Subject: A1) What is a hurricane, typhoon, or tropical cyclone?
NOAA
Contributed by Chris Landsea
The terms "hurricane" and "typhoon" are regionally specific names for a strong "tropical cyclone". A tropical cyclone is the generic term for a non-frontal synoptic scale low-pressure system over tropical or sub-tropical waters with organized convection (i.e. thunderstorm activity) and definite cyclonic surface wind circulation (Holland 1993).
Tropical cyclones with maximum sustained surface winds of less than 17 m/s (34 kt, 39 mph) are called "tropical depressions" (This is not to be confused with the condition mid-latitude people get during a long, cold and grey winter wishing they could be closer to the equator ;-)). Once the tropical cyclone reaches winds of at least 17 m/s (34 kt, 39 mph) they are typically called a "tropical storm" and assigned a name. If winds reach 33 m/s (64 kt, 74 mph)), then they are called:
• "hurricane" (the North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline, or the South Pacific Ocean east of 160E)
• "typhoon" (the Northwest Pacific Ocean west of the dateline)
• "severe tropical cyclone" (the Southwest Pacific Ocean west of 160E or Southeast Indian Ocean east of 90E)
• "very severe cyclonic storm" (the North Indian Ocean)
• "tropical cyclone" (the Southwest Indian Ocean)
(Neumann 1993).
Subject: A2) What is a "Cape Verde" hurricanes? Contributed by Chris Landsea
Cape Verde-type hurricanes are those Atlantic basin tropical cyclones
that develop into tropical storms fairly close (<1000 km [600 mi.] or so)
of the Cape Verde Islands and then become hurricanes before reaching the
Caribbean. (That would be my definition, there may be others.) Typically,
this may occur in August and September, but in rare years (like 1995)
there may be some in late July and/or early October. The numbers range
from none up to around five per year - with an average of around 2.
Subject: A3) What is a super-typhoon? What is a major hurricane ? What is
an intense hurricane ?
Contributed by Stan Goldenberg
"Super-typhoon" is a term utilized by the U.S. Joint Typhoon Warning
Center for typhoons that reach maximum sustained 1-minute surface winds
of at least 65 m/s (130 kt, 150 mph). This is the equivalent of a strong
Saffir-Simpson category 4 or category 5 hurricane in the Atlantic basin
or a category 5 severe tropical cyclone in the Australian basin.
"Major hurricane" is a term utilized by the National Hurricane Center for
hurricanes that reach maximum sustained 1-minute surface winds of at
least 50 m/s (96 kt, 111 mph). This is the equivalent of category 3, 4
and 5 on the Saffir-Simpson scale.
"Intense hurricane" is an unofficial term , but is often used in the
scientific literature. It is the same as "major hurricane".
Subject: A4) What is an easterly wave ? Contributed by Chris Landsea
It has been recognized since at least the 1930s (Dunn 1940) that lower
tropospheric (from the ocean surface to about 5 km [3 mi.] with a maximum
at 3 km [2 mi.]) westward traveling disturbances often serve as the
"seedling" circulations for a large proportion of tropical cyclones over
the North Atlantic Ocean. Riehl (1945) helped to substantiate that these
disturbances, now known as African easterly waves, had their origins over
North Africa. While a variety of mechanisms for the origins of these
waves were proposed in the next few decades, it was Burpee (1972) who
documented that the waves were being generated by an instability of the
African easterly jet. (This instability - known as baroclinic-barotropic
instability - is where the value of the potential vorticity begins to
decrease toward the north.) The jet arises as a result of the reversed
lower-tropospheric temperature gradient over western and central North
Africa due to extremely warm temperatures over the Saharan Desert in
contrast with substantially cooler temperatures along the Gulf of Guinea
coast.
