Wind patterns in the Pacific Ocean fluctuate seasonally, annually, and decadally, resulting in changes in the dispersal of volcanic smog (vog) across the Hawaiian Islands. A variety of synoptic-scale weather patterns can affect the Islands, creating variability in the direction and intensity of wind patterns. Recent changes in wind profiles were analyzed to identify possible patterns that could influence and increase the dispersion of vog over time on Hawai’i Island and the other Hawaiian Islands to the northwest. Historically, Northeast Trade Winds prevailed for much of the year, shifting vog into the Pacific Ocean southwest of Hawaii and away from the state’s principal population centers, but Northeast Trade Winds have shown a 20+% reduction over the past several decades. An increase in the southerly source of prevailing wind increased the frequency and intensity of vog and its impacts on the environment and health and well-being of people across the Islands.

Este trabalho teve como objetivo analisar as condições meteorológicas favoráveis e desfavoráveis à dispersão de poluentes, com ênfase no ozônio, nas áreas urbanas associadas à Região Metropolitana de São Paulo (RMSP) e a Grande Santiago (GS). Medidas de concentração de estações localizadas em ambas as áreas de estudo foram utilizadas para determinar o comportamento médio, o número de ocorrências de ultrapassagens de padrão de qualidade do ar e o aumento noturno das concentrações de ozônio num período de dez anos. Foi encontrado que o máximo horário acontece próximo das 14 horas (horário local) nas duas regiões, e que existe uma tendência à diminuição no número de ultrapassagens do padrão de qualidade do ar associado a este poluente. Na RMSP é possível observar um máximo secundário durante a madrugada, enquanto que este fenômeno não é claramente observado nos valores médios de concentração na GS. Com dados da Reanálise do NCEP/NCAR foi visto que a ocorrência deste fenômeno está associada a configurações sinóticas específicas na GS, enquanto que na RMSP não existe padrão sinótico específico que diferencie casos com e sem aumento de ozônio noturno. Um novo módulo fotoquímico (NPM), contendo 95 reações foi inserido no modelo BRAMS na tentativa de melhorar o prognóstico das concentrações de poluentes, principalmente em relação ao ozônio. O NPM mostrou melhor desempenho que o módulo fotoquímico original do modelo (SPM; Simple Photochemical Module) para todos os poluentes considerados, salvo o ozônio, para o qual o SPM apresentou melhores índices estatísticos em 5 das 7 estações avaliadas. As características locais associadas à ocorrência simultânea ou não de aumento na concentração de ozônio foram estudadas utilizando o modelo BRAMS com o módulo SPM ativado, sendo verificado que o aumento de ozônio na RMSP está associado, principalmente, ao transporte vertical deste poluente a partir de níveis mais elevados da atmosfera, enquanto que o transporte horizontal é a maior contribuição para o aumento das concentrações de ozônio durante o período noturno na GS. / This work aimed to analyze the favorable and unfavorable meteorological conditions for pollutant dispersion, especially ozone, at the Metropolitan Area of São Paulo (MASP) and the Greater Santiago (GS). Ozone concentration measurements from stations located in both urban areas were used to determinate the mean behavior, the number of standard air quality exceedances and the increase on nocturnal ozone concentration over a 10 years period. It was found that the diurnal maximum occurs around 14 local time in both urban areas, and that there is a diminishment tendency of the total cases of standard air quality exceedances during all period. A secondary maximum of ozone concentration was observed in MASP at night, which was not clearly seen on mean values over GS. By using the NCEP/NCAR Reanalysis data it was seen that the occurrence of this phenomenon is associated to specific synoptic patterns for GS, while there is no specific synoptic pattern for the MASP cases, with or without nocturnal increase in ozone concentrations. A New Photochemical Module (NPM), containing 95 reactions was coupled to BRAMS model in an attempt to improve the prediction of air pollutants, especially ozone. The NPM showed a better performance than the original module, the SPM (Simple Photochemical Module), for all considered pollutants but ozone. For this pollutant, the SPM presents better statistical indexes for 5 of the 7 stations analyzed. The local characteristics related to simultaneous occurrence of nocturnal ozone increase was studied using the SPM-BRAMS model, which showed that the secondary nocturnal maximum of ozone concentrations in MASP are related to vertical transport of this pollutant from higher levels of the atmosphere to the surface, while the horizontal transport is a major contributor to the increase of ozone concentrations in the GS during the night.

