Natural events following the activity of the Tropical Cyclone Seroja in April 2021 are investigated. During its active phase, Tropical Cyclone Seroja generated extreme rainfall events in some sub-provinces of East Nusa Tenggara (NTT): Ngada, Alor, Belu, Rote Ndao on 4 April, 2021, Kupang on 4 to 5 April, 2021, East Sumba on 4 to 6 April, 2021. Moreover, these extreme rainfall events triggered flood in Alor, East Flores, Lembata, The City of Kupang, Kupang, East Sumba, Malaka, Belu, and North Central Timor. The maximum sea wave height of the Indian Ocean at the Southern part of NTT was also increasing, from 4 meters on 1 to 2 April, 2021 up to 6 meters on 3 April, 2021, and rose to higher than 7 meters on 4 to 6 April, 2021. On 7 to 9 April, 2021, the sea wave height declined as the Tropical Cyclone Seroja moved to the Southwest of NTT.

Informasi karakteristik gelombang merupakan hal yang sangat penting dalam berbagai bidang seperti pelayaran, industri, pariwisata dan keamanan struktur lepas pantai. Penelitian mengenai karakteristik gelombang telah dilakukan oleh banyak peneliti baik dalam skala global maupun regional. Metode yang digunakan umumnya adalah perata-rataan yang menghilangkan variasi selama data tersebut diproduksi. Variasi data tersebut ada kalanya dibutuhkan untuk melihat tingkat kestabilan data selama kurun waktu tertentu. Tujuan penelitian ini adalah untuk mengetahui distribusi spasial dan temporal tinggi gelombang signifikan di wilayah Indonesia serta mengetahui tingkat kestabilannya. Pada penelitian ini dilakukan analisa kestabilan tinggi gelombang melalui perhitungan koefisien variasi, indeks variabilitas bulanan dan indeks variabilitas musiman. Data yang digunakan adalah luaran model WAVEWATCH-III pada jangka waktu tahun 1991-2015 dengan time step 3 jam. Hasil penelitian menunjukkan bahwa gelombang signifikan memiliki pola variabilitas berdasarkan siklus monsunal. Gelombang signifikan di Indonesia mencapai puncaknya bersamaan dengan aktifnya monsun Asia dan Australia di bulan DJF dan JJA. Tingkat kestabilan yang direpresentasikan oleh koefisien variasi mencapai nilai terendah juga pada musim ini. Hal ini berarti pada saat monsun aktif, gelombang signifikan cenderung stabil dibandingkan dengan musim peralihan. Distribusi spasial menunjukkan bahwa daerah laut lepas lebih tinggi gelombangnya dibanding di laut tertutup. Koefisien variasi dan indeks variabilitas musiman dan bulanan menunjukkan daerah laut lepas memiliki tingkat kestabilan yang lebih tinggi. Daerah yang paling stabil yaitu S. Hindia diikuti S. Pasifik dan Laut Cina Selatan. Sedangkan perairan yang tidak stabil meliputi Teluk Tomini, Laut Flores dan Selat Malaka.

<p>To explore the characteristics of Northerly Cold Surge during Years of the Maritime Continent Campaign 2021, intensive observation was used to detect the modification processes of the air mass at the head of cold surge, convection development, and severe weather including torrential rainfall using several methods such as the intensive upper-air observation at Jakarta and Pangkal Pinang, vapor variation observation with GNSS network, and precipitation radar network. During this campaign, 7 CENS (Cross-Equatorial Northerly Surge) events were observed according to Hattori’s criteria. The results of the intensive observation show that all of 7 CENS events occurred in association with the negative SST anomaly over the Java Sea with CENS6 (18 – 21 February 2021) induced extreme rainfall (over 150 mm/day) in the southern part of Jakarta. The significant negative SST anomaly was continued over the inland & marginal seas of Indonesia under the strong northerly surge condition during this campaign.</p>

Pada kajian ini dilakukan evaluasi penggunaan beberapa skema konvektif pada model WRF (Weather Research and Forecasting) untuk prediksi cuaca di wilayah Indonesia. Terdapat tiga skema konvektif yang akan dievaluasi yaitu; skema konvektif cumulus BMJ (Betts Miller Janjic), KF (Kain Fritsch), dan GD (Grell 3D ensemble). Data yang digunakan untuk evaluasi adalah data curah hujan per 3 jam dan data angin per 12 jam (level ketinggian; permukaan, 850, 500, 250 mb) dari hasil pengolahan model WRF dan observasi selama periode bulan Agustus 2011 dan Februari 2012 di stasiun Juanda-Surabaya dan Cengkareng-Jakarta. Hasil verifikasi dari tiga skema konvektif pada model WRF terhadap data observasi menunjukkan bahwa untuk prakiraan curah hujan, penggunaan skema konvektif BMJ lebih baik dari skema KF dan GD, dan untuk prakiraan arah dan kecepatan angin skema BMJ dan GD relatif lebih baik dari skema KF. Berdasarkan analisis hasil verifikasi yang diperoleh, pemilihan skema konvektif cumulus BMJ cenderung lebih baik dari skema konvektif KF dan GD untuk di aplikasikan pada model WRF.

In early April 2021, the territory of Indonesia, around the province of East Nusa Tenggara in particular, was severely damaged due to being hit by tropical cyclone Seroja. The impact of tropical cyclone Seroja does not only occur in Nusa Tenggara but also in Australia. In fact, the impact that hit Australia exceeded the damage that occurred in East Nusa Tenggara. The impacts caused by tropical cyclone Seroja in East Nusa Tenggara included 181 deaths and 74,222 houses damaged. Tropical cyclones are extreme weather anomalies that hit many countries, especially in the middle latitudes associated with vast oceans, such as the area around the South China Sea, the Pacific Ocean and the Atlantic Ocean, such as the Philippines, Japan, America, Australia, Europe, etc. Early detection systems for the genesis of tropical cyclones are still being developed by international collaborations such as The Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA) in the Indian Ocean, Tropical Atmosphere Ocean (TAO) in the Pacific Ocean, and Prediction and Research Moored, Array in the Tropical Atlantic (PIRATA). To find out the early sign of a tropical cyclone, it is characterized by sea surface temperatures > 26.5 C, the growth of very broad and thick convective clouds, and rotating wind speeds of > 63 km/hour. For this reason, continuous observations are needed in the area where the tropical cyclone first developed. Observation equipment required includes satellite observations, buoys, and weather radar. Unfortunately, in the territory of Indonesia, especially in the Indian and Pacific oceans around Indonesia, this equipment is not equipped with such equipment due to very expensive funding factors and vandalism constraints. For this reason, in the future, national and international cooperation will be needed to start building an early warning system for the emergence of tropical cyclones among research centers globally.