Federico A. Norte

Analysis of Three Situations of the Foehn Effect over the Andes (Zonda Wind) Using the Eta–CPTEC Regional Model

Abstract The zonda is a warm and extremely dry wind that occurs east of the Andes Cordillera in the extratropical latitudes of South America. Its orographic origin is similar to the foehn that blows in Germany and Austria and the chinook that occurs east of the Rocky Mountains. Three typical zonda events of different categories (surface and elevated) are described, through observational and Eta–Centro de Previsao de Tempo e Estudos Climaticos (CPTEC) model output. During the events the temperature rises sharply by 10°–15°C and the dewpoint temperature drops by 15°–20°C in an interval of a few hours. The sustained wind strength at the surface increases to 30 kt, with gusts of more than 40 kt. The episodes generally start around midday and last for about 10 h. The Eta–CPTEC model was able to forecast several aspects of the three analyzed zonda cases, such as wind strength, temperature, and humidity changes, and their starting and ending times. Some relationships between the intensity of the windward static ...

Influence of the Subtropical Andes on Baroclinic Disturbances: A Cold Front Case Study

Abstract The Andes Cordillera produces a significant disruption to the structure and evolution of the weather systems that cross South America. In particular, cold fronts tend to be “channeled” to the north immediately to the east of the Andes, fostering the advance of cold air incursions (cold surges) well into subtropical, and sometimes tropical, latitudes. In contrast, active cold fronts hardly reach subtropical latitudes along the western side of the Andes (Pacific sea border). Instead, as a cold front moves equatorward along the east side of the Andes, a marked low-level warming tends to appear along the west side of the subtropical Andes, leading to the formation of a mesoscale coastal low (or trough) in this region. To further understand the processes that lead to a contrasting evolution of the cold front at each side of the Andes, a typical frontal passage is studied in this work, using synoptic observations and a regional model [Eta–Centro de Previsao de Tempo e Estudos Climaticos (CPTEC)] simula...

Strong Cross-Barrier Flow under Stable Conditions Producing Intense Winter Orographic Precipitation: A Case Study over the Subtropical Central Andes

Abstract The most intense orographic precipitation event over the subtropical central Andes (36°–30°S) during winter 2005 was examined using observational data and a regional model simulation. The Eta-Programa Regional de Meteorologia (PRM) model forecast was evaluated and used to explore the airflow structure that generated this heavy precipitation event, with a focus on orographic influences. Even though the model did not realistically reproduce any near-surface variables, nor the precipitation shadow in the leeside lowlands, its reliable forecast of heavy precipitation over the windward side and the wind fields suggests that it can be used as a valuable forecasting tool for such events in the region. The synoptic flow of the 26–29 August 2005 storm responded to a well-defined dipole from low to upper levels with anomalous low (high) geopotential heights at midlatitudes (subtropical) latitudes located off the southeast Pacific coast, resulting in a large meridional geopotential height gradient that drov...

Análisis de una ola de calor extrema en la región subtropical de américa del sur

ANALYSIS OF AN EXTREME HEAT WAVE OVER THE SUBTROPICAL REGION OF SOUTH AMERICA This work analyzes the meteorological conditions that caused the occurrence of an intense heat wave over subtropical South America, with temperatures higher than 40°C immediately to the east of the Andes, during the last week of January 2003. To that purpose the terms of the thermodynamic equation were evaluated through numerical simulations performed with the Eta/CPTEC regional model. The strong temperature increase was caused by adiabatic warming (subsidence) as well as by the net positive surface heat fl ux, especially to the east of the Andes. The qualitative analysis of the Quasi-Geostrophic Omega Equation revealed that the large-scale subsidence was the result of the cold horizontal temperature advection and the advance of a wide ridge at midlevels. Additionally, the presence of forced orographic subsidence (Zonda wind) is evident from observations and numerical simulations in connection with the passage of a short trough immerse in the wider ridge, the approach of a cold front and the southward progress of the thermal-orographic low. Since this phenomenon is quite uncommon during summer, its incidence in combination to the other processes previously described would explain the occurrence of extreme maximum temperatures over western Argentina.