The waves move generally toward the west in the lower tropospheric
tradewind flow across the Atlantic Ocean. They are first seen usually in
April or May and continue until October or November. The waves have a
period of about 3 or 4 days and a wavelength of 2000 to 2500 km [1200 to
1500 mi.], typically (Burpee 1974). One should keep in mind that the
"waves" can be more correctly thought of as the convectively active
troughs along an extended wave train. On average, about 60 waves are
generated over North Africa each year, but it appears that the number
that is formed has no relationship to how much tropical cyclone activity
there is over the Atlantic each year.
While only about 60% of the Atlantic tropical storms and minor hurricanes
( Saffir-Simpson Scale categories 1 and 2) originate from easterly waves,
nearly 85% of the intense (or major) hurricanes have their origins as
easterly waves (Landsea 1993). It is suggested, though, that nearly all
of the tropical cyclones that occur in the Eastern Pacific Ocean can also
be traced back to Africa (Avila and Pasch 1995).
It is currently completely unknown how easterly waves change from year to
year in both intensity and location and how these might relate to the
activity in the Atlantic (and East Pacific).
Subject: A5) What is a tropical disturbance, a tropical depression, or a
tropical storm ?
Contributed by Chris Landsea
These are terms used to describe the progressive levels of organized
disturbed weather in the tropics that are of less than hurricane status.
Tropical Disturbance
A discrete tropical weather system of apparently organized convection -
generally 200 to 600 km (100 to 300 nmi.) in diameter - originating in
the tropics or subtropics, having a non-frontal migratory character, and
maintaining its identity for 24 hours or more. It may or may not be
associated with a detectable perturbation of the wind field. Disturbances
associated with perturbations in the wind field and progressing through
the tropics from east to west are also known as easterly waves .
Tropical Depression
A tropical cyclone in which the maximum sustained wind speed (using the
U.S. 1 minute average standard) is 33 kt (38 mph, 17 m/s). Depressions
have a closed circulation.
Tropical Storm
A tropical cyclone in which the maximum sustained surface wind speed
(using the U.S. 1 minute average standard) ranges from 34 kt (39 mph,17.5
m/s) to 63 kt (73 mph, 32.5 m/s). The convection in tropical storms is
usually more concentrated near the center with outer rainfall organizing
into distinct bands.
Hurricane
When winds in a tropical cyclone equal or exceed 64 kt (74 mph, 33 m/s)
it is called a hurricane (in the Atlantic and eastern and central Pacific
Oceans). Hurricanes are further designated by categories on the
Saffir-Simpson scale. Hurricanes in categories 3, 4, 5 are known as Major
Hurricanes or Intense Hurricanes.
The wind speed mentioned here are for those measured or estimated as the
top speed sustained for one minute at 10 meters above the surface. Peak
gusts would be on the order of 10-25% higher.
Last updated January 30, 2006
Subject: A6) What is a sub-tropical cyclone?
Contributed by Chris Landsea and Sandy Delgado.
A sub-tropical cyclone is a low-pressure
system existing in the tropical or subtropical latitudes (anywhere
from the equator to about 50°N) that has characteristics of both
tropical cyclones and mid-latitude (or extratropical) cyclones.
Therefore, many of these cyclones exist in a weak to moderate
horizontal temperature gradient region (like mid-latitude cyclones),
but also receive much of their energy from convective clouds (like
tropical cyclones). Often, these storms have a radius of
maximum winds which is farther out (on the order of 100-200 km
[60-125 miles] from the center) than what is observed for purely
"tropical" systems. Additionally, the maximum
sustained winds for sub-tropical cyclones have not been observed
to be stronger than about 33 m/s (64 kts, 74 mph)).
Many times these subtropical storms transform into true tropical
cyclones. A recent example is the Atlantic basin's Hurricane
Florence in November 1994 which began as a subtropical cyclone
before becoming fully tropical. Note there has been at least
one occurrence of tropical cyclones transforming into a
subtropical storm (e.g. Atlantic basin storm 8 in 1973).
Subtropical cyclones in the Atlantic basin are classified
by the maximum sustained surface winds:
- less than 18 m/s (34 kts, 39 mph) : "subtropical depression",
- greater than or equal to 18 m/s (34 kts, 39 mph) : "subtropical
storm"
Prior to 2002 subtropical storms were not given names, but
the Tropical Prediction Center issued forecasts and warnings
on them similar to those for tropical cyclones. Since 2003 they are
given names from the tropical cyclone list.
