[Atlantic Oceanographic and Meteorological Laboratory] Image of lightning image of coral reef image of coastal mangroves image of airplane flying


AOML
•Home
•Projects
Hurricane
  Overview

•Publications

Research Divisions

•Hurricane
  Research

•Ocean and
  Chemistry

•Physical
  Oceanography

[Coastal & Regional] [Hurricanes]

Site Map

Staff Data Sets Contact  Information Research Divisions

Hurricanes

[Tropical Meteorology:plane in storm image]Tropical Meteorology: Tropical cyclones move with the surrounding wind and draw their energy from the sea. The hurricane problem has historically been framed in terms of track forecasts and emphasis on forcing by the synoptic-scale atmosphere. As society has begun to ask meteorologists to predict variations in intensity or seasonal levels of activity, ocean thermal structure emerged as a key, perhaps dominant, factor. When one takes account of storm-induced cooling of the surface, it is upper ocean heat content that controls intensity fluctuations. Annual to decadal changes in the Atlantic thermohaline circulation correspond to variations of the numbers of hurricanes, most especially of the most intense "major hurricanes" that cause greatest damage. Hurricane wind and rain impacts on the built (Figure 26) and natural environments make human and economic effects an emerging field of study that spans architecture, engineering, sociology, economics, biology, chemistry, and even geology, in addition to meteorology and oceanography. Scientists at AOML, working with colleagues at Florida International University, RSMAS, the National Hurricane Center (NHC), the Environmental Modeling Center, and elsewhere, are in a unique situation to pursue all aspects of the hurricane problem.

Hurricane Track Forecasting: Hurricane track forecasts are the success story of tropical meteorology. If current trends continue, early in the new century vector errors at all forecast periods will be half what they were in 1970. Motion is a combination of propagation due to the vortex's asymmetric structure and advection by the surrounding winds (PTM1). Synoptic surveillance missions in which aircraft observe this "steering flow" with dropsondes deployed around hurricanes are HRD's contribution to the improvement of operational forecasts (PTM2). These flights, which began in 1982, provided the justification for procurement of NOAA's high-level Gulfstream IV jet aircraft. In their present form (PTM3), they incorporate targeting based upon NCEP's ensemble forecasts and reduce track errors by 10-15% in data-denial simulations (Figure 27). This improvement is equivalent to about a decade of business-as-usual progress in the forecaster's art.

Oceanic Forcing: By contrast, Hurricane Opal of 1985 illustrates the limited progress in intensity forecasting (PTM4). Opal intensified rapidly overnight as it accelerated toward the United States Gulf Coast (Figure 28). It would have been a repeat of Hurricane Camille's devastating landfall if it had not weakened equally abruptly. Neither the intensification nor the weakening were forecast with enough lead time to permit appropriate response by emergency managers. A simple air-sea interaction model developed at Massachusetts Institute of Technology appears to demonstrate a dominant role for oceanic forcing relative to atmospheric forcing or the cyclones' internal dynamics. As impressive as this result is, it is accurate only to about one Saffir-Simpson category. One possible improvement is replacement of the climatological representation of upper ocean structure with one observed through satellite altimetry (PTM5). Detailed observations of ocean response from aircraft (PTM6), combined with buoy and dropsonde observations of the hurricane's atmospheric boundary layer, also show promise for further improvement (PTM7). Shear of the environmental wind appears to be the mechanism by which atmospheric teleconnections modulate Atlantic hurricane activity and it is generally thought to have a significant role in day-to-day intensity changes of individual storms. Airborne Doppler and reflectivity radar show that an environmental shear >10 m s-1 imposes a wavenumber-one structure on the eyewall convection (Figure 29). Individual cells form ~45° to the right of the down-shear direction, reach maturity with reflectivities >45 dBZ on the left side of the shear vector, and have largely rained out by the time they detach from the eyewall as they advect back to the right side of the shear (PTM8). Observations of chemical tracers offer an opportunity to validate meteorological theories of hurricane development (PTM9). Already, these insights have led to a reevaluation of hurricane eye thermodynamics in which air has a long residence time inside the eye, in contrast with the rapid recycling through the eye postulated earlier. The boundary layer is the place where hurricanes impact people and property. Hurricane surface winds are the main emphasis of the Hurricanes at Landfall (HaL) focus of the United States Weather Research Project (USWRP). Since the mid-1980s, HRD has provided forecasters with quasi-operational analyses that are based on data from reconnaissance aircraft and surface anemometers (PTM10) . Recent addition of satellite cloud-drift winds and surface winds deduced from both spaceborne and airborne remote sensing extends the domain and accuracy of this product. It is used routinely as guidance for watches and warnings and is proving useful for early evaluation of insurance losses and impacts on infrastructure during individual landfalls. The GPS-based dropsondes developed for synoptic surveillance are superb boundary layer probes (Figure 30) because they report independent wind and thermodynamic observations every 5 m as they fall to the surface (PTM11). In the convective regions of hurricanes, GPS sondes have revealed previously unsuspected low-level jets at 100-300 m altitude with winds 20-40% stronger than those at 3 km or the surface. Another operationally important property of hurricanes is the onset of gale-force winds (17 m s-1) after which preparations for landfall generally must cease. The surface wind analyses provide forecasts and validation of this key parameter, particularly so since their augmentation with remote sensing (Figure 31). Dedicated aircraft missions (PTM12) and detailed radar observations (PTM13) in hurricanes as they pass onshore, combined with land-based intercept teams from universities such as Texas Tech, Clemson, and the University of Oklahoma, and careful post-storm damage assessments (PTM14) will produce increasingly refined quantitative models of hurricane wind impacts. Evacuation in response to effective and timely warnings have reduced deaths from storm surge to the point that inland flooding is the primary cause of U.S. mortality, as the somber experience of Hurricane Floyd this past season dramatizes.

