Floods are a major contributor to natural disasters in Sumatra. However, atmospheric conditions leading to floods are not well understood due, among other factors, to the lack of a complete record of floods. Here, the 5 year flood record for Sumatra derived from governmental reports, as well as from crowd-sourcing data, based on Twitter messages and local newspapers’ reports, is created and used to analyze atmospheric phenomena responsible for floods. It is shown, that for the majority of analyzed floods, convectively coupled Kelvin waves, large scale precipitation systems propagating at ∼12 m/s along the equator, play the critical role. While seasonal and intraseasonal variability can also create conditions favorable for flooding, the enhanced precipitation related to Kelvin waves was found in over 90% of flood events. In 30% of these events precipitation anomalies were attributed to Kelvin waves only. These results indicate the potential for increased predictability of flood risk.

On the basis of detailed analysis of a case study and long-term climatology, it is shown that equatorial waves and their interactions serve as precursors for extreme rain and flood events in the central Maritime Continent region of southwest Sulawesi, Indonesia. Meteorological conditions on January 22, 2019 leading to heavy rainfall and devastating flooding in this area are studied. It is shown that a convectively coupled Kelvin wave (CCKW) and a convectively coupled equatorial Rossby wave (CCERW) embedded within the larger-scale envelope of the Madden-Julian Oscillation (MJO) enhanced convective phase, contributed to the onset of a mesoscale convective system which developed over the Java Sea. Low-level convergence from the CCKW forced mesoscale convective organization and orographic ascent of moist air over the slopes of southwest Sulawesi. Climatological analysis shows that 92% of December-January-February floods and 76% of extreme rain events in this region were immediately preceded by positive low-level westerly wind anomalies. It is estimated that both CCKWs and CCERWs propagating over Sulawesi double the chance of floods and extreme rain event development, while the probability of such hazardous events occurring during their combined activity is eight times greater than on a random day. While the MJO is a key component shaping tropical atmospheric variability, it is shown that its usefulness as a single factor for extreme weather-driven hazard prediction is limited.

ABSTRAKKetersediaan air tanah yang sebagian besar berasal dari curah hujan merupakan faktor pembatas yang penting bagi peningkatan produksi suatu tanaman. Neraca air merupakan suatu metode yang dapat digunakan untuk melihat ketersediaan air tanah bagi tanaman pada waktu tertentu. Prosedur perhitungan neraca air dibuat berdasarkan sistem tata buku Thorntwaite dan Mather (1957) dengan satuan tinggi air (mm). Analisis ketersediaan air tanah secara grafik disajikan dalam periode bulanan, mulai Januari hingga Desember secara klimatologis. Data yang digunakan dalam analisis ini adalah data rata-rata jumlah curah hujan bulanan dan suhu rata-rata bulanan tahun 1985-2011 dari Stasiun Klimatologi Pulau Baai Bengkulu yang mewakili daerah penelitian yaitu wilayah Kota Bengkulu dan data tanah Kota Bengkulu. Wilayah Kota Bengkulu memiliki ketersediaan air yang sangat cukup dengan persentase air tersedia setiap bulannya 100%, yang dibuktikan dari hasil perhitungan, oleh karena itu Kota Bengkulu memiliki nilai surplus sepanjang tahun. Pada kondisi jumlah curah hujan bulanan yang cukup tinggi berkisar 179 -458 mm, suhu yang relatif rendah berkisar 25,9 -26,9 ˚C, dan lahan dengan nilai surplus yang tinggi seperti ini, di wilayah Kota Bengkulu baik untuk ditanami padi, sedangkan untuk penanaman jagung kurang baik.Kata kunci: neraca air, curah hujan, kesesuaian lahan, suhu udara, klasifikasi oldeman ABSTRACTThe availability of groundwater most of which comes from rainfall is an important limiting factor for increasing the production of a plant. Water balance is a method that can be used to see the availability of ground water for plants at a certain time. The water balance calculation procedure is based on the Thorntwaite and Mather book keeping system (1957) with water unit (mm). Analysis of groundwater availability graphically is presented in monthly periods, from January to December climatologically. The data used in this analysis is the average data of the amount of monthly rainfall and the average monthly temperature of 1985-2011 from Bengkulu Island Climatology Station representing the research area of Bengkulu City and Bengkulu City land data. Bengkulu City has a very good water supply with percentage of water available every month 100%, as evidenced from the calculation, therefore Bengkulu City has surplus value throughout years. While conditions of high monthly rainfall is quite high ranging from 179 -458 mm , Relatively low temperatures ranging from 25.9 -26.9 ˚C, and land with a high surplus value like this, in the area of Bengkulu City is suitable for planting rice, whereas for the planting of maize is less suitable.

Teknik RGB (Red-Green-Blue) merupakan salah satu teknik intepretasi citra satelit dengan mengombinasikan beberapa kanal secara tumpang tindih warna merah, hijau dan biru untuk menyajikan informasi yang lebih mudah dipahami. Teknik RGB dapat digunakan dalam kajian analisis cuaca, terutama untuk mengidentifikasi kondisi khusus seperti bencana hidrometeorologi. Kejadian banjir pada tanggal 20 - 21 Februari 2017 yang merendam sekurangnya 7 kabupaten dan 1 kota di Provinsi Lampung yang dindikasikan terjadi karena hujan ekstrim yang merata di wilayah Lampung. Hasil pelaporan curah hujan di stasiun Klimatologi Masgar terukur 107.0 mm/hari, Pos Pengamatan Politeknik Negeri Lampung terukur 159.6 mm/hari dan Pos Pengamatan Kemiling Bandar Lampung terukur 154.0 mm/hari dimana curah hujan termasuk dalam kategori hujan sangat lebat BMKG (> 100 mm/hari). Hasil analisis kondisi regional menunjukan adanya tekanan udara rendah di barat lampung dan daerah konvergensi serta shearline di Lampung bagian barat dan tengah. Analisis citra satelit menunjukan adanya kumpulan awan dengan suhu puncak yang sangat dingin, teknik RGB menggunakan identifikasi mikrofisis atmosfer pada malam hari (Night Microphysics) dan sebaran massa udara (Air Mass) menunjukan adanya proses mikrofisis yang intensif serta aliran massa udara penyebab awan hujan yang tumbuh dan meluas di wilayah Lampung sebelum dan saat terjadinya banjir. Hasil produk olahan HCAI (Highresolution Cloud Analysis Information) menunjukan awan didominasi oleh awan Comulonimbus (Cb) dan awan konvektif padat (Dense Cloud).

In 2017, BMKG has 41 weather radars covering most of Indonesia region and most of its data are automatically sent to the BMKG headquarter every 10 minutes. There are four different weather radar brands with its specific data format and software analysis. In recent years, the weather radar community has developed open-source software to handle several radar data formats. Based on this, BMKG has developed the Indonesia In-House Radar Integration System (InaRAISE) of BMKG using the open-source weather radar software. InaRAISE has been developed using Python-based libraries Wradlib and Py-ART for processing weather radar data. BMKG radar data have been successfully extracted and transformed into Cartesian coordinates for post-processing. The multiple radars have been successfully composited by comparing column-maximum reflectivity. Web-based near real-time radar images has been experimentally operated, but not officially launched. The main constraint is the susceptible communication network between radar sites and BMKG headquarter causing real-time data transfer problems. InaRAISE serves feasible data radar extraction for data assimilation in the numerical weather prediction model. InaRAISE could serve as a supporting of the existing radar integration system or possibly as a replacement.