For more information see
http://www.e-education.psu.edu/public/meteo/subtropical_cyclones.html
Penn State University's write up on the Subtropical Cyclones.
Last Revised : March 16, 2010
Subject: A7) What is an extra-tropical cyclone ?
Contributed by Stan Goldenberg
An extra-tropical cyclone is a storm system that primarily gets its
energy from the horizontal temperature contrasts that exist in the
atmosphere. Extra-tropical cyclones (also known as mid-latitude or
baroclinic storms) are low pressure systems with associated cold fronts,
warm fronts, and occluded fronts.
Tropical cyclones, in contrast, typically have little to no temperature
differences across the storm at the surface and their winds are derived
from the release of energy due to cloud/rain formation from the warm
moist air of the tropics ( Holland 1993, Merrill 1993).
Structurally, tropical cyclones have their strongest winds near the
earth's surface , while extra-tropical cyclones have their strongest
winds near the tropopause - about 8 miles (12 km) up. These differences
are due to the tropical cyclone being "warm-core" in the troposphere
(below the tropopause) and the extra-tropical cyclone being "warm-core"
in the stratosphere (above the tropopause) and "cold-core" in the
troposphere. "Warm-core" refers to being relatively warmer than the
environment at the same pressure surface ("pressure surfaces" are simply
another way to measure height or altitude).
Often, a tropical cyclone will transform into an extra-tropical cyclone
as it recurves poleward and to the east. Occassionally, an extra-tropical
cyclone will lose its frontal features, develop convection near the
center of the storm and transform into a full-fledged tropical cyclone.
Such a process is most common in the North Atlantic and Northwest Pacific
basins. The transformation of tropical cyclone into an extra-tropical
cyclone (and vice versa) is currently one of the most challenging
forecast problems (i.e., Jones et al. 2003).
References:
Jones, S.C., Harr, P.A., Abraham, J., Bosart, L.F., Bowyer, P.J., Evans,
J.L., Hanley, D.E., Hanstrum, B.N., Hart, R.E., Lalaurette, F., Sinclair,
M.R., Smith, R.K., Thorncroft, C. 2003: The Extratropical Transition of
Tropical Cyclones: Forecast Challenges, Current Understanding, and Future
Directions. Weather and Forecasting, 18, 1052-1092.
Merrill, R. T., (1993): "Tropical Cyclone Structure" - Chapter 2, Global
Guide to Tropical Cyclone Forecasting, WMO/TC-No. 560, Report No. TCP-31,
World Meteorological Organization; Geneva, Switzerland
Web version of Guide
Last updated August 13, 2004
Subject: A8) What is storm surge and how is it different from storm tide ?
Contributed by NHC Storm Surge Unit
Storm surge is an abnormal rise of water generated by a storm, over
and above the predicted astronomical tide.
Storm Tide is the water level rise during a storm due to the
combination of storm surge and the astronomical tide.
Storm surge inundation refers to the storm surge as height
above ground level. For the (see F7.html) SLOSH model, this is done
by subtracting the average elevation of each grid cell from the water level
computed by the model referenced to a vertical datum. This helps alleviate
confusion inherent in past use of of surge above an abstract geophysical
reference level.
Last Updated : May 14, 2010
Subject: A9) What is a "CDO" ? NRL - Monterey Contributed by Chris Landsea
"CDO" is an acronym that stands for "central dense overcast". This is the
cirrus cloud shield that results from the thunderstorms in the eyewall of
a tropical cyclone and its rainbands. Before the tropical cyclone reaches
hurricane strength (33 m/s, 64 kts, 74 mph), typically the CDO is
uniformly showing the cold cloud tops of the cirrus with no eye apparent.
Once the storm reaches the hurricane strength threshold, usually an eye
can be seen in either the infrared or visible channels of the satellites.
