Estudo das características das perturbações EOF extratropicais e seu impacto na previsão de tempo de médio prazo por conjunto

Abstract

The characteristics of the extratropical EOF perturbations are studied and their impact on the medium range ensemble weather prediction is evaluated, using simulations with the Center for Weather Prediction and Climate Studies Ensemble Weather Prediction System (CPTEC-EPS). The EOF method, originally applied over tropical regions and for horizontal wind and temperature fields, uses empirical orthogonal functions (EOF) to determine the fast growing initial perturbations, named optimal perturbations. Seven experiments are performed in order to investigate the main mechanisms related to the EOF perturbations growth in the extratropics. This study is relevant since the mechanisms for perturbation growth in the tropics are different from those of midlatitudes. Also, the characteristics of extratropical EOF perturbations were not studied yet. Besides the investigation on the extratropical EOF characteristics, analyses about the inclusion of additional initial perturbations in the surface pressure (P) and specific humidity (Q) fields, perturbations on target areas, and modifications in the methodology for selecting the fast growing modes are also conducted. A test using a simplified version of the breeding of growing modes method for initial perturbations is also performed. The results are presented for a period of austral summer (December 15, 2004 to February 15, 2005) and for a case study of a severe event, the cyclone Catarina, occurred in March 2004. Suitable statistical scores for ensemble weather forecasting are calculated to assess objectively the impact that modifications proposed in each experiment has on the quality of ensemble weather predictions. The results show that extratropical EOF perturbations develops preferentially near regions with high baroclinicity in midlatitudes and close to areas of important synoptic systems that act in the tropics-extratropics of the Southern Hemisphere during the austral summer. Perturbations on P and Q are important in obtaining perturbations that are more organized and with more baroclinic structures in midlatitudes. This modification also produces perturbations that grow faster than in those cases in which no perturbations are applied to P and Q, during the nonlinear model integration. Statistical evaluation indicates that extratropical EOF perturbations span a subspace in the phase space representing fast-growing errors, contributing to the enhancement of the ensemble forecast quality in both deterministic and probabilistic sense and, moreover, including more diversity in the initial perturbations. Regional EOF Perturbations computed separately over northern South America and southern South America in addition to hemispheric perturbations contribute in a general sense to the improvement of local forecasts over the target region. The simplified breeding method presents a performance similar to that of EOF method over extratropics, showing slight advantage only in the ensemble spread in this region. However, the EOF method is more skillful over tropics and South America, indicating that it is more suitable for both regions. The results of the case study for the cyclone Catarina show that additional EOF perturbations over the extratropics and on P and Q fields increase the ability of the ensemble members in simulating the system and contribute to produce better predictions of the cyclone central pressure.