Jose Paulo Bonatti

Diurnally Forced Tropical Tropospheric Circulation over South America

Abstract The impact of diurnally varying thermal forcing on the tropical circulation is studied with a linearized shallow water model on an equatorial beta plane. The mass and flow fields are interpreted as an internal mode with equivalent depth of the order of 250 m. This is the preferred vertical mode excited by typical cumulus heating profiles in the tropics. The diurnal forcing with the horizontal scale of the Amazon source partitions approximately 40% of the input energy to slow modes. The fast modes are trapped by the critical latitudes contributing significantly to oscillations in the divergence field, which emanate out of the source region. The divergence field at the source is approximately in phase with the forcing; the vorticity field lags the forcing by approximately 10 hours. A comparison of the model results with the 500 mb level vertical motion, derived from the National Meteorological Center analysis, shows a similar pattern of propagating oscillations. It is suggested that the diurnally f...

Experiments with EOF-Based Perturbation Methods and Their Impact on the CPTEC/INPE Ensemble Prediction System

Abstract The impact of modifications of the perturbation method based on empirical orthogonal functions (EOFs) used operationally upon the ensemble prediction system (EPS) at the Center for Weather Prediction and Climate Studies/National Institute for Space Research (CPTEC/INPE) is evaluated. The main changes proposed in this study are to apply the EOF method to perturb the midlatitudes, apply additional perturbations to the surface pressure (P) and specific humidity (Q) fields, and compute regional perturbations over South America. The impact of these modifications in the characteristics of the initial perturbations and in the quality of the EPS forecasts is investigated. The EPS forecasts are evaluated through average statistical scores over the period 15 December 2004–15 February 2005. The statistical scores used in the evaluation are pattern anomaly correlation, root-mean-square error, ensemble spread, Brier skill score, and perturbation versus error correlation analysis (PECA). Results indicate that ...

IMPACTO DA UTILIZAÇÃO DE PREVISÕES "DEFASADAS" NO SISTEMA DE PREVISÃO DE TEMPO POR CONJUNTO DO CPTEC/INPE

THE IMPACT OF USING LAGGED FORECASTS ON THE CPTEC/INPE ENSEMBLE PREDICTION SYSTEM In this work we report the application of the lagged average forecasting technique to CPTEC/INPE ensemble forecast. The CPTEC/INPE data consist of two months samples of 15 days forecast for the variables: geopotential height at 500 hPa, air temperature at 850 hPa and mean sea level atmospheric pressure. We focus on the following: 1) Does the lagged averaged ensemble forecast improve forecast skill compared to the CPTEC/INPE operational ensemble? 2) Is the dispersion of the ensemble useful in predicting forecast skill? The results indicate that the utilization of 12h-lagged average forecasts improves the performance of the operational ensemble contributing to increase the ensemble spreading and, consequently, to reduce the under-dispersion of the system. Also we observed that lagged average forecast (LAF) shows similar performance of the operational EPS-CPTEC/INPE and that there is a tendency to higher performance when spread forecast is low, for 5 and 7 day forecast. These results provide the basis for the operational implementation of the LAF technique, which has low computational cost, and contribute to a more efficient utilization of the CPTEC/INPE ensemble predictions.

Estudo da energética modal para episódios de ZCAS. Parte I: análise observacional

The horizontal and vertical energy exchanges for a composite of seven South Atlantic Convergence Zone (SACZ) episodes were studied by expansion into normal mode functions, with an emphasis on the vertical energy partition between external and internal modes and on the energy interactions within and among various horizontal oscillation modes: Rossby, Kelvin, Mixed Rossby-Gravity and West and East Gravity. A maximum share of the total energy (about 60%) was found in the 4th to 7th internal modes at equivalent highs between 100 and 600 meters, especially over large part of the central South America and near the equator including the SACZ region. As the latitude increases, the energy is distributed towards the lower order modes (n = 1 to 3). For the horizontal energy partition, the most expressive contributions were obtained for the self-interactions of the Rossby and Kelvin modes and for Rossby-Kelvin cross interactions in all vertical mode categories. The Rossby-Kelvin cross interactions constituted the main process for constructive energy interferences in the SACZ region. The vertical mode interactions indicated that the percentage of total energy increases from low levels to the stratosphere with maximum positive (negative) interferences in high levels (stratosphere), for the 4th to 7th internal modes.