Tropical cyclones that have nearly circular CDO's are indicative of
favorable, low vertical shear environments.
Subject: A10) What is a "TUTT"?
Fitzpatrick et al. 1995 Contributed by Chris Landsea
A "TUTT" is a Tropical Upper Tropospheric Trough. A TUTT low is a TUTT
that has completely cut-off. TUTT lows are more commonly known in the
Western Hemisphere as an "upper cold low". TUTTs are different than
mid-latitude troughs in that they are maintained by subsidence warming
near the tropopause which balances radiational cooling. TUTTs are
important for tropical cyclone forecasting as they can force large
amounts of vertical wind shear over tropical disturbances and tropical
cyclones which may inhibit their strengthening. There are also
suggestions that TUTTs can assist tropical cyclone genesis and
intensification by providing additional forced ascent near the storm
center and/or by allowing for an efficient outflow channel in the upper
troposphere. For a more detailed discussion on TUTTs see the article by
Fitzpatrick et al. (1995).
Subject: A11) What is the "eye"? How is it formed and maintained ? What is the "eyewall"? What are "spiral bands"?
(Written with major assistance by Chris Landsea)
The "eye" is a roughly circular area of comparatively light winds and fair weather found at the center of a severe tropical cyclone. Although the winds are calm at the axis of rotation, strong winds may extend well into the eye. There is little or no precipitation and sometimes blue sky or stars can be seen. The eye is the region of lowest surface pressure and warmest temperatures aloft - the eye temperature may be 10°C [18°F] warmer or more at an altitude of 12 km [8 mi] than the surrounding environment, but only 0-2°C [0-3°F] warmer at the surface (Hawkins and Rubsam 1968) in the tropical cyclone. Eyes range in size from 8 km [5 mi] to over 200 km [120 mi] across, but most are approximately 30-60 km [20-40 mi] in diameter (Weatherford and Gray 1988).
The eye is surrounded by the "eyewall", the roughly circular ring of deep convection which is the area of highest surface winds in the tropical cyclone. The eye is composed of air that is slowly sinking and the eyewall has a net upward flow as a result of many moderate - occasionally strong - updrafts and downdrafts. The eye's warm temperatures are due to compressional warming of the subsiding air. Most soundings taken within the eye show a low-level layer which is relatively moist, with an inversion above - suggesting that the sinking in the eye typically does not reach the ocean surface, but instead only gets to around 1-3 km [ 1-2 mi] of the surface.
The exact mechanism by which the eye forms remains somewhat controversial. One idea suggests that the eye forms as a result of the downward directed pressure gradient associated with the weakening and radial spreading of the tangential wind field with height (Smith, 1980). Another hypothesis suggests that the eye is formed when latent heat release in the eyewall occurs, forcing subsidence in the storm's center (Shapiro and Willoughby, 1982). It is possible that these hypotheses are not inconsistent with one another. In either case, as the air subsides, it is compressed and warms relative to air at the same level outside the eye and thereby becomes locally buoyant. This upward buoyancy approximately balances the downward directed pressure gradient so that the actual subsidence is produced by a small residual force.
Another feature of tropical cyclones that probably plays a role in forming and maintaining the eye is the eyewall convection. Convection in tropical cyclones is organized into long, narrow rainbands which are oriented in the same direction as the horizontal wind. Because these bands seem to spiral into the center of a tropical cyclone, they are sometimes called "spiral bands". Along these bands, low-level convergence is a maximum, and therefore, upper-level divergence is most pronounced above. A direct circulation develops in which warm, moist air converges at the surface, ascends through these bands, diverges aloft, and descends on both sides of the bands. Subsidence is distributed over a wide area on the outside of the rainband but is concentrated in the small inside area. As the air subsides, adiabatic warming takes place, and the air dries. Because subsidence is concentrated on the inside of the band, the adiabatic warming is stronger inward from the band causing a sharp contrast in pressure falls across the band since warm air is lighter than cold air. Because of the pressure falls on the inside, the tangential winds around the tropical cyclone increase due to increased pressure gradient. Eventually, the band moves toward the center and encircles it and the eye and eyewall form (Willoughby 1979, 1990a, 1995).
