Potter T.D., Colman B.R. (co-chief editors). The handbook of weather, climate, and water: dynamics, climate physical meteorology, weather systems, and measurements
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Mohr and Zipser (1996a,b) mapped MCSs by size and by ‘‘intensity,’’ defined by
the minimum brightness temperature (i.e., by the pixel with largest optical depth of
ice in a given MCS). They are mapped by season (Fig. 6) and by sunrise vs sunset
observation time (not shown). The resulting maps give some additional insi ghts into
the properties of precipitating systems in different p arts of the world. Unlike MCCs,
Figure 6 January, April, July, and October, 1993, top to bottom distribution of MCSs
according to minimum (polarization-corrected) brightness temperatures. Symbols increase in
size and thickness with decreasing temperature, which represents more intense convective
pixels. The coldest range, <120 K, has the largest and thickest cross (after Mohr and Zipser,
1996a).
636 TROPICAL PRECIPITATING SYSTEMS
which are nearly absent in tropical oceans, the high rainfall regions of the oceans
have abundant (ice-scattering) MCSs, including numerous MCSs with large areas.
The MCSs with large (ice-scattering) intensity, however, favor land areas, especially
at sunset, a time when growth of MCSs from intense thunderstorms is common.
Oceanic regions have more MCSs at sunrise than at sunset.
Using Lightning Measurements
Satellite-derived maps of lightning discharges have been used increasingly in
studies of global distributions of thunderstorms. Since 1995 such data have been
available from a near-polar orbit, and since 1997 on TRMM. Research using these
data is relatively new, and comparative studies of lightning and other databases
only beginning. The distribution of lightning is even more skewed than the distri-
bution of rainfall, with a relatively small number of storms producing a dispropor-
tionate fraction of total lightning. Lightning mapping clearly demonstrates a
scarcity of lightning over tropical oceans, resembling the MCC distributions to
some extent (Fig. 7). Comparing the lightning, MCS, and MCC distributions, one
could hypothesize that the lightning distribution faithfully represents the distribu-
tion of strong MCSs and MCCs, which are common over continents and rare over
oceans. Recent research that attempts to test this hypothesis indicates that it fails.
That is, MCSs over land and ocean with similar brightness temperatures and
Figure 7 Climatological lightning imaging sensor (LIS instrument on TRMM) flash rate
density, Dec. 1997–Nov. 1999. Data have been normalized by sensor view time and an
assumed detection efficiency of 75% (after Boccippio et al., 2000).
7 DISTRIBUTION OF TROPICAL=SUBTROPICAL RAINFALL 637
similar radar echoes (from TRMM) still have higher lightning frequency over land,
for reasons that are under investigation but must involve details of the microphy-
sics.
REFERENCES
Adler, R. F., G. J. Huffman, D. V. Bolvin, S. Curtis, and E. J. Nelkin (2000). Tropical rainfall
distributions deter mined using TRMM combined with other satellite and rain gauge
information, J. Appl. Meteor. 39, 2007–2023.
Barnes, G. M. (2001). Severe weather in the tropics, in Severe Convective Storms, Meteor.
Monographs, 28(50), C. Doswell (Ed.)., Am. Meteor. Soc., Boston MA.
Boccippio, D. J., S. J. Goodman, and S. Heckman (2000). Regional differences in tropical
lightning distributions, J. Appl. Meteor. 39, 2231–2248.
Chappell, C. F. (1986). Quasi-stationary convective events. Mesoscale analysis and forecasting,
P. S. Ray (Ed.), Am. Meteor. Soc. 289–310.
Godfrey, J. S., R. A. Houze, Jr., R. H. Johnson, R. Lukas, J.-L. Redelsberger, A. Sumi, and
R. Weller (1998). Coupled Ocean–Atmosphere Response Experiment: An interim report,
J. Geophys. Res. 103(C7), 14,395–14,450.
Houze, R. A., Jr. (1993). Cloud Dynamics, Academic Press, San Diego, CA.
Laing, A. G., and J. M. Fritsch (1997). The global population of mesoscale convective
complexes, Quart. J. Roy. Meteor. Soc. 123, 389–405.
