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.