Thus the cloud-free eye may be due to a combination of dynamically forced centrifuging of mass out of the eye into the eyewall and to a forced descent caused by the moist convection of the eyewall. This topic is certainly one that can use more research to ascertain which mechanism is primary.
Some of the most intense tropical cyclones exhibit concentric eyewalls, two or more eyewall structures centered at the circulation center of the storm ( Willoughby et al. 1982,Willoughby 1990a ). Just as the inner eyewall forms, convection surrounding the eyewall can become organized into distinct rings. Eventually, the inner eye begins to feel the effects of the subsidence resulting from the outer eyewall, and the inner eyewall weakens, to be replaced by the outer eyewall. The pressure rises due to the destruction of the inner eyewall are usually more rapid than the pressure falls due to the intensification of the outer eyewall, and the cyclone itself weakens for a short period of time.
Last updated May 20, 2011
Subject: A12) What is a moat in a hurricane ?
Contributed by Frank Marks
The term "moat" usually refers to the region between the eyewall and an
outer rainband, such as a secondary eyewall rainband. The moat is the
relatively light rain region between the rainband and the eyewall.
Subject: A13) What is UTC ? How do I tell at what time a satellite
picture was taken ?
Contributed by Neal Dorst
UTC stands for Universal Time Coordinated, what used to be called
Greenwich Mean Time (GMT) and Zulu Time (Z). This is the time at the
Prime Meridian (0¡ Longitude) given in hours and minutes on a 24 hour
clock. For example, 1350 UTC is 13 hours and 50 minutes after midnight or
1:50 PM at the Prime Meridian.
The Greenwich Royal Observatory at Greenwich, England (at 0¡ Longitude)
was where naval chronometers (clocks) were set, a critical instrument for
calculating longitude. This is why GMT became the standard for world
time. Meteorologists have used UTC or GMT times for over a century to
ensure that observations taken around the globe are taken simultaneously.
On most satellite pictures and radar images the time will be given. If
it's not in local time then it will usually be given as UTC, GMT, or Z time.
To convert this to your local time it is necessary to subtract the
appropriate number of hours for the Western Hemisphere or add the correct
number of hours for the Eastern Hemisphere. And don't forget the extra
hour adjustment for Daylight Savings Time or Winter Time over Standard
Time for your zone.
Local Time Zone Time Adjustment
(hours)
Atlantic Daylight Time (ADT) -3
Atlantic Standard Time (AST)
Eastern Daylight Time (EDT) -4
Eastern Standard Time (EST)
Central Daylight Time (CDT) -5
Central Standard Time (CST)
Mountain Daylight Time (MDT) -6
Mountain Standard Time (MST)
Pacific Daylight Time (PDT) -7
Pacific Standard Time (PST)
Alaskan Daylight Time (ADT) -8
Alaskan Standard Time (ASA) -9
Hawaiian Standard Time (HAW) -10
New Zealand Standard Time (NZT)
International Date Line Time (IDLE) +12
Guam Standard Time (GST)
Eastern Australian Standard Time (EAST) +10
Japan Standard Time (JST) +9
China Coast Time (CCT) +8
West Australia Standard Time (WAST) +7
Russian Time Zone 5 (ZP5) +6
Russian Time Zone 4 (ZP4) +5
Russian Time Zone 3 (ZP3) +4
Bagdad Time (BT)
Russian Time Zone 2(ZP2) +3
Eastern European Time (EET)
Russian Time Zone 1(ZP1) +2
Central European Time (CET)
French Winter Time (FWT)
Middle European Time (MET)
Swedish Winter Time (SWT)
Middle European Winter Time (MEWT)
+1
Western European Time (WET)
Greenwich Mean Time (GMT) 0
Last updated August 13, 2004
Subject: A14) How do I convert from mph to knots (or m/s), from inches of
mercury to mb (or hPa), or convert degrees of latitude to miles (or
kilometers) ? Contributed by Neal Dorst
For winds:
1 mile per hour = 0.869 international nautical mile per hour (knot)
1 mile per hour = 1.609 kilometers per hour
1 mile per hour = 0.4470 meter per second
1 knot = 1.852 kilometers per hour
1 knot = 0.5144 meter per second
1 meter per second = 3.6 kilometers per hour
For pressures:
1 inch of mercury = 25.4 mm of mercury = 33.86 millibars
= 33.86 hectoPascals
For distances:
1 foot = 0.3048 meter
1 international nautical mile = 1.1508 statute miles
= 1.852 kilometers = .99933 U.S nautical mile (obsolete)
1¡ latitude = 69.047 statute miles = 60 nautical miles
= 111.12 kilometers
For longitude the conversion is the same as latitude except the value is
multiplied by the cosine of the latitude.
