The Effects of Tropical Waves on the Formation and Structure of Tropical Cyclones

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Five types of large-scale tropical waves in the atmosphere travel around Earth's equatorial regions. In his previous work, the PI analyzed these waves in observations and determined that four of the five wave types have a significant influence on the formation of tropical cyclones (called hurricanes in the Western Hemisphere). His and others' recent research have shown that the propagating waves play major roles in modulating patterns of rainfall and winds. They have also been shown to be integrally related to the formation of many or most of the tropical cyclones that occur throughout the world. They almost certainly affect the structure and intensity of mature hurricanes as well. Building upon that work, this research project focuses on determining the role of large-scale tropical waves in forming and modulating hurricanes in the North Atlantic and Northeast Pacific basins. Under this award the PI will focus on storms that formed in the years 2003-2005. The final year was an active year for destructive North Atlantic hurricanes. Observational analyses will determine what tropical waves were present during the formation and development of each storm. Relationships will be sought between the presence of different types of waves and storm formation and changes in storm structure. Each case study will consist of observational analyses and a series of numerical simulation experiments. The observational analyses will use spectral filtering techniques to isolate the wave perturbation fields occurring throughout the storm's life cycle. The filter bands will be designed to capture the activity of the five major tropical wave types that are most prominent in the large-scale flow (the Madden-Julian Oscillation, equatorial Rossby waves, mixed-Rossby-gravity waves, Kelvin waves, and tropical-depression-type waves). These filtered data, along with the hurricane structure data from the available data sources, will be used to determine relationships between wave activity and the formation and structural changes of the hurricanes. The storms will be simulated using a high-resolution version of the Weather Research and Forecasting (WRF) model. Control simulations, using the full observed state of the atmosphere as initial conditions will be compared with simulations in which the different types of waves have been removed from the initial state or included with modified phases or strengths. From the results of these experiments, the influences of tropical waves on cyclone formation and structural evolution will be determined quantitatively. In addition to improving our knowledge of important physical mechanisms governing the behavior of hurricanes, this study should lead to direct improvements in the capacity to forecast hurricane formation and structure change. The tropical waves to be studied are large-scale and have periods ranging from about 4-90 days. These waves are inherently predictable using statistical and/or deterministic models. Understanding how these waves influence tropical cyclones will lead to improved predictions of tropical cyclone formation and development. Two graduate students will be trained at the PhD level.