Domain choice in an experimental nested modeling prediction system for South America

The purposes of this paper are to evaluate the new version of the regional model, RegCM3, over South America for two test seasons, and to select a domain for use in an experimental nested prediction system, which incorporates RegCM3 and the European Community-Hamburg (ECHAM) general circulation model (GCM). To evaluate RegCM3, control experiments were completed with RegCM3 driven by both the NCEP/NCAR Reanalysis (NNRP) and ECHAM, using a small control domain (D-CTRL) and integration periods of January–March 1983 (El Niño) and January–March 1985 (La Niña). The new version of the regional model captures the primary circulation and rainfall differences between the two years over tropical and subtropical South America. Both the NNRP-driven and ECHAM-driven RegCM3 improve the simulation of the Atlantic intertropical convergence zone (ITCZ) compared to the GCM. However, there are some simulation errors. Irrespective of the driving fields, weak northeasterlies associated with reduced precipitation are observed over the Amazon. The simulation of the South Atlantic convergence zone is poor due to errors in the boundary condition forcing which appear to be amplified by the regional model. To select a domain for use in an experimental prediction system, sensitivity tests were performed for three domains, each of which includes important regional features and processes of the climate system. The domain sensitivity experiments were designed to determine how domain size and the location of the GCM boundary forcing affect the regional circulation, moisture transport, and rainfall in two years with different large scale conditions. First, the control domain was extended southward to include the exit region of the Andes low level jet (D-LLJ), then eastward to include the South Atlantic subtropical high (D-ATL), and finally westward to include the subsidence region of the South Pacific subtropical high and to permit the regional model more freedom to respond to the increased resolution of the Andes Mountains (D-PAC). In order to quantify differences between the domain experiments, measures of bias, root mean square error, and the spatial correlation pattern were calculated between the model results and the observed data for the seasonal average fields. The results show the GCM driving fields have remarkable control over the RegCM3 simulations. Although no single domain clearly outperforms the others in both seasons, the control domain, D-CTRL, compares most favorably with observations. Over the ITCZ region, the simulations were improved by including a large portion of the South Atlantic subtropical high (D-ATL). The methodology presented here provides a quantitative basis for evaluating domain choice in future studies

Baixa do Noroeste Argentino e Baixa do Chaco: caracterísitcas, diferenças e semelhanças The Northewestern Argentinean Low and The Chaco Low: their characteristics, differences and similarities

A Baixa do Chaco e a Baixa do Noroeste Argentino são frequentemente confundidas ou tratadas como um único sistema. Isso se deve a vários fatores: o pouco conhecimento que se tem sobre elas, especialmente sobre a Baixa do Chaco, a baixa resolução das reanálises globais que as mostram como um único sistema e a relativamente escassa cobertura de dados sobre a região que ocupam. Este trabalho tenta contribuir para o melhor entendimento das diferenças e semelhanças entre ambos os sistemas, analisando algumas características dinâmicas e termodinâmicas da região em que elas atuam, com a finalidade de aprofundar seu conhecimento e de possibilitar a sua melhor identificação e diferenciação. O estudo utiliza as reanálises do modelo regional Eta/CPTEC do período (2000-2004) e considera os meses de verão, período no qual as duas baixas estão ativas. Os resultados mostraram que a Baixa do Noroeste Argentino está mais relacionada com a atividade transiente, o que pode ser comprovado através da influência do jato subtropical e da variabilidade temporal de algumas variáveis, como a pressão e a temperatura potencial equivalente. A Baixa do Chaco se desenvolve numa atmosfera mais instável do ponto de vista termodinâmico, porém menos afetada pela dinâmica das latitudes médias.<br>The Chaco Low and the Northwestern Argentinean Low are frequently confounded or treated as a single system, due to our limited knowledge about them (especially concerning the Chaco Low), the low resolution of global analyses -that show them as a single low pressure system-, and the relatively poor data coverage over the region where they are usually located. This article aims to contribute to a better understanding of the differences and similarities between them, as well as to analyze the atmospheric dynamical and thermo-dynamical conditions where they evolve. It is considered that this study will aid to a better recognition of both systems. With this purpose, regional reanalyses from the Eta/CPTEC model for the 2000-2004 period has been used. The article focuses in summer months, since both low-pressure systems are more active during this season. Results show that the Northwestern Argentinean Low is more related to transient activity, as can be deducted from the subtropical jet influence on this system and also from the temporal variability of some key variables -such as the sea level pressure and the equivalent potential temperature-. From a thermo-dynamic point of view, the Chaco Low develops in a more unstable environment and is less influenced by middle latitude dynamics