Subject: A15) How do tropical cyclones form ?
Contributed by Chris Landsea
To undergo tropical cyclogenesis, there are several favorable pre cursor
environmental conditions that must be in place (Gray 1968,1979) :
Warm ocean waters (of at least 26.5 degC [80 degF]) throughout a
sufficient
depth (unknown how deep, but at least on the order of 50 m [150 ft]).
Warm waters are necessary to fuel the heat engine of the tropical
cyclone.
An atmosphere which cools fast enough with height such that it is
potentially unstable to moist convection. It is the thunderstorm activity
which allows the heat stored in the ocean waters to be liberated for the
tropical cyclone development.
Relatively moist layers near the mid-troposphere (5 km [3 mi.]). Dry mid
levels are not conducive for allowing the continuing development of
widespread thunderstorm activity.
A minimum distance of at least 500 km [300 mi.] from the equator. For
tropical cyclogenesis to occur, there is a requirement for non-negligible
amounts of the Coriolis force to provide for near gradient wind balance
to occur. Without the Coriolis force, the low pressure of the disturbance
cannot be maintained.
A pre-existing near-surface disturbance with sufficient vorticity and
convergence. Tropical cyclones cannot be generated spontaneously. To
develop, they require a weakly organized system with sizable spin and low
level inflow.
Low values (less than about 10 m/s [20 kts 23 mph]) of vertical wind
shear between the surface and the upper troposphere. Vertical wind shear
is the magnitude of wind change with height. Large values of vertical
wind shear disrupt the incipient tropical cyclone and can prevent
genesis, or, if a tropical cyclone has already formed, large vertical
shear can weaken or destroy the tropical cyclone by interfering with the
organization of deep convection around the cyclone center.
Having these conditions met is necessary, but not sufficient as many
disturbances that appear to have favorable conditions do not develop.
Recent work (Velasco and Fritsch 1987, Chen and Frank 1993, Emanuel 1993)
has identified that large thunderstorm systems (called mesoscale
convective complexes [MCC]) often produce an inertially stable, warm core
vortex in the trailing altostratus decks of the MCC. These mesovortices
have a horizontal scale of approximately 100 to 200 km [75 to 150 mi.],
are strongest in the mid-troposphere (5 km [3 mi.]) and have no
appreciable signature at the surface. Zehr (1992) hypothesizes that
genesis of the tropical cyclones occurs in two stages:
stage 1 occurs when the MCC produces a mesoscale vortex.
stage 2 occurs when a second blow up of convection at the mesoscale
vortex initiates the intensification process of lowering central
pressure and increasing swirling winds.
Subject: A16) Why do tropical cyclones require 80¡F (26.5¡C) ocean
temperatures to form ?
Contributed by Chris Landsea
Tropical cyclones can be thought of as engines that require warm, moist
air as fuel (Emanuel 1987). This warm, moist air cools as it rises in
convective clouds (thunderstorms) in the rainbands and eyewall of the
hurricane The water vapor in the cloud condenses into water droplets
releasing the latent heat which originally evaporated the water. This
latent heat provides the energy to drive the tropical cyclone
circulation, though actually very little of the heat released is utilized
by the storm to lower its surface pressure and increase the wind speeds.
