Subject: H1) What is the Dvorak technique and how is it used?
Contributed by Chris Landsea

The Dvorak technique is a methodology to get estimates of tropical 
cyclone intensity from satellite pictures. Vern Dvorak developed the 
scheme using a pattern recognition decision tree in the early 1970s 
(Dvorak 1975, 1984) .

Utilizing the current satellite picture of a tropical cyclone, one 
matches the image versus a number of possible pattern types: Curved band 
Pattern, Shear Pattern, Eye Pattern, Central Dense Overcast (CDO) 
Pattern, Embedded Center Pattern or Central Cold Cover Pattern. If 
infrared satellite imagery is available for Eye Patterns (generally the 
pattern seen for hurricanes, severe tropical cyclones and typhoons), then 
the scheme utilizes the difference between the temperature of the warm 
eye and the surrounding cold cloud tops. The larger the difference, the 
more intense the tropical cyclone is estimated to be. From this one gets 
a "T-number" and a "Current Intensity (CI) Number". CI numbers have been 
calibrated against aircraft measurements of tropical cyclones in the 
Northwest Pacific and Atlantic basins. On average, the CI numbers 
correspond to the following intensities:

Current Intensity Numbers	   Central Pressure
CI 	Maximum Sustained 		(mb)
Number	One Minute Winds	Atlantic	NW Pacific
	(kts)
0.0	<25			----		----
0.5	25			----		----
1.0	25			----		----
1.5	25			----		----
2.0	30			1009		1000
2.5	35			1005		997
3.0	45			1000		991
3.5	55			994		984
4.0	65			987		976
4.5	77			979		966
5.0	90			970		954
5.5	102			960		941
6.0	115			948		927
6.5	127			935		914
7.0	140			921		898
7.5	155			906		879
8.0	170			890		858

Note that this estimation of both maximum winds and central pressure 
assumes that the winds and pressures are always consistent. However, 
since the winds are really determined by the pressure gradient, small 
tropical cyclones (like the Atlantic's Andrew in 1992, for example) can 
have stronger winds for a given central pressure than a larger tropical 
cyclone with the same central pressure. Thus caution is urged in not 
blindly forcing tropical cyclones to "fit" the above pressure- wind 
relationships. (The reason that lower pressures are given to the 
Northwest Pacific tropical cyclones in comparison to the higher pressures 
of the Atlantic basin tropical cyclones is because of the difference in 
the background climatology. The Northwest Pacific basin has a lower 
background sea level pressure field. Thus to sustain a given pressure 
gradient and thus the winds, the central pressure must accordingly be 
smaller in this basin.)

The errors for using the above Dvorak technique in comparison to aircraft 
measurements taken in the Northwest Pacific average 10 mb with a standard 
deviation of 9 mb (Martin and Gray 1993). Atlantic tropical cyclone 
estimates likely have similar errors. Thus an Atlantic hurricane that is 
given a CI number of 4.5 (winds of 77 kt and pressure of 979 mb) could in 
reality be anywhere from winds of 60 to 90 kt and pressures of 989 to 969 
mb. These would be typical ranges to be expected; errors could be worse. 
However, in the absence of other observations, the Dvorak technique does 
at least provide a consistent estimate of what the true intensity is.

While the Dvorak technique was calibrated for the Atlantic and Northwest 
Pacific basin because of the aircraft reconnaissance data ground truth, 
the technique has also been quite useful in other basins that have 
limited observational platforms. However, at some point it would be 
preferable to re-derive the Dvorak technique to calibrate tropical 
cyclones with available data in the other basins.

Lastly, while the Dvorak technique is primarily designed to provide 
estimates of the current intensity of the storm, a 24 h forecast of the 
intensity can be obtained also by extrapolating the trend of the CI 
number. Whether this methodology provides skillful forecasts is unknown.

