ABSTRACT The unprecedented 2024 April-May floods in the State of Rio Grande do Sul have affected over 90% of the state, displacing hundreds of thousand people and causing 171 deaths. The El Niño Southern Oscillation phenomenon was found to be important to explain the variability in the observed rainfall, consistent with previous research that show previous heavy rainfall events in the area occurred during El Niño years. Cold front fronts bringing rainfall remained stationary over southern Brazil, since they could not move into central Brazil because of atmospheric blocking with warm and dry air and highest temperatures in that region. The flood event in Porto Alegre was triggered by heavy rainfall over the Lake Guaíba Basin, with accumulated surpassing 500 mm in 5 days, leading to record high levels of 5,35 m on May 5, highest that the previous flood in 1941. In addition to the extreme nature of this event, this contributed to the significant impacts of flooding in vulnerable areas of the Porto Alegre Metropolitan Region and nearby municipalities. This highlights the need to objectively assess risk and reinforce flood infrastructure to increase resilience to this and future, even more extreme, floods. 202 AprilMay April 90 state 17 deaths years region Basin 50 days 535 35 5,3 1941 municipalities future 20 9 1 53 3 194 2 19

RESUMO As inundações sem precedentes de abril a maio de 2024 no Rio Grande do Sul afetaram mais de 90% do estado, deslocando centenas de milhares de pessoas e causando 172 mortes. O fenômeno El Niño Oscilação Sul foi considerado importante para explicar a variabilidade nas chuvas observadas, consistente com pesquisas anteriores que mostram que eventos anteriores de fortes chuvas na área ocorreram durante anos de El Niño. As frentes frias que trazem chuvas permaneceram estacionadas no sul do Brasil, uma vez que não puderam se mover para o centro do Brasil devido ao bloqueio atmosférico com ar quente e seco e às temperaturas mais altas naquela região. A inundação em Porto Alegre foi desencadeada por fortes chuvas na Bacia do Guaíba, com acumulações superiores a 500 mm em 5 dias, levando a níveis recordes de 5,35 m em 5 de maio, valores mais altos que a enchente anterior, em 1941. Além do caráter extremo deste evento, isso contribuiu para os impactos significativos de enchentes em áreas vulneráveis da Região Metropolitana de Porto Alegre e municípios próximos. Isso mostra a necessidade de avaliar objetivamente o risco e reforçar a infraestrutura para inundações para aumentar a resiliente a esta e futuras enchentes, ainda mais extremas. 202 90 estado 17 mortes observadas região Guaíba 50 dias 535 35 5,3 anterior 1941 evento próximos extremas 20 9 1 53 3 5, 194 2 19

ABSTRACT Drought is a natural and recurrent phenomenon. It is considered ‘a natural disaster’ whenever it occurs in an intensive manner in highly populated regions, resulting in significant damage (material and human) and loss (socioeconomic). This paper presents the efforts developed to monitor the impact of drought in the semiarid region of Northeast Brazil. In this scope, information from different sources is compiled to support the evaluation and identification of impacted municipalities, with the main objective of supporting emergency actions to mitigate their impact. In the semiarid region of Brazil there are frequent occurrences of dry periods during the rainy season, which, depending on the intensity and duration, can cause significant damage to family-farmed crops, with a farming system characterized by low productivity indices. However, rain-fed agriculture has great economic expression and high social importance due to the region is densely occupied, and contributes to the establishment of communities in the countryside. Specifically, in the present study, the methodology adopted to monitor the impact of agricultural droughts, including an analysis of the hydrological year 2015-2016, is presented, considering different water stress indicators for the identification of the affected municipalities and assessment of the methods and tools developed.

ABSTRACT This study discusses the climatological aspects of the most severe drought ever recorded in the semiarid region Northeast Brazil. Droughts are recurrent in the region and while El Nino has driven some of these events others are more dependent on the tropical North Atlantic sea surface temperature fields. The drought affecting this region during the last 5 years shows an intensity and impact not seen in several decades in the regional economy and society. The analysis of this event using drought indicators as well as meteorological fields shows that since the middle 1990s to 2016, 16 out of 25 years experienced rainfall below normal. This suggests that the recent drought may have in fact started in the middle-late 1990s, with the intense droughts of 1993 and 1998, and then the sequence of dry years (interrupted by relatively wet years in 2007, 2008, 2009 and 2011) after that may have affected the levels of reservoirs in the region, leading to a real water crisis that was magnified by the negative rainfall anomalies since 2010.

O experimento de campo do TRMM-LBA ocorreu conteceu durante o verão austral de 1999, na região do sudeste de Amazonia. Entre os principais objetivos deste trabalho pode-se citar a identificação e descrição da variabilidade diurna da chuva nesta região, associada a diferentes fenômenos meteorológicos e sistemas de tempo que ocorreram durante o período de Janeiro-Fevereiro. Usando uma rede de 40 pluviômetros instalados no estado de Rondônia, ,juntamente com outras observações de circulação atmosférica e convecção, foi possível identificar detalhes do ciclo diurno de chuva e os mecanismos de circulação associados. Os eventos de chuva foram caracterizados por regimes de vento nos níveis baixos e no contexto da circulação de grande escala: ventos de oeste e de leste. O regime "de oeste" se associa a Zona de Convergência do Atlântico Sul (ZCAS) e a episódios intensos de jatos de baixos níveis ao leste dos Andes (LLJ), que podem estender seus efeitos até Rondônia. Episódios de eventos "de leste" se associam a sistemas meteorológicos que se propagam desde a Foz do Rio Amazonas em direção oeste (e.g. linhas de instabilidade) e podem ser acompanhados por episódios fracos de LLJ e de ZCAS. A variabilidade diurna de chuva durante episódios de circulação "de oeste" apresenta um máximo característico á tarde (1200-1400 hora local) com um máximo secundário á noite (2000-2200 hora local). O regime de circulação "de leste" mostra um máximo á tarde (1200-1400 hora local), precedido de um máximo secundário durante a madrugada (0000-0200 hora local).

The TRMM-LBA field campaign was held during the austral summer of 1999 in southwestern Amazonia. Among the major objectives, was the identification and description of the diurnal variability of rainfall in the region, associated with the different rain producing weather systems that occurred during the January-February season. By using a network of 40 digital rain gauges implemented in the state of Rondônia, and together with observations and analyses of circulation and convection, it was possible to identify details of the diurnal cycle of rainfall and the associated rainfall mechanisms. Rainfall episodes were characterized by regimes of "low-level easterly" and "westerly" winds in the context of the large-scale circulation. The westerly regime is related to an enhanced South Atlantic Convergence Zone (SACZ) and an intense and/or wide Low Level Jet (LLJ) east of the Andes, which can extend eastward towards Rondônia, even though some westerly regime episodes also show a LLJ that remains close to the foothill of the Andes. The easterly regime is related to easterly propagating systems (e.g. squall-lines) with possible weakened or less frequent LLJs and a suppressed SACZ. Diurnal variability of rainfall during westerly surface wind regime shows a characteristic maximum at late afternoon followed by a relatively weaker second maximum at early evening (2100 Local Standard Time LST). The easterly regime composite shows an early morning maximum followed by an even stronger maximum in the afternoon.