Laing, A. G., and J. M. Fritsch (2000). The large-scale environments of the global populations
of mesoscale convective complexes, Mon. Wea. Rev. 128, 2756–2776.
LeMone, M. A., E. J. Zipser, and S. B. Trier (1998). The role of environmental shear and CAPE
in determining the structure and evolution of mesoscale convective systems during TOGA
COARE, J. Atmos. Sci. 55, 3493–3518.
Madden, R. A., and P. R. Julian (1972). Description of global-scale circulation cells in the
Tropics with a 40–50 day period, J. Atmos. Sci. 29, 1109–1123.
Madden, R. A., and P. R. Julian (1994). Observations of the 40–50 day tropical oscillation—A
review. Mon. Wea. Rev., 122, 814–837.
McCollum, J. R., A. Gruber, and M. B. Ba (2000). Discrepancy between gauges and satellite
estimates of rainfall in equatorial Africa, J. Appl. Meteor. 39, 666–679.
Mohr, K. I., and E. J. Zipser (1996a). Defining mesoscale convective systems by their ice
scattering signature, Bull. Am. Meteor. Soc. 77, 1179–1189.
Mohr, K. I., and E. J. Zipser (1996b). Mesoscale convective systems defined by their 85 GHz
ice scattering signature: Size and intensity comparison over tropical oceans and continents,
Mon. Wea. Rev. 124, 2417–2437.
Reed, R. J., D. C. Norquist, and E. E. Recker (1977). The Structure and Properties of African
Wave Disturbances as Observed During Phase III of GATE, Mon. Wea. Rev. 105, 317–333.
Riehl, H., and J. S. Malkus (1958). On the heat balance in the equatorial trough zone,
Geophysica 6, 503–538.
Rutledge, S. A., E. R. Williams, and T. D. Keenan (1992). The Down Under Doppler and
Electricity Experiment (DUNDEE): Overview and preliminary results, Bull. Am. Meteor.
Soc. 73, 3–16.
638
TROPICAL PRECIPITATING SYSTEMS
Zipser, E. J. (1977). Mesoscale and convective-scale downdrafts as distinct components of
squall-line structure, Mon. Wea. Rev. 105, 1568–1589.
Zipser, E. J. (1994). Deep cumulonimbus cloud systems in the tropics with and without
lightning, Mon. Wea. Rev. 122, 1837–1851.
Zipser, E. J., and K. Lutz (1994). The vertical profile of radar reflectivity of convective cells:
A strong indicator of storm intensity and lightning probability? Mon. Wea. Rev. 122,
1751–1759.
REFERENCES 639
CHAPTER 31
HURRICANES
FRANK D. MARKS, Jr.
1 INTRODUCTION
The term hurricane is used in the Western Hemisphere for the general class of strong
tropical cyclones, including western Pacific typhoons and similar systems, known
simply as cyclones in the Indian and southern Pacific Oceans. A tropical cyclone is a
low-pressure system that derives its energy primarily from evaporation from the sea
in the presence of 1-min sustained surfa ce wind speeds > 17 m=s and the associated
condensation in convective clouds concentrated near its center. In contrast, midlati-
tude storms (low-pressure systems with associated fronts) primarily get their energy
from the horizontal temperature gradients that exist in the atmosphere. Structurally,
the strongest winds in tropical cyclones are near Earth’s surface (a consequence of
being ‘‘warm core’’ in the troposphere), while the strongest winds in midlatitude
storms are near the tropopause (a consequence of being ‘‘warm core’’ in the strato-
sphere and ‘‘cold core’’ in the troposphere). Warm core refers to being warmer than
the environment at the same pressure surface.
A tropical cyclone with the highest sustained wind speeds between 17 and 32 m=s
is referred to as a tropical storm, whereas a tropical cyclone with sustained wind
speeds 33 m=s is referred to as a hurricane or typhoon. Once a tropical cyclone has
sustained winds 50 m=s, it is referred to as a major hurricane or supertyphoon. In
the Atlantic and eastern Pacific Oceans hurricanes are also classified by the damage
they ca n cause using the Saffir–Simpson scale (Table 1).
