Session H6: Understanding Hurricanes and Severe Storms: Patterns, Prediction and Mitigation

Understanding Severe Hurricanes

Hurricanes are complex phenomena, whose understanding involves many facets, of which my presentation will provide an overall flavor and review. Understanding the physical hurricane involves a complex amalgam of fluid dynamics, thermodynamics and scale interactions. The basic structure is one of a fluid vortex, which dictates everything from the characteristic spiral shape to the clear eye region. Energetically, once formed a hurricane is a self sustaining heat engine, one that extracts energy from the enthalpy difference between the warm ocean surface and the cold upper atmosphere, and one that will continue its merry way until it is destroyed by some external influence (such as landfall). Hurricanes also are a response to the global climate in which they develop and can feed back to influence and perhaps even change that climate. For example a series of hurricanes moving into the higher latitudes in the Pacific can set off a train of events that are still affecting European weather a year later. From a societal perspective they are the most dangerous and deadly of all natural atmospheric systems, capable of causing widespread destruction and long-term disruptions to entire societies. The damage wreaked by Katrina in New Orleans provides a canonical example, but this was by no means the worse cyclone in history. Even lesser damage on a small island nation can be much more catastrophic and exceed their entire gross domestic product. This capacity for disruption arises from three main mechanisms: the high surface winds, the response of the ocean to these winds, and the intense rainfall. These have widely different contributions in different storms: the extended region of high winds and particularly the storm-surge response were dominant factors in Katrina; whereas the $>$10,000 deaths by Hurricane Mitch arose entirely from rainfall and the associated flooding and landslides. Societal response to this danger involves complex interplays of warning, communication culture, previous experiences and perceptions, interplays that are neither well understood nor adequately predictable.   

Tornadoes and Severe Thunderstorms: Physical Understanding and Climate Questions

Severe thunderstorms (those that produce large hail, high winds and/or tornadoes) are of importance because of the threat to life and property they pose. This talk will review our understanding of the physical processes that lead to them and their distribution in time and space. The basic approach follows that of weather forecasting, focusing on the atmospheric ``ingredients'' in the environment necessary to produce severe thunderstorms and tornadoes, particularly the thermodynamic state of the atmosphere and the organizing effects of vertical wind shear that leads to the most severe storms. We will look at the challenges of reconciling our limited reporting databases of events and our physical expectations derived from the distribution of those environmental conditions. Consistent (and inconsistent) aspects of the various databases around the world will be discussed with their implications for what we can and cannot say about the basic physical processes. Of particular interest is the record from the United States. Some simple efforts to deal with the spatial and temporal inhomogeneities in the observational record will be developed with the limits that are implied on our ability to detect past changes. Finally, the talk will close with a discussion of the possible effects of anthropogenic global warming on severe thunderstorms, particularly in the United States. Global climate model studies of this problem are very recent, with the first peer-reviewed results appearing in 2007. The limitations of the climate models and possible scenarios for the future will be discussed.   

Factors Influencing Hurricane Surges along the Louisiana-Mississippi Coast

The effects of recent hurricanes along U.S. Coastlines, along with expected future sea level rise and the potential for increased storm activity all point to a critical need for improved methods for estimating coastal hazards and associated risks. Many of the models used today to assess hazards and risk incorporate considerable empiricism in their formulation. Unfortunately, most of the empirical evidence is drawn from small to moderate storm events and cannot be effectively extrapolated to extreme storms such as Hurricane Katrina. This presentation will critique the state of the art in hurricane surge prediction, including the adequacy of numerical models, coefficients within these models, and the wind fields utilized to force them. Once the predictive system and its physical basis are introduced and discussed, a methodology will be described for utilizing information from such a system to estimate risk for coastal areas, including the effect of uncertainties in both the modeling system and storm climate,. Using the methodology introduced here, maps of estimated storm surge levels for selected recurrence intervals in the New Orleans area will be presented along with a comparison to some previously derived values to provide perspective.   

Wetland Loss and Restoration Options in Southern Louisiana

Wetlands are productive landscape features of the broad Mississippi River Delta system. In addition to their ecological services of providing habitats for a variety of species including juvenile commercial and recreational fish, they provide a valuable wave reduction role during severe storm events characterized by elevated water levels and high waves. Currently, these wetlands are stressed by a combination of natural and human-related forces resulting in rapid loss rates. Although many factors contribute to wetland loss rates, the single greatest factor is the shunting of river borne sediments offshore into deep water. Navigational interests benefit greatly from the present fixed location of the main navigation channel at Southwest pass with its terminus at the edge of the continental shelf such that the sediment load is discharged into deep water. The Mississippi River Delta region is subsiding at up to more than 10 times the Eustatic rate of sea level rise and thus the wetland and barrier island systems require these sediments for maintenance and growth. With the increasing scarcity and costs of energy, it is highly desirable that wetland restoration be done using natural forces to the degree possible. Absent legal issues, a pragmatic approach could be to identify those areas where progress can be made with realistic investments of economic and energy resources and to accept that areas with less benefit per investment will continue to degrade. The paper will review various options and discuss obstacles and opportunities.