Este trabalho teve como objetivo analisar as condições meteorológicas favoráveis e desfavoráveis à dispersão de poluentes, com ênfase no ozônio, nas áreas urbanas associadas à Região Metropolitana de São Paulo (RMSP) e a Grande Santiago (GS). Medidas de concentração de estações localizadas em ambas as áreas de estudo foram utilizadas para determinar o comportamento médio, o número de ocorrências de ultrapassagens de padrão de qualidade do ar e o aumento noturno das concentrações de ozônio num período de dez anos. Foi encontrado que o máximo horário acontece próximo das 14 horas (horário local) nas duas regiões, e que existe uma tendência à diminuição no número de ultrapassagens do padrão de qualidade do ar associado a este poluente. Na RMSP é possível observar um máximo secundário durante a madrugada, enquanto que este fenômeno não é claramente observado nos valores médios de concentração na GS. Com dados da Reanálise do NCEP/NCAR foi visto que a ocorrência deste fenômeno está associada a configurações sinóticas específicas na GS, enquanto que na RMSP não existe padrão sinótico específico que diferencie casos com e sem aumento de ozônio noturno. Um novo módulo fotoquímico (NPM), contendo 95 reações foi inserido no modelo BRAMS na tentativa de melhorar o prognóstico das concentrações de poluentes, principalmente em relação ao ozônio. O NPM mostrou melhor desempenho que o módulo fotoquímico original do modelo (SPM; Simple Photochemical Module) para todos os poluentes considerados, salvo o ozônio, para o qual o SPM apresentou melhores índices estatísticos em 5 das 7 estações avaliadas. As características locais associadas à ocorrência simultânea ou não de aumento na concentração de ozônio foram estudadas utilizando o modelo BRAMS com o módulo SPM ativado, sendo verificado que o aumento de ozônio na RMSP está associado, principalmente, ao transporte vertical deste poluente a partir de níveis mais elevados da atmosfera, enquanto que o transporte horizontal é a maior contribuição para o aumento das concentrações de ozônio durante o período noturno na GS. / This work aimed to analyze the favorable and unfavorable meteorological conditions for pollutant dispersion, especially ozone, at the Metropolitan Area of São Paulo (MASP) and the Greater Santiago (GS). Ozone concentration measurements from stations located in both urban areas were used to determinate the mean behavior, the number of standard air quality exceedances and the increase on nocturnal ozone concentration over a 10 years period. It was found that the diurnal maximum occurs around 14 local time in both urban areas, and that there is a diminishment tendency of the total cases of standard air quality exceedances during all period. A secondary maximum of ozone concentration was observed in MASP at night, which was not clearly seen on mean values over GS. By using the NCEP/NCAR Reanalysis data it was seen that the occurrence of this phenomenon is associated to specific synoptic patterns for GS, while there is no specific synoptic pattern for the MASP cases, with or without nocturnal increase in ozone concentrations. A New Photochemical Module (NPM), containing 95 reactions was coupled to BRAMS model in an attempt to improve the prediction of air pollutants, especially ozone. The NPM showed a better performance than the original module, the SPM (Simple Photochemical Module), for all considered pollutants but ozone. For this pollutant, the SPM presents better statistical indexes for 5 of the 7 stations analyzed. The local characteristics related to simultaneous occurrence of nocturnal ozone increase was studied using the SPM-BRAMS model, which showed that the secondary nocturnal maximum of ozone concentrations in MASP are related to vertical transport of this pollutant from higher levels of the atmosphere to the surface, while the horizontal transport is a major contributor to the increase of ozone concentrations in the GS during the night.

