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- Author or Editor: A. Celeste Saulo x
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Abstract
A low pressure system [known as the northwestern Argentinean low (NAL)] is commonly observed over northwestern Argentina near the Andean slopes. This study describes two NAL episodes for summer and winter, with emphasis on the characterization of their three-dimensional structure and temporal variability. With the aid of a high-resolution regional model [Eta/Centro de Previsão de Tempo e Estudos Climáticos (CPTEC)], the main mechanisms involved in the NAL life cycle were studied in order to examine how the thermal topographical processes influenced the system behavior.
Surface pressure changes in the NAL are mostly dominated by the 600–900-hPa thickness variability, suggesting its thermal character. Based on this result, the temperature tendency equation has been used to quantify all the contributions to thermal changes.
The summer NAL has a significant diurnal cycle that has been dominated by surface warming. This single mechanism can explain the low pressure system formation by itself, suggesting that the NAL could always be present during this season unless an adverse process counteracts the positive contribution by the surface sensible heat flux. Other favoring processes found in the analyzed cases were the Foehn effect (warming by subsidence) and the latent heat release. The intermittent behavior of the NAL is associated with a suppression of net warming in the 600–900-hPa layer, due to a cold-air outbreak.
In the winter case, the Foehn effect has been mainly responsible for the NAL development. This mechanism acts in connection with an upper-level cyclonic disturbance approaching the Andes, indicating that the thermal response is triggered by a dynamical forcing. As the Foehn effect (locally known as Zonda wind) is a frequent winter phenomenon, the NAL intermittence during this season could be related to transient baroclinic activity, which modulates both the intensification and the decay stages.
The NAL has been regarded as a thermal-orographic system. This study suggests that the analyzed NALs behave as an almost “pure” surface thermally driven low in summer, while dynamical-orographic forcing is the organizing mechanism in winter.
Abstract
A low pressure system [known as the northwestern Argentinean low (NAL)] is commonly observed over northwestern Argentina near the Andean slopes. This study describes two NAL episodes for summer and winter, with emphasis on the characterization of their three-dimensional structure and temporal variability. With the aid of a high-resolution regional model [Eta/Centro de Previsão de Tempo e Estudos Climáticos (CPTEC)], the main mechanisms involved in the NAL life cycle were studied in order to examine how the thermal topographical processes influenced the system behavior.
Surface pressure changes in the NAL are mostly dominated by the 600–900-hPa thickness variability, suggesting its thermal character. Based on this result, the temperature tendency equation has been used to quantify all the contributions to thermal changes.
The summer NAL has a significant diurnal cycle that has been dominated by surface warming. This single mechanism can explain the low pressure system formation by itself, suggesting that the NAL could always be present during this season unless an adverse process counteracts the positive contribution by the surface sensible heat flux. Other favoring processes found in the analyzed cases were the Foehn effect (warming by subsidence) and the latent heat release. The intermittent behavior of the NAL is associated with a suppression of net warming in the 600–900-hPa layer, due to a cold-air outbreak.
In the winter case, the Foehn effect has been mainly responsible for the NAL development. This mechanism acts in connection with an upper-level cyclonic disturbance approaching the Andes, indicating that the thermal response is triggered by a dynamical forcing. As the Foehn effect (locally known as Zonda wind) is a frequent winter phenomenon, the NAL intermittence during this season could be related to transient baroclinic activity, which modulates both the intensification and the decay stages.
The NAL has been regarded as a thermal-orographic system. This study suggests that the analyzed NALs behave as an almost “pure” surface thermally driven low in summer, while dynamical-orographic forcing is the organizing mechanism in winter.
Abstract
This paper concentrates on the analysis of the life cycle of the low-level jet (LLJ) during a summer Chaco jet event. This is accomplished through the use of the Eta/Centro de Previsão del Tempo e Estudos Climáticos (CPTEC) regional model, in order to obtain high temporal and spatial detail of the main processes taking place. Both the low-level circulation and the geopotential height evolution at different latitudes are analyzed to provide a more detailed description of the effects of topography and differential warming on the evolution of this current.