In 1948 Erik Palmen observed that tropical cyclones required ocean
temperatures of at least 80¡F (26.5¡C) for their formation and growth.
Later work (e.g., Gray 1979) also pointed out the need for this warm
water to be present through a relatively deep layer (~150 ft, 50 m) of
the ocean. This 80¡F value is tied to the instability of the atmosphere
in the tropical and subtropical latitudes. Above this temperature deep
convection can occur, but below this value the atmosphere is too stable
and little to no thunderstorm activity can be found ( Graham and Barnett
1987).
See Question G3 for how this value might change if a significant global
warming occurs.
References:
Graham, N. E., and T. P. Barnett, 1987: Sea surface temperature, surface
wind divergence, and convection over tropical oceans. Science, No.238,
pp. 657-659.
Gray, W.M. 1979 : "Hurricanes: Their formation, structure and likely role
in the tropical circulation" Meteorology Over Tropical Oceans. D. B. Shaw
(Ed.), Roy. Meteor. Soc., James Glaisher House, Grenville Place,
Bracknell, Berkshire, RG12 1BX, pp. 155-218
Palmen, E. H., 1948: On the formation and structure of tropical cyclones.
Geophysica , Univ. of Helsinki, Vol. 3, 1948, pp. 26-38.
Last updated August 13, 2004
Subject: A17) What is the Saharan Air Layer (SAL) ?
How does it affect tropical cyclones ?
Contributed by Jason Dunion
The Saharan Air Layer (SAL) is a mass of very dry, dusty air which forms over
the Sahara Desert during the late spring, summer, and early fall and usually
moves out over the tropical North Atlantic Ocean every 3-5 days. The SAL
extends between ~5,000-20,000 ft (~1500-6000 m) in the atmosphere and is
associated with large amounts of mineral dust, dry air (~50% less moisture
than a typical moist tropical sounding), and strong winds (~25-55 mph or
~10-25 m/s). These strong winds, or jets, are usually found between
6,500-14,500 ft (2000-4500 m) above the surface in the central and western
North Atlantic and have a depth of ~1-2 miles (~1.6-3.2 km).
The SAL can have a significant negative impact on tropical cyclone intensity
and formation. Its dry air can act to weaken a tropical cyclone by promoting
downdrafts around the storm, while its strong winds can substantially increase
the vertical wind shear in and around the storm environment. It is not yet
clear what effect the SAL's dust has on tropical cyclone intensity, though some
recent studies have suggested that it can actually impact the formation of
clouds.
The SAL can cover an area the size of the continental U.S. and has been tracked
as far west as the Caribbean Sea, Central America, and the Gulf of Mexico.
Real-time satellite imagery for tracking the SAL can be found at
http://cimss.ssec.wisc.edu/tropic2/real-time/salmain.php?&prod=splitEW&time
References:
Dunion, J.P., and C.S. Velden, 2004: The impact of the Saharan Air
Layer on Atlantic tropical cyclone activity. Bull. Amer. Meteor.
Soc. vol. 85 no. 3, 353-365.
Last updated March 17, 2010
Subject: A18) What is a neutercane?
Contributed by Neal Dorst
A neutercane is a small (meso-)scale (< 100 miles in diameter)
low-pressure system that has characteristics of both tropical cyclone and
mid-latitude or extratropical cyclone. A subclass of sub-tropical
cyclone, neutercanes are distinguished by their small size and their
origination, sometimes forming within mesoscale convective complexes.
The term was coined by Robert Bundgaard, after he participated in a
research flight in the early 1970's. He witnessed a small cyclonic
circulation over land, which appeared to have both tropical and
extratropical characteristics. He used the term in later discussions with
Dr. Bob Simpson, then director of the OAR. 'Neutercane' was meant to
synthesize the word 'neutral' and 'hurricane' to imply a hurricane-like
vortex which was midway between tropical and extratropical.