MENVSC / Department of Geography and Geo-Information Sciences / Out of all rain-producing weather systems, cut-off lows (COLs) are linked with the occurrence of high impact rainfall and in some cases short-lived floods which can last for 24 hours over South Africa. This study examined the characteristics associated with the present occurrence of the severe COL systems over South Africa from 2011 to 2017. The accuracy of the 4.4 km Unified Model (UM) which is currently in use for simulating areas of deep moist convection in South Africa was evaluated. The UM simulated geopotential height at 500 hPa as well as the associated 24 hours precipitation which were compared against the daily fields of geopotential height and 6-hourly precipitation from the European Centre for Medium-Range Weather Forecasts (ECMWF). COL events were categorized and analyzed according to the associated surface circulation patterns at 850 hPa. The seasonal distribution and duration of the systems over northern (10°E-33°E //22°-32°S) and southern (10°E-33°E //32°-35°S) regions of the study area were also analyzed. Results show COL systems shifting with season towards the north eastern parts of the country, with an increased number of events during the austral winter season during the study period. Systems which lasted for long time were observed during the austral winter and spring seasons. The UM tends to simulate areas of heavy precipitation accurately with poor simulation during the initial stages of the systems. The UM provided a more realistic-looking closed geopotential height and rainfall fields for systems which are coupled with a cold front at the surface. Application of the knowledge about the evolution in the characteristics of COL events from this study can improve the operational forecasting of these weather systems over the country. / NRF

<p dir="ltr">It is known that tornadic storms favor environments characteristic of high values of thermal instability, adequate vertical wind shear, abundant near-surface moisture supply, and strong storm-relative helicity at the lowest 1-km boundary layer. These mesoscale environmental conditions and associated storm behaviors are strongly governed by large-scale synoptic patterns and sensitive to variabilities in near-surface characteristics, which are less known in the current research community. This study aims to advance the relatively underexplored area regarding the interaction between surface characteristics, mesoscale environmental conditions, and large-scale synoptic patterns driving tornadic storms in the U.S. </p><p dir="ltr">We first investigate the impact of surface drag on the structure and evolution of these boundaries, their associated distribution of near-surface vorticity, and tornadogenesis and maintenance. Comparisons between idealized simulations without and with drag introduced in the mature stage of the storm prior to tornadogenesis reveal that the inclusion of surface drag substantially alters the low-level structure, particularly with respect to the number, location, and intensity of surface convergence boundaries. Substantial drag-generated horizontal vorticity induces rotor structures near the surface associated with the convergence boundaries in both the forward and rear flanks of the storm. Stretching of horizontal vorticity and subsequent tilting into the vertical along the convergence boundaries lead to elongated positive vertical vorticity sheets on the ascending branch of the rotors and the opposite on the descending branch. The larger near-surface pressure deficit associated with the faster development of the near-surface cyclone when drag is active creates a downward dynamic vertical pressure gradient force that suppresses vertical growth, leading to a weaker and wider tornado detached from the surrounding convergence boundaries. A conceptual model of the low-level structure of the tornadic supercell is presented that focuses on the contribution of surface drag, with the aim of adding more insight and complexity to previous conceptual models.</p><p dir="ltr">We then examine the behaviors and dynamics of TLVs in response to a range of surface drag strengths in idealized simulations and explore their sensitivities to different storm environments. We find that the contribution of surface drag on TLV development is strongly governed by the interaction between surface rotation, surface convergence boundaries, and the low-level mesocyclone. Surface drag facilitates TLV formation by enhancing near-surface vortices and low-level lifting, mitigating the need for an intense updraft gradient developing close to the ground. As surface drag increases, a wider circulation near the surface blocks the inflow from directly reaching the rotating core, leading to a less tilted structure that allows the TLV position beneath the pressure minima aloft. Further increase in drag strength discourages TLV intensification by suppressing vertical stretching due to a negative vertical pressure perturbation gradient force, and it stops benefiting from the support of surrounding convergence boundaries and the overlying low-level updraft, instead becoming detached from them. We hence propose a favorable condition for TLV formation and duration where a TLV forms a less tilted structure directly beneath the low-level mesocyclone but also evolves near surrounding surface boundaries, which scenario strongly depends on underlying surface drag strength. </p><p dir="ltr">Beyond near-surface characteristics, we further explore how these storm-favorable environmental conditions may interact with the larger-scale synoptic patterns and how these interactions may affect the tornadic storm potential in the current warming climate. We employ hierarchical clustering analysis to classify the leading synoptic patterns driving tornadic storms across different geographic regions in the U.S. We find that the primary synoptic patterns are distinguishable across geographic regions and seasonalities. The intense upper-level jet streak described by the high values of eddy kinetic energy (EKE) associated with the dense distribution of Z500 contours dominates the tornado events in the southeast U.S. in the cold season (November-March). Late Spring and early Summer Tornado events in the central and south Great Plains are dominated by deep trough systems to the west axes of the tornado genesis position, while more summer events associated with weak synoptic forcing are positioned closer to the lee side of Rocky Mountain. Moreover, the increasing trend in tornado frequency in the southeastern U.S. is mainly driven by synoptic patterns with intense forcing, and the decreasing trends in portions of the Great Plains are associated with weak synoptic forcing. This finding indicates that the physical mechanisms driving the spatial trends of tornado occurrences differ across regions in the U.S.</p>