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What is a ‘hurricane’?. Where do they form? Why? Why do they move east to west rather than west to east? Why do they have names?!? And how do they pick the names? What is the ‘eye”? The ‘eye wall”? How do hurricanes compare to other storms? Typhoons? mid-latitude cyclones?

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What is a ‘hurricane’?

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What is a ‘hurricane’?

  • Where do they form? Why?

  • Why do they move east to west rather than west to east?

  • Why do they have names?!? And how do they pick the names?

  • What is the ‘eye”?

  • The ‘eye wall”?

  • How do hurricanes compare to other storms? Typhoons? mid-latitude cyclones?

  • http://www.aoml.noaa.gov/hrd/tcfaq/tcfaqHED.html


Sept 06 2004

“IVAN IS FORECAST TO SLOWLY DEEPEN OVER THE NEXT 36 HOURS REACHING 130 KT IN 48 HOURS “

DEEPEN? What does this mean? How does it relate to hurricane intensity?

“ESTIMATED MINIMUM CENTRAL PRESSURE 951 MB MAX SUSTAINED WINDS 110 KT WITH GUSTS TO 130 KT. “

What is an “MB”? A “KT”? What is the significance of these values?


What does ‘warm water’ have to do with hurricanes?

  • NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 5 AM EDT MON SEP 06 2004

    “NOW THAT FRANCES HAS MOVED OVER THE WARM WATERS OF THE NORTHEAST GULF OF MEXICO...THUNDERSTORMS ACTIVITY HAS STEADILY INCREASED NEAR THE TIGHT CIRCULATION CENTER DURING THE PAST 6 HOURS. THE LAST RECON REPORT AT AROUND 05Z INDICATED A RAGGED EYE WAS TRYING TO FORM…

    NOW THAT THE CENTER OF FRANCES IS BACK OVER WARM WATER...SOME RE-STRENGTHENING IS POSSIBLE. WATER TEMPERATURES ARE NEAR 83-84F AND UPPER-LEVEL OUTFLOW IS STILL QUITE IMPRESSIVE.”


What are the ‘levels’ or ‘layers’ of the of the atmosphere?

  • “UPPER-LEVEL OUTFLOW IS STILL QUITE IMPRESSIVE”

  • “A STRONG DEEP-LAYER ANTICYCLONE IS FORECAST TO REMAIN TO THE NORTH OF THE HURRICANE STEERING IVAN INTO THE EASTERN CARIBBEAN SEA BY DAY TWO AND TOWARD THE SOUTHWESTERN PORTION OF HISPANIOLA ON DAY FOUR.”

  • “STRONG UPPER LEVEL SHEAR…”


How are Hurricanes tracks established?

  • THE NHC MODEL GUIDANCE IN GOOD AGREEMENT ON FRANCES MOVING WEST-NORTHWESTWARD AROUND THE SOUTHWESTERN PORTION OF THE SUBTROPICAL RIDGE AXIS THAT EXTENDS EAST-WEST ALONG 32N LATITUDE. THE PRIMARY DIFFERENCE IN THE MODELS IS HOW SHARP OF A TURN FRANCES WILL MAKE UPON REACHING 32N. THE GFS APPEARS TO MAKE TOO SHARP OF A NORTHEASTWARD TURN GIVEN THE CURRENT LARGE DIAMETER OF FRANCES. THE OFFICIAL FORECAST IS CLOSE TO THE PREVIOUS FORECAST TRACK AND IS CLOSE THE GUNA MODEL CONSENSUS.


Huh?

  • “THERE IS VERY GOOD AGREEMENT THROUGH DAY FIVE WITH NOGAPS TO THE RIGHT OF THE AIDS ENVELOPE AND THE UKMET TO THE LEFT OF THE AIDS ENVELOPE. THE OFFICIAL FORECAST IS SLIGHTLY RIGHT OF THE PREVIOUS FORECAST TRACK AND IN VERY GOOD AGREEMENT WITH GUNS AND GUNA”.


So what is a ‘MODEL” and how does it work?


How does RADAR and Satellite imagery assist in evaluating hurricanes?


06 Sept. 04 Visible Satellite Image

A SMALL EYE IS EVIDENT ON THE VISIBLE IMAGERY. …. AND SOMEWHAT RAGGED-LOOKING BANDING FEATURES


Hermine?

Francis

IVAN

06 Sept 04 IR Satellite Image


The Dvorak Technique Explained

The Dvorak technique is a method using enhanced Infrared and/or visible satellite imagery to quantitatively estimate the intensity of a tropical system. Cloud patterns in satellite imagery normally show an indication of cyclogenesis before the storm reaches tropical storm intensity. Indications of continued development and/or weakening can also be found in the cloud features. Using these features, the pattern formed by the clouds of a tropical cyclone, expected systematic development, and a series of rules, an intensity analysis and forecast can be made. This information is then standardized into an intensity code


How do statistics and probability relate to hurricane forecasts?

Note the difference in size between the forecast cones… note the degradation of certainty over time, and the higher certainty of the track for hurricane ‘Ivan’


First question….