Tropical Rainfall: Tropical rainfall is a key element of tropical weather both in hurricanes and generally. At sea, airborne radar and passive radiometers can measure the distribution of rainfall (PTM15). In-situ measurements of precipitation microphysics are essential to understanding the remotely sensed observations (PTM16) . Acoustic sensing of wind and rainfall at sea (PTM17) is an extremely promising avenue of investigation. Collaboration with NASA through their Third Convection and Mesoscale Experiment (CAMEX3) with planned follow-on in 2001 or 2002 directs extensive resources at this important problem. The Tropical Rainfall Measuring Mission (TRMM), which measures rainfall from orbit worldwide, is finding application to hurricane rainfall distribution (PTM18) . Another powerful tool is the National Weather Services' fully deployed network of operational Doppler radars, the WSR-88D, which is proving invaluable for measurement of precipitation in hurricanes (PTM19) . Numerical modeling also has a vital role in understanding (PTM20) and prediction of rainfall processes and amounts both for tropical cyclones (PTM21) and for ecological impacts (PTM22).

Remote Sensing: Recent satellite measurements promise additional information, both for real-time analysis and forecasting and for research. The 1400-km wide swath of the SeaWinds instrument aboard QuikSCAT, which was launched into polar orbit in June 1999, provides surface winds at 25 km resolution that can aid in the early detection of Atlantic Ocean tropical depressions (PTM23) and provide data for assimilation into the AOML real-time surface wind analysis for the Hurricanes at Landfall (HAL) project (PTM24). Features in the surface winds observed at about 100 m resolution by the Canadian RadarSat Synthetic Aperture Radar (SAR) during four storms in the 1998 hurricane season provide evidence of deep secondary flows in the hurricane boundary layer (PTM25). Climatic change modulates hurricane activity. Hurricane landfalls on the U.S. east coast were common during the 1940s through the mid 1960s. In the 1970s and 1980s, landfalls were few (Figure 32). The 1995-1999 seasons inclusive have been the five most active in the >100 year quantitative record (PTM26) . The fluctuations in activity are most pronounced for major hurricanes, the strongest 20% of hurricanes with winds >50 m s-1 that account for 80% of U.S. economic loss. They also correlate with the observed "North Atlantic Mode" of global sea-surface temperatures (PTM27) Geological cores from Florida Bay show clear indications of historical hurricanes (PTM28). "Paleotempestology" is a propitious avenue for extension of the climatological record into the past. The pressing need to understand human and economic impact of hurricanes should not obscure the role of high winds and huge influxes of fresh water on the fragile South Florida ecosystem.

A Vision of the Future: There is a reasonable expectation that the elevated level of hurricane activity that has characterized the late 1990s will continue into the second decade of the new century. The Hurricane Research Division will formalize and extend collaborations with the National Weather Service, the media, universities and other government agencies. If it is to be a viable organization, it will need to hire young PIs and secure stable funding for a staff of approximately 35. Hurricane track forecasts will continue to improve until they reach a plateau with errors about half of those achieved in the late 20th century. Hurricane intensity forecasting will come to show considerable skill, achieving accuracies of less than one Saffir-Simpson category at 48 h and beyond, contingent upon knowledge of the cyclone's track. Similarly, forecasts of seasonal activity will become more precise and useful. Knowledge of oceanic forcing and response will prove essential to these advances. Meteorological progress will be converted to neighborhood-level forecasts through quantitative understanding of the surface boundary layer and precipitation processes. Finely-tuned appreciation of hurricanes' impacts on humans and their property will make forecasts more useful. Meteorological advances notwithstanding, damage (corrected for inflation) will double about every 30 years and a few tens of U.S. citizens will die, primarily by drowning in fresh water, each year. There remains a small, but nonzero, probability that a poorly forecast hurricane striking a vulnerable shore might kill thousands.

[Horizontal Rule]

US Department of Commerce Logo Office of Oceanic and Atmospheric Research Logo Atlantic Oceanographic and Meteorological Laboratory Logo National Oceanic and Atmospheric Administration Logo US Department of Commerce National Oceanic and Atmospheric Administration Office of Oceanic and Atmospheric Research Atlantic Oceanographic and Meteorological Laboratory

  Disclaimer | Privacy Notice
  DOC/NOAA/AOML

aoml.webmaster@noaa.gov  
  

Hurricane Research Division Physical Oceangraphy Division Ocean Chemistry Division AOML Home