This study shows that the life cycle of the particular event analyzed is not the same at the different latitudes swept by this well-organized northerly current, expanding from 15° to 32°S during two consecutive days. A common feature to all the examined latitudes is the presence of a diurnal cycle linked to local effects, which is more evident during the first day and a half of the simulation. This cycle was identified not only by a nocturnal maximum of the wind, but also by the oscillations of the geostrophic wind close to the surface in response to differential warming over sloping terrain. However, during the second day, the diurnal oscillation is superseded by synoptic-scale forcing. The meridional growth of this northerly current reacts basically to a deepening of the northwestern Argentinean low, consequently becoming a geostrophic response to a synoptic perturbance. However, during the final stages of this event, a northerly wind area located over the southern tip of the current, which notably increases the northerlies' penetration toward higher latitudes, develops. This last extension is mainly due to a component of ageostrophic origin. Evidence is provided in support of the hypothesis that this secondary development is a feedback between the LLJ and the precipitation at the exit region.
Abstract
This paper concentrates on the analysis of the life cycle of the low-level jet (LLJ) during a summer Chaco jet event. This is accomplished through the use of the Eta/Centro de Previsão del Tempo e Estudos Climáticos (CPTEC) regional model, in order to obtain high temporal and spatial detail of the main processes taking place. Both the low-level circulation and the geopotential height evolution at different latitudes are analyzed to provide a more detailed description of the effects of topography and differential warming on the evolution of this current.
This study shows that the life cycle of the particular event analyzed is not the same at the different latitudes swept by this well-organized northerly current, expanding from 15° to 32°S during two consecutive days. A common feature to all the examined latitudes is the presence of a diurnal cycle linked to local effects, which is more evident during the first day and a half of the simulation. This cycle was identified not only by a nocturnal maximum of the wind, but also by the oscillations of the geostrophic wind close to the surface in response to differential warming over sloping terrain. However, during the second day, the diurnal oscillation is superseded by synoptic-scale forcing. The meridional growth of this northerly current reacts basically to a deepening of the northwestern Argentinean low, consequently becoming a geostrophic response to a synoptic perturbance. However, during the final stages of this event, a northerly wind area located over the southern tip of the current, which notably increases the northerlies' penetration toward higher latitudes, develops. This last extension is mainly due to a component of ageostrophic origin. Evidence is provided in support of the hypothesis that this secondary development is a feedback between the LLJ and the precipitation at the exit region.
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 Previsão de Tempo e Estudos Climáticos (CPTEC)] simulation.
The passage of the postfrontal anticyclone over southern South America produces a poleward-pointing pressure gradient and, hence, geostrophic easterly flow at low levels. The tall and steep mountains block the flow, leading to a very small zonal wind component close to the slopes. Convergence (divergence) of the zonal flow to the east (west) of the subtropical Andes is largely compensated for by upward (downward) motion, and the associated cooling (warming) over this region. The weak zonal wind component near the Andes also breaks down the geostrophic balance over this region, giving rise to an acceleration of the southerly winds (i.e., along-barrier flow) and the consequent increase in cold advection. Therefore, to the east of the subtropical Andes both horizontal and vertical advection cool the lower troposphere, fostering the equatorward propagation of the cold front. To the west of the Andes, horizontal advection is largely offset by the strong warming associated with the enhanced subsidence over that region hindering the advance of the cold front into subtropical latitudes.
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 Previsão de Tempo e Estudos Climáticos (CPTEC)] simulation.
The passage of the postfrontal anticyclone over southern South America produces a poleward-pointing pressure gradient and, hence, geostrophic easterly flow at low levels. The tall and steep mountains block the flow, leading to a very small zonal wind component close to the slopes. Convergence (divergence) of the zonal flow to the east (west) of the subtropical Andes is largely compensated for by upward (downward) motion, and the associated cooling (warming) over this region. The weak zonal wind component near the Andes also breaks down the geostrophic balance over this region, giving rise to an acceleration of the southerly winds (i.e., along-barrier flow) and the consequent increase in cold advection. Therefore, to the east of the subtropical Andes both horizontal and vertical advection cool the lower troposphere, fostering the equatorward propagation of the cold front. To the west of the Andes, horizontal advection is largely offset by the strong warming associated with the enhanced subsidence over that region hindering the advance of the cold front into subtropical latitudes.
