Rosana Nieto Ferreira

Barotropic Aspects of ITCZ Breakdown

Abstract In satellite images the ITCZ (intertropical convergence zone) is sometimes observed to undulate and break down into a series of tropical disturbances. Tropical cyclones may later develop within these disturbances and move into higher latitudes allowing the ITCZ to reform. It has been proposed that ITCZ breakdown results from a convectively modified form of combined barotropic and baroclinic instability of the mean flow. An unstable mean flow can be produced by ITCZ convection in just a couple of days. In this sense, the ITCZ produces favorable conditions for its own instability and breakdown. In this study, a nonlinear shallow water model on the sphere is used to simulate barotropic aspects of ITCZ breakdown. In the shallow-water model, the ITCZ is simulated by a prescribed zonally elongated mass sink near the equator. The mass sink produces a cyclonic potential vorticity (PV) anomaly that has a reversed meridional PV gradient on its poleward side. According to the Ripa theorem, a flow that has a...

Dynamical Aspects of Twin Tropical Cyclones Associated with the Madden–Julian Oscillation

Abstract A nonlinear shallow-water model on the sphere is used to study barotropic aspects of the formation of twin tropical disturbances by Madden–Julian oscillation (MJO) convection. In the model, the effect of MJO convection upon the lower-tropospheric tropical circulation was simulated by an eastward moving, meridionally elongated mass sink straddling the equator. The intensity and propagation speed of the mass sink were chosen to simulate observations that MJO convection intensifies while nearly stationary in the eastern equatorial Indian Ocean, weakens while moving eastward over the Maritime Continent, again intensifies once it reaches the west Pacific Ocean, and finally becomes stationary and dies off near the date line. This mass sink produced twin cyclones in the two regions where it was stationary, namely, where it was initially turned on and where it was turned off. In addition, the mass sink produced two zonally elongated cyclonic potential vorticity anomalies straddling the equator in the reg...

The Role of Tropical Cyclones in the Formation of Tropical Upper-Tropospheric Troughs

Abstract Tropical upper-tropospheric troughs (TUTTs), also known as midoceanic troughs, are elongated troughs that appear in summer monthly averaged maps of the upper-tropospheric flow over the oceans. The transient part of these climatological features is composed of TUTT cells and their origin is the subject of this study. TUTT cells often occur to the east of tropical cyclones. A nonlinear shallow water model on the sphere was used in a simplified study of the interactions between tropical cyclones and the circumpolar vortex. Based on the results of these simulations and keeping in mind their limitations, it is proposed that dispersion of short Rossby wave energy is a possible mechanism to explain the formation of TUTT cells to the east of tropical cyclones. The model simulations suggest that two types of TUTT cells may form to the east of tropical cyclones. When embedded in cyclonic or weak anticyclonic shear, the trough to the east of the tropical cyclone may broaden, resulting in the formation of an...

Application of a linear spectral model to the study of Amazonian squall lines during GTE/ABLE 2B

A linear nonhydrostatic spectral model is run with the basic state, or large scale, vertical profiles of temperature and wind observed prior to convective development along the northern coast of South America during the GTE/ABLE 2B. The model produces unstable modes with mesoscale wavelength and propagation speed comparable to observed Amazonian squall lines. Several tests with different vertical profiles of low-level winds lead to the conclusion that a shallow and/or weak low-level jet either does not produce a scale selection or, if it does, the selected mode is stationary, indicating the absence of a propagating disturbance. A 700-mbar jet of 13 m/s, with a 600-mbar wind speed greater or equal to 10 m/s, is enough to produce unstable modes with propagating features resembling those of observed Amazonian squall lines. However, a deep layer of moderate winds (about 10 m/s) may produce similar results even in the absence of a low-level wind maximum. The implications in terms of short-term weather forecasting are discussed.

A Climatology of the Structure, Evolution, and Propagation of Midlatitude Cyclones in the Southeast United States

AbstractThe seasonal and interannual variability of the structure, evolution, and propagation of midlatitude cyclones in the southeast United States are studied using a composite analysis. In the upper levels, the composites show that the axis of the wintertime upper-level trough remains north–south oriented and propagates eastward along 40°N, while the summertime upper-level trough has a much slower propagation at a farther north latitude and an axis that is tilted in the northeast–southwest direction. Upper-level circulation changes are consistent with a shift from wintertime “cyclonic behavior” to summertime “anticyclonic behavior” midlatitude cyclones. Significant changes in the low-level structure and precipitation patterns of midlatitude cyclones ensue from these upper-level changes. While the winter composite is characterized by eastward-propagating midlatitude cyclones that extend deep into the subtropics, the summer composite is characterized by semistationary midlatitude troughs that only briefl...

Radar Observations of Convective System Variability in Relationship to African Easterly Waves during the 2006 AMMA Special Observing Period

Abstract A radar-based analysis of the structure, motion, and rainfall variability of westward-propagating squall-line mesoscale convective systems (SLMCSs) in Niamey, Niger, during the African Monsoon Multidisciplinary Activities (AMMA) 2006 special observing period is combined with an analysis of 700-mb (hPa) winds and relative vorticity to study the relationship between SLMCSs and African easterly waves (AEWs). Radar results show that SLMCSs were the most important rainmakers in Niamey and accounted for about 90% of the rainfall despite being present less than 17% of the time. Analysis of the 700-mb synoptic-scale flow revealed that during the 2006 West African monsoon season the African easterly jet vacillated between about 10° and 15°N on time scales of 1–2 weeks. AEWs followed the jet as it vacillated north and south, thereby producing two preferred paths for AEWs propagating past Niamey’s longitude, a northern track along 8°–16°N and a southern track along 2°–6°N. It was found that Niamey SLMCSs oc...

Climate and Geomorphologic-related Disasters in Latin America

Because most of its territory is located in tropical and equatorial areas, climatic factors are a main driver of natural disasters in Latin America. For instance, intense tropical rainfall in mountain areas is responsible for some of the most catastrophic floods and landslides in Latin America, such as those that occur in northern Venezuela, Colombia, and southeastern Brazil. The El Nino-Southern Oscillation also triggers a variety of geomorphologic hazards along the continent such as the floods, landslides, and coastal erosion that affect desert areas of the Pacific coast and the floods and landslides that affect southeastern Brazil and the La Plata Basin. In Central America and Mexico, hurricane landfalls from the Eastern Pacific and Atlantic Ocean bring intense rainfall, winds, and storm surges that cause devastating floods and landslides. Many of the devastating effects of natural hazards and disasters have been aggravated by increased human-induced vulnerability in various Latin American regions. Human-induced desertification in Patagonia and accelerated fluvial erosion in south and southeastern Brazil are examples of negative human effects on the Latin American landscape that aggravate the effects of natural disasters. Climatic trends in South America during the last few decades, in particular in the Argentinean plains, have also contributed to flood aggravation.

Influence of the North Atlantic Subtropical High on wet and dry sea-breeze events in North Carolina, United States

This project was partially funded by the Climate and Large-Scale Dynamics and the Physical and Dynamic Meteorology programs of the National Science Foundation’s Division of Atmospheric and Geospatial Sciences, Award AGS-1118141.