Subject: H2) Who are the "Hurricane Hunters" and what are they looking for?
Contributed by Neal Dorst

In the Atlantic basin (Atlantic Ocean, Gulf of Mexico, and Caribbean Sea) 
and in the eastern and central Pacific, as required, hurricane 
reconnaissance is carried out by two government agencies

U.S. Air Force Reserves' 53rd Weather Reconnaissance Squadron
NOAA's Aircraft Operations Center.
The U.S. Navy stopped flying hurricanes in 1974.

USAF   
The 53rd WRS is based at Keesler AFB in Mississippi and maintains a fleet 
of ten WC-130 planes. These cargo airframes have been modified to carry 
weather instruments to measure wind, pressure, temperature and dew point 
as well as drop instrumented sondes and make other observations.


NOAA   
AOC is presently based at MacDill AFB in Tampa, Florida and among its 
fleet of planes has two P-3 Orions, originally made as Navy sub hunters, 
but modified to include three radars as well as a suite of meteorological 
instruments and dropsonde capability. Starting in 1996 AOC added to its 
fleet a Gulfstream IV jet that is able to make hurricane observations 
from much higher altitudes (up to 45,000 feet). It is used primarily to 
drop sondes around the hurricane's environment to measure synoptic-scale 
parameters in the usually data-free oceanic areas.


The USAF planes are the workhorses of the hurricane hunting effort. They 
are often deployed to a forward base, such as Antigua, and carry out most 
of the reconnaissance of developing waves and depressions. Their mission 
in these situations is to look for signs of a closed circulation and any 
strengthening or organizing that the storm might be showing. This 
information is relayed by satellite to the OAR for the hurricane 
specialists to evaluate.

The NOAA planes are more highly instrumented and are generally reserved 
for when developed hurricanes are threatening landfall, especially on 
U.S. territory. They are also used to conduct scientific research on storms.

The planes carry between six to fifteen people, both the flight crew and 
the meteorologists. Flight crews consist of a pilot, co-pilot, flight 
engineer, navigator, and electrical technicians. The weather crew might 
consist of a flight meteorologist, lead project scientist, cloud 
physicist, radar specialist, and dropsonde operators.

The primary purpose of reconnaissance is to track the center of 
circulation, these are the co-ordinates that the National Hurricane 
Center issues, and to measure the maximum winds. But the crews are also 
evaluating the storm's size, structure, and development and this 
information is also relayed to NHC via radio and satellite link. Most of 
this data, which is critical in determining the hurricane's threat, 
cannot be obtained from satellite.

Subject: H3) What is it like to fly into a hurricane?
   

Contributed by Chris Landsea

The most incredible sight that I've ever seen is in the middle of a 
strong hurricane. One might not believe this, but most hurricane flights 
are fairly boring. They last 10 hours, there are clouds above you and 
clouds below - so all you see is gray, and you don't feel the winds 
swirling around the hurricane.

    
But what does get interesting is flying through the hurricane's rainbands 
and the eyewall, which can get a bit turbulent. The eyewall is a 
donut-like ring of thunderstorms that surround the calm eye. The winds 
within the eyeall can reach as much as 200 mph [325 km/hr] at the flight 
level, but you can't feel these aboard the plane. But what makes flying 
through the eyewall exhilarating and at times somewhat scary, are the 
turbulent updrafts and downdrafts that one hits. Those flying in the 
plane definitely feel these wind currents (and sometimes makes us reach 
for the air-sickness bags). These vertical winds may reach up to 50 mph 
[80 km/hr] either up or down, but are actually much weaker in general 
than what one would encounter flying through a continental supercell 
thunderstorm.
 

But once the plane gets into the calm eye of a hurricane like Andrew or 
Gilbert, it is a place of powerful beauty: sunshine streams into the 
windows of the plane from a perfect circle of blue sky directly above the 
plane, surrounding the plane on all sides is the blackness of the 
eyewall's thunderstorms,

  
... and directly below the plane peeking through the low clouds one can 
see the violent ocean with waves sometimes 60 feet high [20 m] crashing 
into one another. The partial vacuum of the hurricane's eye (where one 
tenth of the atmosphere is gone) is like nothing else on earth. I would 
much rather experience a hurricane this way - from the safety of a plane 
- than being on the ground and having the hurricane's full fury hit 
without protection.

