This study presents a climatological and composite analysis of elevated mixed layers (EMLs) in South and North America derived from the NCEP Climate Forecast System Reanalysis. The EMLs are identified based on objective criteria applied to the reanalysis data. Composite analyses of synoptic-scale conditions and severe weather parameters associated with spring EML cases are presented. EMLs are more frequent immediately to the east of the Andes and the Rockies. The North American EMLs form by surface heating over the higher terrain of the Rockies, with peak frequency occurring in spring and summer. EMLs in South America are generated by differential temperature advection due to ageostrophic circulations east of the Andes, as indicated by the temperature lapse rate tendency equation, which relates to the higher frequency of EMLs during the cold season in South America. EMLs over North America are about 100 hPa lower than over South America due to the lower height of the Rockies in comparison to the Andes. The synoptic conditions associated with EMLs in South and North America are characterized by an upper-level trough upstream and low-level moisture flux convergence due to poleward-directed flow, favoring synoptic-scale ascent poleward of the EML location, where the convective inhibition is relatively low. When EMLs occur, higher surface-based convective available potential energy and low-level storm-relative helicity, in association with lower lifting condensation level heights observed in North America, indicate that surface-based supercell storms and tornadoes are more likely to occur on this continent in comparison with South America, corroborating observations.
This study shows a synoptic climatology of warm fronts in Southeastern South America (SESA). Data from Climate Forecast System Reanalysis (CFSR) was used to identify warm fronts from 1979 to 2010. The identification method was based on the magnitude of meridional gradient of 850-hPa equivalent potential temperature ( e) and 850-hPa wind fields. Composites of the most important atmospheric variables were constructed from 1 day before until 1 day after the formation of the warm front. An average frequency of two warm fronts per month is observed, with higher frequencies in austral winter. Most warm fronts precede the formation of extratropical cyclones over Uruguay and form because of the southward movement of previous cold/stationary fronts. Warm fronts form on average around southern Paraguay, northeastern Argentina and western part of southern Brazil and Uruguay, coupled to the eastern edge of the Chaco Low (CL) and the Northwestern Argentinean Low (NAL) where north/northwesterly flow predominates. An upper-level wave of wavenumber eight supports warm frontogenesis. Location and intensity of synoptic systems associated with a warm front event differ from winter to summer. Elevated instability is commonly present near warm fronts, and the average warm-front slope is 1 : 110, agreeing with other studies. Instability indices increase after the warm-front passage, leading to greater rainfall 1 day after the warm front forms.
ResumoO objetivo desta pesquisa é estudar os ciclones e os padrões de circulação atmosférica a eles associados no Oceano Atlântico Sul, próxima à costa das Regiões Sul e Sudeste do Brasil, usando dados da Reanálise ERA-Interim nos meses de outubro a abril dos anos de 2003 a 2013. Foi desenvolvido um método objetivo para obter as datas, a localização e a intensidade dos ciclones, o que permitiu gerar compostos de algumas variáveis meteorológicas. Os resultados mostraram que em média ocorreram 17 casos de ciclones entre os meses de outubro a abril e que o período de vida dentro da região de estudo chegou até a 8 dias em alguns casos. O período de out/08 a abr/09 chama atenção por ter apresentado 27 casos. Os compostos para o período mostraram que a combinação de difluência no escoamento em altos níveis, associada a um cavado em médios níveis e a presença de anomalias positivas do vento zonal são favoráveis à formação de ciclones no Atlântico Sul, próximo à costa das Regiões Sul e Sudeste do país. A convergência do escoamento em baixos níveis é decisiva para estimular a ciclogênese na região. O estudo de caso mostrou uma aplicação da ferramenta desenvolvida que pode ter utilidade no aperfeiçoamento dos modelos numéricos de previsão desse tipo de sistema meteorológico.Palavras-chave: ciclones, ciclogêneses no Oceano Atlântico Sul, reanálise ERA-Interim. Cyclones and Atmosferic Circulation Patterns Study in the South Atlantic Ocean Near the Coast of South and a Southest of Brazil Using Reanalysis Era-Interim Data AbstractThe main objective of this research is to study the cyclones, and the atmospheric circulation pattern associated with them, occurring in South Atlantic Ocean, using data from ERA-Interim Reanalysis from October to April in the [2003][2004][2005][2006][2007][2008][2009][2010][2011][2012][2013] period. An objective method was developed and applied to identify the dates, location and intensity of cyclones, which allowed to generate composites of some meteorological variables. The results showed that, on average, 17 cases of cyclones occurred in the months from October to April and the life within the studied area reaches up to 8 days.The period of October/2008 to April /2009 calls attention for recording 27 cases. Composites for the entire period showed a combination of the difluence in the flow at upper levels, associated with an mid-tropospheric trough and the presence of positive zonal-wind anomalies are favorable to formation of cyclone in the Atlantic Ocean near South and Southeastern Brazil. Convergence of low-level flow were decisive to stimulate cyclogenesis in the region. The case studied showed an application of the developed tool that can be useful in improving the numerical prediction models of this type of weather system.