Classificação sinótica dos campos de pressão atmosférica na América do Sul e sua relação com as baixas do chaco e do noroeste argentino Synoptic classification of the atmospheric pressure fields over South America and its relation to the Chaco and the Argentinean Northwest Lows

O trabalho apresenta uma classificação dos campos de pressão atmosférica ao nível médio do mar sobre América do Sul, durante as estações de verão e inverno, com o intuito de identificar os principais padrões sinóticos associados à Baixa do Noroeste da Argentina (BNOA) e à Baixa do Chaco (BCH). Para isso, a técnica de Análise de Componentes Principais rotacionadas foi aplicada à serie de reanálises do NCEP para o período 1985-2010. Um segundo objetivo foi determinar as principais frequências de variabilidade relacionadas com a atuação de ambos os sistemas, mediante a utilização do método de Análise Espectral. Os resultados destacam cinco padrões de circulação que representam aproximadamente 80% dos casos analisados. Durante o verão, a BCH, localizada entre o norte da Argentina, Paraguai, Mato Grosso do Sul e o sul da Bolívia, está associada principalmente com dois padrões sinóticos. A BNOA está associada apenas com um padrão sinótico, similar ao campo médio sazonal. Já no inverno, a BCH não aparece, confirmando seu caráter térmico, gerada principalmente pela liberação de calor latente, produto da convecção observada na área de atuação deste sistema. A BNOA também está associada apenas com um padrão sinótico durante esta estação. Este padrão é similar ao obtido no verão, porém representando menor porcentagem de variância e relacionado principalmente com perturbações sinóticas. A análise de frequência mostra que o modo de circulação associado à BCH tem um pico de energia em torno dos 17 dias, enquanto que a BNOA apresenta um pico de energia em torno dos 22 dias durante o verão e um relativo aumento da variabilidade sinótica no inverno.<br>This work performs a classification of sea level pressure fields for South America during summer and winter, in order to identify the main synoptic patterns associated with the Northwestern Argentinean Low (NAL) and the Chaco Low (CHL). To this purpose, the Principal Component Analysis technique in rotated mode was applied to the NCEP reanalysis for the period from 1985 to 2010. A second objective was to determine the main variability frequencies related to both systems, by employing Spectral Analysis. The results provide five circulation patterns, representative of about 80% of all cases. During summer, the CHL, located between northern Argentina, Paraguay, Bolivia and southern MS, is mainly associated with two synoptic patterns. The NAL is associated with a single synoptic mode, similar to the seasonal mean field. During winter, the CHL is not recognized, in agreement with its thermal nature related to latent heat released by tropical convection. The NAL is also associated with a unique synoptic mode during this season. This mode is similar to that obtained in the summer and is also related to synoptic scale disturbances but explaining a lesser percentage of variance. The Frequency Analysis show that the circulation mode associated to the CHL has an energy peak at around 17 days, while the NAL has an energy peak around 22 days in summer and a relative increase of synoptic variability in winter

Sahelian Heat Wave Characterization From Observational Data Sets

International audienceObservational data sets are used to statistically document processes involved in spring time heat waves in the Sahel Nighttime heat waves are linked to an increase in precipitable water as well as a frequent high aerosol load. Daytime heat waves are generally associated with cleaner skies. The more intense events show in addition a higher amount of precipitable water