Subject: H4) Can I get a seat on a hurricane flight?
Contributed by Neal Dorst

Sorry, but only people who are part of the mission are allowed on 
military and public aircraft. This may include accredited members of the 
press, provided they are working on a current story involving the storm. 
If you are an accredited reporter and want to know how to arrange for 
your involvement in future flights with the

	Air Force Reserve Command's Hurricane Hunters, please contact Lt. Col. 
	Michael Odom of the 403rd Wing (228) 377-2056.

	NOAA civilian hurricane aircraft contact Lori Bast (813) 828-3310 ext. 
	3072 or click here for information.

Please note that seats are not always available on every flight, and that 
there is a limit of two seats per media outlet on a given flight. NOAA 
maintains a lengthy list of requests to fly aboard their aircraft during 
hurricane missions. If a hurricane is threatening landfall, local media 
will be given the first opportunity to fly. Due to the dynamics of 
hurricanes, flight plans can and do change right up until the last minute 
and flights are often cancelled. All of your contact information (cell 
numbers, pagers, home/office numbers) is extremely helpful in alerting 
you to changes.

Subject: H5) What's it like to go through a hurricane on the ground ? 
What are the early warning signs of an approaching tropical cyclone ?
Contributed by Neal Dorst

Just as every person is an individual, every hurricane is different. So 
every experience with such a storm will be unique. The summary below is 
of a general sequence of events one might expect from a Category 2 
hurricane approaching a coastal area. What you might experience could be 
vastly different.

96 hours before landfall
At first there aren't any apparent signs of a storm. The barometer is 
steady, winds are light and variable, and fair weather cumulus clouds dot 
the sky. But the perceptive observer will note a swell on the ocean 
surface of about a meter (3 feet) in height with a wave coming ashore 
every ten seconds. These waves race out far ahead of a storm at sea, but 
could easily be masked by locally wind driven waves.

72 hours before landfall
Little has changed, except that the swell has increase to about 2 meters 
(6 feet) in height and the waves now come in every nine seconds. This 
means that the storm, still far over the horizon, is approaching.

48 hours before landfall
If anything, conditions have improved. The sky is now clear of clouds, 
the barometer is steady, and the wind is almost calm. The swell is now 
about 3 m (9 feet) and coming in every 8 seconds. A hurricane watch is 
issued, and areas with long evacuation times are given the order to 
begin.

36 hours before landfall
The first signs of the storm appear. The barometer is falling slightly, 
the wind is around 5 m/s (10 kts, 11 mph), and the ocean swell is about 
4m (13 feet) in height and coming in 7 seconds apart. On the horizon a 
large mass of white cirrus clouds appear. As the veil of clouds 
approaches it covers more of the horizon.

30 hours before landfall
The sky is now covered by a high overcast. The barometer is falling at .1 
millibar per hour (.003 inches of Hg/hr), and the winds pick up to about 
10 m/s (20 kts, 23 mph). The ocean swell, coming in only 5 seconds apart, 
is beginning to be obscured by wind driven waves, and small whitecaps 
begin to appear on the ocean surface. A hurricane warning is issued and 
low lying areas and people living in mobile homes are ordered to 
evacuate.

24 hours before landfall
In addition to the overcast, small low clouds streak by overhead. The 
barometer is falling by .2 mb/hr (.006"Hg/hr), the wind picks up to 15 
m/s (30 kts, 34 mph). The wind driven waves are covered in whitecaps and 
streaks of foam begin to ride over the surface. Evacuations should be 
completed and final preparations made by this time.

18 hours before landfall
The low clouds are thicker and bring driving rain squalls with gusty 
winds. The barometer is steadily falling at half a millibar per hour 
(.015 "Hg/hr), and the winds are whistling by at 20 m/s (40 kts, 46 mph). 
It is hard to stand against the wind.