The GOES-16 mesoscale domain sector (MDS) scans with 1-min intervals are used in this study to analyze a severe thunderstorm case that occurred in southeastern Brazil. The main objective is to evaluate the GOES-16 MDS rapid scans against the operational full-disk scans with lower temporal resolution for nowcasting. Data from a C-band radar, observed sounding, and a ground-based lightning network are also used in the analysis. A group of thunderstorms formed in the afternoon of 29 November 2017 in an environment of moderate convective available potential energy (CAPE) and deep-layer shear. The storms presented supercell characteristics and intense lightning activity with peak rates in excess of 150 flashes per 5 min. The satellite-derived trends with 1-min interval were skillful in detecting thunderstorm intensification, mainly in the developing stage. The decrease in cloud-top 10.35-μm brightness temperature was accompanied by increases in ice mass flux, concentration of small ice particles at cloud top, and storm depth. In the mature stage, there is no evident trend in the satellite-derived parameters that could indicate storm intensification, but the cluster area expands suggesting cloud-top divergence. The 1-min rapid scans indicate greater lead time to severe weather relative to 10- and 15-min-resolution imagery, but also presented numerous false alarms (indication of severe weather but no occurrence) due to oscillations in the satellite-derived parameters. The parameters calculated every 5 min presented better skill than 10 and 15 min and fewer false alarms than 1 min.
We applied the Goddard Earth Observing System for subseasonal to seasonal climate prediction to assess the impact of inclusion biomass burning (BB) aerosols over South America (SA) during the austral winter. We also evaluated the model sensitivity to the BB emissions prescription using no emissions, monthly climatological, and daily emissions. Each hindcast consisted of four members running from June to November of each year between 2000 and 2015. Our results indicated that interactive BB aerosols improve the seasonal climate prediction performance over SA. More realistic daily based emissions significantly further improve the performance in comparison with the climatological ones. Therefore, improvements in the BB emissions representation are urged to represent the aerosol impacts on seasonal climate prediction performance adequately.
This study presents an analysis of a period with positive precipitation anomalies and anomalous severe weather activity in parts of the Southeastern Brazil during the dry season (austral winter). The objective of this work is to identify the synoptic pattern associated with the severe episodes on August 2018. The analysis was based on observational data and the CFSR (Climate Forecast System Reanalysis). Standardized anomalies of the main meteorological variables were used in the analysis. An anomalous trough in the mid and upper troposphere and the associated low-level flow from the Amazon Basin to Southeastern Brazil provided high amount of precipitable water, and were the main responsible for the rainfall markedly above the climatology along with severe hail and strong winds. The atmospheric environment was conditionally unstable due to a stationary front. These type of analyzes should be taken into consideration at this time of year to improve weather forecasts and minimize impacts in such circumstances.
This research explores the benefits of radar data assimilation for short-range weather forecast in Southeastern Brazil using the Weather Research and Forecasting (WRF) model’s three-dimensional variational data assimilation (3DVAR) system. Different data assimilation options are explored, including the cycling frequency, the number of outer loops and the use of null-echo assimilation. Initially, four microphysics parameterizations are evaluated (Thompson, Morrison, WSM6 and WDM6). The Thompson parameterization produces the best results, while the other parameterizations generally overestimate the precipitation forecast, especially WDSM6. Additionally, the Thompson scheme tends to overestimate snow, while the Morrison scheme overestimates graupel. Regarding the data assimilation options, the results deteriorate and more spurious convection occurs when using a higher cycling frequency, i.e., 30 minutes instead of 60 minutes. The use of two outer loops produces worse precipitation forecasts than the use of one outer loop, and the null-echo assimilation is shown to be an effective way to suppress spurious convection. However, in some cases, the null-echo assimilation also removes convective clouds that are not observed by the radar and/or are still not producing rain, but have the potential to grow into an intense convective cloud with heavy rainfall. Finally, a cloud convective mask was implemented using ancillary satellite data to prevent null-echo assimilation from removing potential convective clouds. The mask demonstrated to be beneficial in some circumstances, but it needs to be carefully evaluated in more cases to have a more robust conclusion regarding its use.
This study presents a 9-year climatology and composite analysis of quasi-linear convective systems (QLCSs) in southern Brazil (SB). QLCSs are identified using radar imagery and defined as a 100-km long, 40-dBZ convective line that lasts for at least 1 hr. QLCS cases associated with at least three severe wind reports are classified as severe. Composites of the synoptic-scale environments and severe convective weather parameters are constructed for severe and non-severe cases of three synoptic patterns found as the most recurrent in QLCS cases. QLCSs are more frequent and more likely to be severe during spring, while very few cases occur during winter. Most cases occur during late night and morning, corroborating previous research of mesoscale convective systems in this area, but the percentages of severe cases are higher in late afternoon and early night. Faster QLCSs, particularly those with velocities greater than 50 km/hr, have higher probability of being severe in comparison to slower systems. The three synoptic patterns more often related to QLCS occurrence in SB are characterized by a mid-level trough upstream of SB (type 1), a mid-level trough west of the Andes (type 2) and predominance of zonal flow over SB (type 3). In general, there is a low-pressure system over northwestern Argentina extending a trough to SB, where low-level moisture flux convergence and warm advection occur. In severe cases, the sea-level pressure is lower in northwestern Argentina and there is greater low-level northwesterly flow over SB. The type 1 is the most frequent pattern when QLCSs are observed in SB and is also the type with higher percentage of severe cases. Convective available potential energy (CAPE) and downdraft CAPE are good discriminators between severe and nonsevere cases in type 1 and 2 environments, while the 1,000-500-hPa bulk wind difference is a better predictor in type 3 cases. K E Y W O R D Squasi-linear convective systems, severe thunderstorms, southern Brazil, synoptic climatology
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