The Saffir–Simpson scale categorizes hurricanes on a scale from 1 to 5, where
category 1 hurricanes are the weakest, and category 5 the most intense. Major
hurricanes correspond to categories 3 and higher. The reasons that some distur-
bances intensify to a hurricane, while others do not, are not well understood. Neither
Handbook of Weather, Climate, and Water: Dynamics, Climate, Physical Meteorology, Weather Systems,
and Measurements, Edited by Thomas D. Potter and Bradley R. Colman.
ISBN 0-471-21490-6 # 2003 John Wiley & Sons, Inc.
641
is it clear why some tropical cyclones become major hurricanes, while others do not.
Major hurricanes produce 80 to 90% of the U.S. hurricane-caused damages despite
accounting for only one-fifth of all landfalling tropical cyclones. Only three category
5 hurricanes have made landfall on the U.S. mainland (Florida Keys, 1935, Camille,
1969, and Andrew, 1992). Recent major hurricanes to make landfall on the United
States were Hurricanes Bonnie and Georges in 1998, and Bret and Floyd in 1999.
As with large-scale extratropical weather systems, the structur e and evolution of a
tropical cyclone is dominated by the fundamental contradiction that while the airflow
within a tropical cyclone represents an approximate balance among forces affecting
each air parcel, slight departures from balance are essential for vertical motions and
resulting clouds and precipitation, as well as changes in tropical cyclone intensity.
As in extratropical weather systems, the basic vertical balance of forces in a tropical
cyclone is hydrostatic except in the eyewall, where convection is superimposed on
the hydrostatic motions. However, unlike in extratropical weather systems, the basic
horizontal balance in a tropical cyclone above the boundary layer is between the
Coriolis force (defined as the horizontal velocity, v, times the Coriolis parameter,* f ),
the centrifugal force (defined as v
2
divided by the radius from the center, r), and the
horizontal pressure gradient force. This balance is referred to as g radient balance,
where the Coriolis and centrifugal force are both proportional to the wind speed.
Centrifugal force is an apparent force that pushes objects away from the center of a
circle. The centrifugal force alters the original two-force geostrophic balance and
creates a nongeostrophic gradient wind.
The inner region of the tropical storm, termed the cyclone core, contains the
spiral bands of precipitation, the eyewall, and the eye that characterize tropical
cyclones in radar and satellite imagery (Fig. 1). The primary circulation—the
tangential or swirling wind—in the core becomes strongly axisymmetric as the
cyclone matures. The strong winds in the core, which occupies only 1 to 4% of
the cyclone’s area, threaten human activities and make the cyclone’s dynamics
unique. In the core, the local Rossby number
{
is always >1 and may be as high
TABLE 1 Saffir–Simpson Scale of Hurricane Intensity
Category Pressure (hPa) Wind (m=s) Storm Surge (m) Damage
1 > 980 33–42 1.0–1.7 Minimal
2 979–965 43–49 1.8–2.6 Moderate
3 964–945 50–58 2.7–3.8 Extensive
4 944–920 59–69 3.9–5.6 Extreme
5 < 920 70 5.7 Catastrophic
*f is the Coriolis parameter ( f ¼2O sin f), where O is the angular velocity of Earth (7.292 10
5
s
1
)
and f is latitude. The Coriolis parameter is zero at the equator and 2O at the pole.
{
The Rossby number indicates the relative magnitude of centrifugal (v
2
) and Coriolis ( fv) accelerations,
Ro ¼V=fr, where V is the axial wind velocity, r the radius from the storm center, and f the Coriolis
parameter. An approximate breakdown of regimes is: Ro < 1 geostrophic flow; Ro > 1 gradient flow; and
Ro > 50 cyclostrophic flow.
642 HURRICANES
as 100. When the Rossby number significantly exceeds unity, the balance in the core
becomes more cyclostrophic, where the pressure gradient force is almost completely
balanced by the centrifugal force. The time scales are such that air swirling around
the center completes an orbit in much less than a pendulum day (defined as 1=f ).
