Browsing by Author "Anantha Aiyyer, Committee Chair"

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Effects of Appalachian Topography on Precipitation from Landfalling Hurricanes
(2009-04-13) Harville, Steven L.; Sandra Yuter, Committee Member; Gary Lackmann, Committee Member; Anantha Aiyyer, Committee Chair
A thorough analysis of rainfall distributions associated with tropical cyclones that have impinged upon or impacted the southern and central Appalachian mountain range is conducted using the North America Regional Reanalysis (NARR) and the Weather Research and Forecasting (WRF) model. The primary objective of this study is to improve the skill and precision of future forecasts by identifying specific areas where enhancement of the precipitation associated with landfalling tropical cyclones due to the direct and indirect effects of orography most frequently occurs. Based on the relative positions between the tropical cyclone tracks and the orientation of the Appalachian Mountains, four storm tracks are classified. We identify locations with the highest potential for flooding using local maximum analysis for each representative track. For storms that run parallel along the eastern side of the Appalachians (Track-B), heavy rainfall is located along eastern slopes with the heaviest precipitation falling across western North Carolina and central Virginia. Storm tracks that run parallel on the western side of the Appalachians (Track-C) show heaviest precipitation falling on the eastern slopes of western North Carolina. For storms that track more perpendicular to the mountain range, maximum rainfall is located over the mountains of central Virginia (Track-A) and across the southern Appalachians (Track-D). A second goal of this work is to document some of the effects of these mountains on landfalling tropical cyclones, on the synoptic environment as a whole, and on the interactions of these tropical and mid-latitude cyclones. Work here is focused on expanding upon the synoptic approach of Atallah et al. (2007). This is accomplished through examination of the precipitation climatology, analysis of composites and case studies, and by numerical simulation. Hart and Evans (2006) find that the orientation of the approaching upper-level mid-latitude trough is one of the most significant factors in determining the occurrence of extratropical transition (ET) and the potential for reintensification. Results suggest that through orographic enhancement of the downstream ridge, these storms play an active role in tilting the approaching mid-latitude trough towards a more negative orientation, thus increasing the likelihood of ET. At the same time synoptic-scale frontal boundaries slow and strengthen as they approach the Appalachians from the west, similar to the findings of O'Handley and Bosart (1996). As a result, numerical simulations run with topography show greater precipitation over areas northwest of the Appalachians than experimental simulations run with flattened terrain.
Nonlinear Structure and Evolution of African Easterly Waves
(2008-07-24) Hardin, Nathan R; Anantha Aiyyer, Committee Chair; Sandra Yuter, Committee Member; Gary Lackmann, Committee Member
Tropical Atlantic Vertical Wind Shear Variability in a Future Climate
(2009-11-23) Talgo, Kevin; Anantha Aiyyer, Committee Chair; Fred Semazzi, Committee Member; Gary Lackmann, Committee Member
Simulations from a suite of 21 fully-coupled global climate models (GCMs) collected for the International Panel on Climate ChangeÃ¢â‚¬â„¢s Fourth Assessment Report (IPCC-AR4) provide a unique opportunity to explore the effects of climate change on tropical cyclone (TC) activity. Vertical wind shear is a key environmental variable that has a detrimental effect on the genesis and intensification of TCs. Variability of shear in the Atlantic is influenced by changes in the large-scale background circulation, forced by teleconnections such as the El Nino-Southern Oscillation and the West African monsoon system. Spatio-temporal variability of ENSO and West African (Sahel) rainfall in the 20th century is examined for the suite of GCMs. This serves as a basis to determine which models have the most reasonable simulations of the 20th century so that they can be used to make predictions about changes to Atlantic vertical wind shear in the 21st century under global warming conditions. Model simulations of the 20th century are compared to observations gathered from various datasets. The models exhibit a wide range of skill in simulating the various features that modulate shear in the tropical Atlantic. Several models have deficient simulations of ENSO and Sahel rainfall in their 20th century simulations. Five models are determined to have accurate simulations of the 20th century climate and will be most useful for making predictions about shear changes in the 21st century. Long-term trends of July-September Sahel rainfall and tropical Atlantic shear under 21st century global warming conditions simulated by the GCMs are examined. There is a strong disagreement across the full suite of models as to the changes in shear and Sahel rainfall in the 21st century. However, four out of the five models determined to have the most accurate simulations of the 20th century climate predict a significant increase in shear in the tropical Atlantic. A statistical approach is used to investigate whether the dichotomy in shear trends in the tropical Atlantic is related to a similar split in the model projections for future rainfall trends in the Sahel. It is suggested that the spread in projections of future Sahel rainfall variability contributes significantly to the uncertainty in tropical Atlantic shear predictions. Atlantic shear and Sahel rainfall are well-correlated and vary together on interannual timescales. We can deduce that Sahel rainfall will continue to be a useful predictor of seasonal TC activity into the 21st century. It appears that the 21st century shear trend is at least partially explained by changes in Sahel rainfall, especially in the eastern tropical Atlantic, closest to the monsoonal forcing in West Africa. However, the degree of association is unclear. It is speculated that other teleconnections, such as ENSO, are becoming more dominant in influencing the multidecadal variability of shear in the tropical Atlantic.