background wind, which shifts closer to northwestern Java during CENS, is not solely responsible for the torrential rainfall in the early morning. The turning flow (landward) that interacts with the dominant wind (seaward) is one of the possible mechanisms simulated in the model that initiates heavy rainfall from southeastern Sumatra that triggers the propagating rainfall towards northwestern Java and produces torrential rainfall in the early morning.
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