12 hours before landfall
The rain squalls are more frequent and the winds don't diminish after 
they depart. The cloud ceiling is getting lower, and the barometer is 
falling at 1 mb/hr (.029 "Hg/hr). The wind is howling at hurricane force 
at 32 m/s (64 kts, 74 mph), and small, loose objects are flying through 
the air and branches are stripped from trees. The sea advances with every 
storm wave that crashes ashore and the surface is covered with white 
streaks and foam patches.

6 hours before landfall
The rain is constant now and the 40 m/s wind (80 kts, 92 mph) drives it 
horizontally. The barometer is falling 1.5 mb/hr (.044 "Hg/hr), and the 
storm surge has advanced above the high tide mark. It is impossible to 
stand upright outside without bracing yourself, and heavy objects like 
coconuts and plywood sheets become airborne missiles. The wave tops are 
cut off and make the sea surface a whitish mass of spray.

1 hour before landfall
It didn't seem possible, but the rain has become heavier, a torrential 
downpour. Low areas inland become flooded from the rain. The winds are 
roaring at 45 m/s (90 kts, 104 mph), and the barometer is free-falling at 
2 mb/hr (.058 "Hg/hr). The sea is white with foam and streaks. The storm 
surge has covered coastal roads and 5 meter (16 foot) waves crash into 
buildings near the shore.

The eye	
Just as the storm reaches its peak, the winds begin to slacken, and the 
sky starts to brighten. The rain ends abruptly and the clouds break and 
blue sky is seen. However the barometer continues falling at 3 mb/hr (.09 
"Hg/hr) and the storm surge reaches the furthest inland. Wild waves crash 
into anything in the grasp of the surge. Soon the winds fall to near 
calm, but the air is uncomfortably warm and humid. Looking up you can see 
huge walls of cloud on every side, brilliant white in the sunlight. 
At this point, the barometer stops falling and in a moment begins to 
rise, soon as fast as it fell. The winds begin to pick up slightly and 
the clouds on the far side of the eyewall loom overhead.

1 hour after landfall
The sky darkens and the winds and rain return just a heavy as they were 
before the eye. The storm surge begins a slow retreat, but the monstrous 
waves continue to crash ashore. The barometer is now rising at 2 mb/hr 
(.058 "Hg/hr). The winds top out at 45 m/s (90 kts, 104 mph), and heavy 
items torn loose by the front side of the storm are thrown about and into 
sides of buildings that had been in the lee before the eye passed.

6 hours after landfall
The flooding rains continue, but the winds have diminished to a 'mere' 40 
m/s (80 kts, 92 mph). The storm surge is retreating and pulling debris 
out to sea or stranding sea borne objects well inland. It is still 
impossible to go outside.

12 hours after landfall
The rain now comes in squalls and the winds begin to diminish after each 
squall passes. The cloud ceiling is rising, as is the barometer at 1 
mb/hr (.029 "Hg/hr). The wind is still howling at near hurricane force at 
30 m/s (60 kts, 69 mph), and the ocean is covered with streaks and foam 
patches. The sea level returns to the high tide mark.

24 hours after landfall
The low clouds break into smaller fragments and the high overcast is seen 
again. The barometer is rising by .2 mb/hr (.006"Hg/hr), the wind falls 
to 15 m/s (30 kts, 34 mph). The surge has fully retreated from land, but 
the ocean surface is still covered by small whitecaps and large waves.

36 hours after landfall
The overcast has broken and the large mass of white cirrus clouds 
disappears over the horizon. The sky is clear and the sun seems 
brilliant. The barometer is rising slightly, the wind are a steady 5 m/s 
(10 kts, 11 mph). All around are torn trees and battered buildings. The 
air stinks of dead vegetation and muck that was dredged by the storm from 
the bottom of the sea to cover the shore. The all clear is given.

