The occurrence of cut-off lows (COLs) over South Pacific can be seen on different synoptic maps and on satellite imageries. However, the formation and maintenance mechanisms of the COLs are not well understood. An energetics analysis of a COL case through the eddy kinetic energy (EKE) equation is the aim of this study. The main terms analysed were ageostrophic flux convergence (AFC), baroclinic (BRC) and barotropic (BRT) conversions. This analysis shows that the formation mechanism of the COL was associated with BRC (COL western side) and AFC (COL eastern side). After the maximum intensity period, the BRC term was negative and the AFC positive.
Brazil and consequently causes the temperature decrease. The energetics shows that the cold events kinetic energy maxima are more intense than those of cool events. For the cold events three maxima are observed, the first (K1) and the third (K3) maxima are developed by baroclinic conversion and ageostrophic flux convergence and the second one (K2) by ageostrophic flux convergence. For the cool events two maxima are found, the first maximum (K4) developed by baroclinic conversion and the second one by ageostrophic flux convergence.
The energetics and behavior of midtropospheric troughs over the Southern Hemisphere and their re lationship with South America surface cyclogenesis were studied during the winters of 1999-2003. All surface cyclogenesis situations over Uruguay and adjacent areas associated with 500-hPa troughs were analyzed. The atmospheric circulation associated with type-B and type-C cyclones form the basis for two composites: composite B (with 25 cases) and composite C (with 13 cases). The results showed that the midtropospheric troughs were more intense in composite C than in composite B before the surface cyclogenesis and that the opposite occurred during the surface cyclogenesis. The baroclinic conversion was dominant in both com posites. In composite B. the ageostrophic flux convergence (AFC) contributed positively to the intensification of the surface cyclone since it imported energy into the area before the cyclogenesis started. But in composite C, the AFC served as a sink because it exported energy. Based on these results, it can be concluded that (i) the trough was crucial for the cyclogenesis; (ii) the variables in the mid-and upper levels did not differ signifi cantly from one composite to another; (iii) the northerly heat and moisture flow acted as a preconditioning for the cyclogenesis, mainly for composite C; (iv) the baroclinic conversion dominated the energetics: and (v) the AFC had only a secondary role, contributing negatively to the development of the cyclone in composite C and positively to the initial development in composite B.
Even though mesoscale convective systems (MCS) are of great importance in precipitation regimes besides being related to severe weather events, they are still not easy to predict. This study builds an objective index for South American MCS based on synoptic features present before the initiation of the MCS between 2005 and 2010. The National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) product is used to access environmental conditions near the MCS-initiation centroid and in unorganized convective environments to obtain a South American MCS index (SA-MCS index). Upper and middle atmospheric levels presented the main patterns used to identify the MCS environments and unorganized convection, with the jet streak downstream of a short trough at medium levels frequently present in the convectively organized environment. The SA-MCS index is shown to depend on a vertical wind shear between 0 and 6 km high, temperature advection at 775 hPa, lifted index (LI) and vertical velocity omega at 800 hPa. When compared with the MCS index developed through the United States MCS climatology, the SA-MCS index was able to predict more than twice as many MCS at a distance of less than 1 ∘ from the point of maximum intensity of the index.
An explosive cyclogenesis over the east coast of South America was simulated during the period from 28 to 30 May 1999, using a limited-area hydrostatic model with 100 km horizontal resolution. The simulations showed that surface heat flux had an important contribution in developing the explosive cyclogenesis, and that the latent heat flux (LHF) has a larger contribution than the sensible heat flux (SHF). In the absence of a total heat flux (THF, ie. LHF plus SHF), the cyclone was 6 hPa shallower than in the control simulation (with THF). Without LHF the cyclone was 4 hPa shallower, while without SHF the cyclone presented the same intensity as the control simulation. The sensitivity experiments show the following effects of surface heat fluxes: i) the magnitudes and extensions of the effects grew with simulation time; ii) the maximum effects appeared in the southeastern part of the cyclone (warm front sector); iii) the LHF effects were 2-3 times larger than the SHF ones; and iv) the LHF effects were observed in the neighborhood of the cyclone, while the SHF effects were more spread out over the domain. In the THF absent experiment, a drier and colder environment was generated, mainly in the lower troposphere over the ocean, decreasing the environmental potential instability, latent heat release and cyclone intensification. Although the THF effects became larger during the most rapid development phase, they must have been present before this period because they preconditioned the environment for explosive development. Thus, to simulate or forecast the explosive cyclogenes on the South American east coast, an adequate representation of the planetary boundary layer is necessary.