Approximation of ensemble members in ocean wave prediction

An efficient method for generating members in a ocean wave ensemble prediction system is proposed. A linearization of the wave model WAM is used to obtain approximations of the ensemble members. This procedure was originally introduced in a dynamical assimilation scheme where Green’s functions play a central role. The evaluation of member approximations can be carried out in a fraction of the time required by the full model integration. This aspect of the method suggests a way of increasing the ensemble size as well as refining the model resolution without increasing computational costs.

Determinação das previsões de temperaturas mínimas e máximas a partir do histórico das previsões de tempo por conjunto do CPTEC

A tecnica de previsao de tempo por conjunto representa uma tentativa de aumentar a previsibilidade dos modelos dinâmicos da atmosfera. Uma forma simples de produzir previsoes de tempo por conjunto e adicionar pequenas perturbacoes as condicoes iniciais do modelo, e integra-lo partindo de cada condicao inicial perturbada para produzir um conjunto de previsoes. O objetivo do presente estudo e apresentar uma metodologia desenvolvida para determinar as temperaturas minimas e maximas a superficie, a partir de informacoes do historico do ponto de grade das previsoes por conjunto do Centro de Previsao de Tempo e Estudos Climaticos (CPTEC) sobre o Brasil. Os resultados indicaram que o modelo de circulacao geral da atmosfera do CPTEC apresenta temperaturas minimas (maximas) sistematicamente mais altas (baixas) dos que as observadas. Um metodo de correcao estatistica, baseado no vies medio das 15 previsoes mais recentes do modelo, foi aplicado para corrigir as previsoes do conjunto. Tais correcoes aumentam o desempenho das previsoes, como e indicado pelos indices estatisticos rms, bias e coeficiente de correlacao de anomalia.

Monthly variation of 200 mb flow in the tropics

SummaryZonal harmonic analysis of stream function suggests the dominance of wave number 1 in the Southern Hemisphere tropics throughout the year. In the Northern Hemisphere, wave numbers 1 and 3 are found to be important in winter while in summer, wave number 1 dominates. Calculation of the rotational part of kinetic energy indicated annual (12 month) variation in the subtropics and a dominant semi-annual (6 month) variation in the tropics. A latitude-time section of zonal wind over the equator revealed interesting implications for interhemispheric coupling. Occurrence of fairly strong westerlies over the Eastern Pacific and Atlantic oceans, during the Northern Hemisphere winter, suggests that these regions are favorable for interhemispheric coupling.ZusammenfassungDie zonale, harmonische Analyse der Stromfunktion zeigt während des ganzen Jahres eine dominierende Wellenzahl 1 in den südhernisphärischen Tropen. In der Nordhemisphäre sind die Wellenzahlen 1 und 3 im Winter von Bedeutung, während im Sommer ebenfalls Wellenzahl 1 dominiert. Die Berechnung des rotationellen Anteils der kinetischen Energie weist eine jährliche (12 Monate) Periode in den Subtropen und eine halbjährige (6 Monate) Periode in den Tropen auf. Der mittlere Zonalwind als Funktion der geographischen Breite und Zeit zeigt über dem Äquator Andeutungen interhemisphärischer Kopplung. Das Auftreten relativ starker Westwinde über dem Ostpazifik und dem Atlantik während des nordhemisphärischen Winters deutet darauf hin, daß diese Regionen günstig für interhemisphärische Kopplung sind.

On some climatological aspects of the zonally averaged atmosphere in the southern hemisphere

SummaryPreliminary estimates of energy fluxes by mean meridional circulations are presented for the Southern Hemisphere. Total diabatic heating has been obtained by assuming balance between the divergence of energy flux and heating. Diabatic heating shows essentially a four latitude belt pattern with heating in the tropics and middle latitudes and cooling in the subtropics and polar latitudes. Although this is qualitatively similar to that obtained for the Northern Hemisphere and also to that obtained for the Southern Hemisphere by direct calculation of heating, there are quantitative differences.ZusammenfassungEs werden vorläufige Abschätzungen des durch die mittlere Meridionalzirkulation bewerkstelligten Energiestromes in der Südhemisphäre vorgelegt. Die diabatische Gesamterwärmung wurde durch die Annahme eines Gleichgewichtes zwischen Energiestromdivergenz und Erwärmung erhalten. Die diabatische Erwärmung weist im wesentlichen vier geographische Breitezonen auf: Aufwärmung in den Tropen und in mittleren Breiten, Abkühlung in den Subtropen und in den Polargebieten. Dies entspricht zwar qualitativ den Ergebnissen für die Nordhemisphäre und auch den durch direkte Berechnung der Erwärmung erhaltenen Resultaten für die Südhemisphäre, doch zeigen sich quantitative Unterschiede.