When the atmosphere is in approximate horizontal and vertical balance, the wind
and mass fields are tightly interconnected. The distribution of a single mass or
momentum variable may be used as a starting point to infer the distribution of all
other such variables. One such variable is potential vorticity (PV), approximately
equal to the vorticity times the thermal stratifica tion, which is related to the three-
dimensional mass and momentum fields through an inverse second-order Laplacian-
like operator. The benefit of such a relationship is that PV variations in a single
Figure 1 (see color insert) NOAA-14 AVHRR multispectral false color image of Hurricane
Floyd at 2041 UTC, September 13 about 800 km east of south Florida. (Photo courtesy of
NOAA Operationally Significant Event Imagery website: http:==www.osei.noaa.gov=.) See ftp
site for color image.
1 INTRODUCTION 643
location are diagnostically related to variations in mass and wind fields at a distance.
Areas of high PV correspond locally to low mass, or cyclones, while areas of low PV
correspond to anticyclones.
Typical extratropical weather systems contain high PV values around
0.5 10
6
m
2
=sK=kg (0.5 PVU) to 5 PVU, whereas, typi cal values in the tropical
cyclone core are 10 PVU. Figure 2 shows the wind and mass fields associated with
an idealized axially symmetric tropical cyclone PV anomaly with the PV concen-
trated near the surface rather than in a vertical column. The cyclonic anomaly
(positive in the Northern Hemisphere) is associated with a cyclonic circulation
that is strongest at the level of the PV anomaly near the surface, and decreas es
upward. Temperatures are anomalously warm above the PV anomaly (isentropic
surfaces are deflected downward). While consistent with the simple PV distribution,
the wind and mass fields are also in horizontal and vertical balance. The tropical
cyclone being a warm-core vortex, the PV inversion dictates that the winds that swirl
about the center decrease with increasing height, but they typically fill the depth of
Figure 2 Gradient wind v (m=s) and perturbation potential temperature y
0
(K, top panel);
and geopotential=height perturbation h
0
(dm) and relative vorticity scale by Coriolis parameter
z=f (bottom panel) for a warm core, lower cyclone. The tropopause location is denoted by the
bold solid line, and the label 0 on the horizontal axis indicates the core (and axis of symmetry)
of the disturbance. The equivalent pressure deviation at the surface in the center of the vortex
is 31 hPa. [From A. J. Thorpe, Mon. Wea. Rev. 114, 1384–1389 (1986). Copyright owned by
the American Meteorological Society.]
644
HURRICANES
the troposphere. If the PV reaches values 10 PVU, the inner region winds can
become intense as in Hurricane Gloria in Figure 3. Gloria had PV values exceeding
60 PVU just inside the radius of maximum winds of 15 km where the axisymmetric
mean tangential winds exceeded 65 m=s.
Many features in the core, however, persist with little change for (pendulum) days
(mean life span of a tropical cyclone is about 5 to 10 days). Because these long
lifetimes represent tens or hundreds of orbital periods (1 h), the flow is nearly
balanced. Moreover, at winds >35 m=s, the local Rossby radius of deformation*
is reduced from its normal 10
3
km to a value comparable with the eye radius. In
very intense tropical cyclones, the eye radius may approach the depth of the tropo -
sphere (15 km), making the aspect ratio unity. Thus, the dynamics near the center of
a tropical cyclone are so exotic that conditions in the core differ from Earth’s day-to-
day weather as much as the atmosphere of another planet does.
Figure 3 Radial-height cross section of symmetric PV for Hurricane Gloria, September 24,
1985. Contours are 0.1 PVU. Values in data sparse region, within 13 km of vortex center, are
not displayed. [From L. J. Shapiro, and J. L. Franklin, Mon. Wea. Rev. 123, 1465–1475,
(1995). Copyright owned by American Meteorological Society.]
*The Rossby radius of deformation is the ratio of speed of the relevant gravity wave mode and the local
vorticity, or, equivalently, the ratios of the Brunt–Vaisala and inertial frequencies. This scale indicates the
amount of energy that goes into gravity waves compared to inertial acceleration of the wind.
1 INTRODUCTION 645