Last updated August 13, 2004

Subject: H6) Are there hurricanes on other planets ?
Contributed by Robert A. Black

There are no other planets known to have warm water oceans from which 
true water cloud hurricanes can form. However, many astronomers and 
planetary meteorologists believe Jupiter exhibits such storms, in which 
ammonia takes the place of water. The principal candidate is the famous 
Great Red Spot, and the numeous whorls that surround it. The Spot 
exhibits an anticyclonic circulation at its top, just as tropical 
cyclones do at the top of the troposphere.

Last updated August 13, 2004

Subject: H7) How does the ocean respond to a hurricane and how does this 
feedback to the storm itself?
Contributed by Joe Cione


The ocean's primary direct response to a hurricane is cooling of the sea 
surface temperature (SST). How does this occur? When the strong winds of 
a hurricane move over the ocean they churn-up much cooler water from 
below. The net result is that the SST of the ocean after storm passage 
can be lowered by several degrees Celsius (up to 10 Fahrenheit).

Figure 1 shows SSTs ranging between 25-27C (77-81F) several days after 
the passage of Hurricane Georges in 1998. As Figure 1 illustrates, 
Georges' post storm 'cold wake' along and to the right of the 
superimposed track is 3-5C (6-9F) cooler than the undisturbed SST to 
the west and south (i.e. red/orange regions are ~30'C [86'F]). The 
magnitude and distribution of the cooling pattern shown in this 
illustration is fairly typical for a post-storm SST analysis.

One important caveat to realize however is that most of the 3-5C (6-9F) 
ocean cooling shown in Figure 1 occurs well after the storm has moved 
away from the region (in this case several days after Georges made 
landfall). The amount of ocean cooling that occurs directly beneath the 
hurricane within the high wind region of the storm is a much more 
important question scientists would like to have answered. Why? 
Hurricanes get their energy from the warm ocean water beneath them. 
However, in order to get a more accurate estimate of just how much energy 
is being transferred from the sea to the storm, scientists need to know ocean temperature conditions directly beneath the hurricane. Unfortunately, with 150kph+ (100mph+) winds, 20m+ (60ft+) seas and heavy cloud cover being the norm in this region of the storm, direct (or even indirect) measurement of SST conditions within the storm's "inner core" environment are very rare. 
Thankfully in this case "very rare" does not mean "once in a lifetime". 
Recently, scientists at the Hurricane Research Division were able to get 
a better idea of how much SST cooling occurs directly under a hurricane 
by looking at many storms over a 28 year period. By combining these rare 
events, HRD scientists put together a "composite average" of ocean 
cooling directly under the storm.

Figure 2 illustrates that, on average, cooling patterns are a lot less 
than the post storm 3-5C (6-9F) cold wake estimates shown in Figure 1. 
In most cases, the ocean temperature under a hurricane will range 
somewhere between 0.2 and 1.2C (0.4 and 2.2F) cooler that the 
surrounding ocean environment. Exactly how much depends on many factors 
including ocean structure beneath the storm (i.e. location), storm speed, 
time of year and to a lesser extent, storm intensity (Cione and Uhlhorn 
2003). 
While the estimates in Figure 2 represent a dramatic improvement when it 
comes to more accurately representing actual SST cooling patterns 
experienced under a hurricane, even small errors in inner core SST can 
result in significant miscalculations when it comes to accurately 
assessing how much energy is transferred from the warm ocean environment 
directly to the hurricane. With all other factors being equal, being 
"off" by a mere 0.5C (1F) can be the difference between a storm that 
rapidly intensifies to one that falls apart! With that much at stake, 
scientists at HRD and other government and academic institutions are 
working to improve our ability to accurately estimate, observe and 
predict "under-the-storm" upper ocean conditions. These efforts include 
statistical studies, modeling efforts and enhanced observational 
capabilities designed to help scientists better assess upper ocean 
thermal conditions under the storm. With such improvements, it is 
believed that future forecasts of tropical cyclone intensity change will 
be significantly improved.

Reference
Cione, J. J., and E. W. Uhlhorn, 2003: Sea Surface Temperature 
Variability in Hurricanes: Implications with Respect to Intensity Change. 
Monthly Weather Review, 131, 1783-1796.

Last updated August 13, 2004