Dr. Simpson observed similar circulations on geostationary satellite
loops, and conducted an investigation with hurricane specialist Banner
Miller. He presented a talk on them at the 8th AMS Conference on
Hurricane and Tropical Meteorology in 1973. During the 1972 hurricane
season, Simpson inaugurated use of the term in official bulletins,
labeling the second (Bravo) and third (Charlie) subtropical cyclones
observed that year as Neutercanes. (Neutercane Bravo transformed into
Hurricane Betty.) However, objections in the press to the term as
possibly sexist led to NOAA management discouraging use of the term, and
ordering Simpson to cease use of any further Government resources in
conducting research on the phenomenon.
From then on, the term "Sub-tropical Cyclone" was used for all such
systems. However, the term entered into several dictionaries, including
the AMS Glossary of Meteorology (which misidentifies them as "large"),
and has been used in the scientific literature.
References
Bull. Amer. Met. Soc., Feb. 1973, Vo. 54 No. 2, p. 153
Glossary of Meteorology 2nd edition, 2000, (AMS, Boston), p. 522
Weatherwise, Sept.-Oct. 2005, Vo. 58 No. 5, p. 60
Revised May 21, 2007
Subject: A19) What does ATCF stand for and how are tropical cyclones
numbered?
Contributed by Neal Dorst
The Automated Tropical Cyclone Forecast (ATCF) system was developed for
the Joint Typhoon Warning Center in 1988. This is a software package used
to plot tropical cyclone information and assist in the generation of forecast
messages. In order to distinguish different tropical cyclones that might be
occurring simultaneously, a distinct alphanumeric code is assigned to each
cyclone as it develops. This code system was adopted by other warning
centers in order to facilitate the passing of storm information and reduce
confusion.
The code designation consists of two letters designating the oceanic basin
("AL" for Atlantic, "EP" for Eastern Pacific, "CP" for Central Pacific
and "WP" for Western Pacific), a two-digit number designating the sequential
number of that particular cyclone for that basin in the year, and lastly
a four-digit year number. So the first depression to form in the Atlantic
for 2001 would be AL012001, the third depression for the Central Pacific
in 1999 would be CP031999.
A cyclone retains its ATCF code designation as long as it remains a
distinct tropical vortex. Even if it becomes a named Tropical Storm or
Hurricane the software will still track it by its ATCF code.
References
Miller, R.J., J. Schrader, C.R. Sampson, and T.L. Tsui, 1990, "The Automated
Tropical Cyclone Forecasting Sytem (ATCF)",Weather and Forecasting,
Vol. 5, (Dec. 1990), p. 653-660
Revised August 14, 2009
Subject: A20) What does AL90, AL91, or 92L refer to in the tropical
discussions?
Contributed by Neal Dorst
Oftentimes, hurricane specialists become curious about disturbances in the
tropics long before they form into tropical depressions and are given a
tropical cyclone number (see A19). In order to alert forecasting centers that
they are investigating such a disturbance and that they wish to have it tracked
by the various forecast models, the specialist will attach a 9-series number
to it. The first such disturbance of the year will be designated 90, the next
91, and so on until 99. After that, they restart the sequence with 90 again.
The purpose of these numbers is to clarify which disturbance they are tracking
as there are often more than one happening at the same time.
To further clarify matters, each number is accompanied by a two-letter code
designating which tropical cyclone basin the disturbance is in. "AL" is
used for the Atlantic basin (including the Caribbean Sea and Gulf of Mexico),
"EP" for the Eastern Pacific, "CP" for Central Pacific, and "WP" for the
Western Pacific.
Many times in discussions these designations will be shortened to 90L, 91L,
and so forth. Also they may be referred to as 'Invest 90L'. However, once
a disturbance is designated a tropical depression this 9-series number will
be dropped and an ATCF code number (see A19) will be assigned in its place.
You may also occasionally see an 8-series number, such as AL82. This means
that this is a test investigation. There is no particular disturbance that
the specialists are interested in, they're just running a test of the system
to make sure communications and software are running properly.
Revised August 14, 2009