RESUMONeste estudo foram realizados experimentos numéricos com um modelo meteorológico regional a fim de verificar o impacto dos Fluxos de Calor Latente (FCL) e Calor Sensível (FCS) em superfície no desenvolvimento de um ciclone intenso ocorrido sobre a costa leste da região sudeste da América do Sul, entre os dias 24 e 26 de julho de 1998. A taxa de intensificação do ciclone foi reduzida em 6 hPa/24 h no experimento em que os FCL e FCS estavam ausentes. Verificou-se que a ausência do FCL apresentou maior impacto do que a ausência do FCS, fazendo com que a taxa de intensificação fosse reduzida em 6 hPa/24 h para o experimento sem FCL e permanecesse inalterada sem FCS, mas neste último a isóbara de menor valor no centro do ciclone ocupou uma área menor. A ausência dos FCL e FCS em superfície gerou uma camada mais seca e mais fria próxima à superfície oceânica, reduzindo a instabilidade potencial do ambiente e diminuindo a intensificação do ciclone. Concluiu-se, portanto que os FCL e FCS em superfície foram importantes antes da fase de mais rápido desenvolvimento do ciclone, agindo no sentido de preparar o ambiente para uma ciclogênese mais intensa, através do fornecimento de energia e umidade para a baixa troposfera. Na fase de mais rápido desenvolvimento, a energia e a umidade disponíveis nas camadas mais baixas da atmosfera (inseridos na fase anterior) também foram importantes, permitindo ao ciclone se desenvolver mais intensamente do que ocorreria em um ambiente mais seco e menos instável termicamente. Palavras-Chave: ciclogênese, fluxos de calor em superfície, modelo regional. ABSTRACT: ROLE OF SURFACE LATENT AND SENSIBLE HEAT FLUXES ASSOCIATED TO A SOUTH AMERICA EAST COAST CYCLOGENESIS CASENumeric experiments were run with meteorological regional model with the purpose to verify the impact of both latent and sensible surface heat fluxes (FCL and FCS, respectively) during the development of an intense east cost cyclone over southeastern region of South America from 24 to 26 July, 1998. The intensification rate of the cyclone was reduced by 6 hPa/24 h in the experiment where the FCL and FCS were absents. It was also verified that the absence of FCL presented more impact than without FCS. The intensification rate was reduced by 6 hPa/24h in the experiment without FCL and became unaltered in the experiment without FCS, but in the later the isobar with minimum value around the cyclone center occupied a smaller area. The absence of the FCL and FCS generated a drier and colder layer near the oceanic surface, reducing the environmental potential instability and decreasing the cyclone intensification. With these results it is possible to conclude that the FCL and FCS were important before the phase of most rapid cyclone development, preparing the environment to a more intense cyclogenesis, supplying both energy and moisture to the lower troposphere. In the phase of the most rapid development, the available energy and moisture (inserted in the previous phase) in the lower levels were also important, allowing the cyclo...
The location and movement of 500-hPa troughs using an automated method are studied with data from a 24-yr period with the objective of determining the trough formation and dissipation regions in the Southern Hemisphere. To identify the 500-hPa mobile troughs, an objective method that uses the Eulerian centripetal acceleration (ECA) is developed. On average, 868 troughs per year were identified by the method, with an increase in trend during the period studied. The troughs have an average lifetime of 4.3 days, being longer (shorter) in subtropical (high) latitudes. The average calculated phase velocity was 13.6 m s-', being higher (lower) in middle (high) latitudes. The troughs are normally found in the 60'-40'S latitudinal band, with maximum occurrence at 50'S. The longitudinal distribution of trough genesis has three maximum regions: over the Drake Strait and the South Atlantic Ocean, over the Indian Ocean around 50oS, and over the southwestern Pacific Ocean between 150'E and 150'W. The trough dissipation regions are less concentrated than the genesis regions and also show three maxima: over the west of the Andes, south of the African continent, and south of Australia. The seasonal variation in the trough dissipation shows that the 30'-40'S band is more active during winter than in summer. The difference between the genesis and dissipation location is that formation occurs more in middle and high latitudes, while dissipation is more common in the 400-30'S latitude belt.
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