  • Where do hurricanes form? Why?


Atmospheric heating….

  • Imbalances in solar NRG are the driving force behind all weather.

  • Direct sunlight imparts more NRG to the surface

  • High angle sunlight has very much lower NRG content

  • The atmosphere is heated from below…


The Earth in Space


Figure 3.3


June 22


Another way of looking at June 22…

www.time.gov


Daylight pattern on 22 September… near the equinox

What would December 22 look like?


What significance do the tropics of Cancer and Capricorn have?


Figure 3.4


Figure 3.6


Figure 3.2


Figure 3.5


Note the influence of clouds… which interrupts the latitudinal patterns


Figure 3.7

DECEMBER 21


Figure 3.8


Figure 3.10


Figure 3.9


Electromagnetic Radiation (EMR) can be described as waves.

Note the generation of a magnetic field at right angles to an electrical field and both are perpendicular to the direction of EMR propagation

http://micro.magnet.fsu.edu/primer/java/electromagnetic/index.html


The wavelength () of EMR is directly related to the amount of NRG in the radiation.

Short () (e.g. gamma rays) have a higher NRG content

Long () (e.g. thermal or microwave (Radio) have a lower NRG content

As the velocity of the propagation of EMR (the speed of light) is believed to be a constant (c)

The number of waves to pass a single point in a given period (the frequency, v) is much higher for shorter  radiation


Blackbody radiance curves…

Wein’s Displacement Law


Figure 2.8


Figure 2.9


Figure 2.14


When the sun is at very low angles, virtually all the shorter  EMR is scattered, allowing only the longer, lower NRG waves (reds) to pass through.


Atmospheric scattering: (skylight or haze)

Rayleigh scatter (molecular scatter) primary factor in upper atmosphere (why the sky is blue)

Mie scatter (non-molecular scattering, water/ice/salt/smoke) primarly in lower atmosphere


Figure 2.15


Figure 2.16


Clouds and Ice have an Albedo approaching 100%


Global albedo is not constant… but the overall average is 35%


NRG is transferred by:

Radiation: (sunlight)

Conduction (illustrated here)

Convection (description follows)

Figure 2.5


Figure 2.6


Figure 2.13


Atmospheric Windows

Some EMR passes through the atmosphere with no interference (an ‘atmospheric window’

Some EMR is absorbed (or blocked) by components in the atmosphere

The most significant absorbers of EMR in the atmosphere are O2 , N2, O3, CO2, H2O


Figure 2.11


Figure 2.2


Figure 2.3


Figure 2.10


45 degrees N… halfway between equator and pole


57.5 degrees North (same latitude as Moscow and Hudson’s Bay… semi-tropical gardens on the coast of Scotland


So how about those names?

  • For several hundred years, hurricanes in the West Indies were often named after the particular saint’s day on which the hurricane occurred.

  • Using women’s names became the practice during World War II

  • In 1979 male and female names were included in lists for the Atlantic and Gulf of Mexico

  • Names associated with storms that have caused significant death and/or damage are usually retired from the list.

  • name lists have been agreed upon at international meetings of the World Meteorological Organization


http://mywebpages.comcast.net/herbwx/hurrsixyr.html


Historical hurricane tracks and nameshttp://weather.unisys.com/hurricane/atlantic/index.html


Tropical Weather & Waves

Tropical winds typically blow from the east, and when they encounter a slow moving trough of low pressure, called a tropical wave, the winds initially converge and lift to produce showers and thunderstorms.

Figure 16.1


Structure of a Hurricane

Tropical cyclones are the international name of hurricanes, which typically form from an organized mass of storms formed along a tropical wave.

In this image of Hurricane Elena, the central area of broken clouds is the eye, surrounded by an eye wall cloud and spiral rain bands, with a total diameter nearing 500 kilometers.

Figure 16.2


Hurricane Wind Profile

Figure 16.3

The low pressure core of the hurricane is surrounded by several thunderstorms, each with updraft and downdraft cycles.

The wind and moisture cycle is repeated as:

surface moist air converges in a counterclockwise pattern at the eye, rises to create high pressure aloft, condenses, precipitates, dries, diverges outward in a clockwise pattern, sinks, and warms.


3-D Radar Image of Hurricane

Figure 16.4

Several key features of a hurricane are shown in this radar composite image, including overshooting clouds, the area of strongest echoes (heaviest rain), and the eyewall.


Formation by Organized Convection

Figure 16.5A

Figure 16.5B

One theory explains that hurricane formation requires cold air above an organized mass of thunderstorms, where the release of latent heat warms the upper troposphere, creates high pressure aloft, which pushes air outward and causes a low to deepen at the surface.

Air moving toward this low intensifies the cycle.


Formation by Heat Engine

Another theory of hurricane development proposes that a heat engine cycle, fueled by warm moist input air and the release of heat when it converts to cool dry air.

Differences in the input and output temperatures determine the amount of work on the ocean and winds that is performed.


Hurricane Stages of Development

The initial components of a hurricane may form as a tropical disturbance, grow into a tropical depression when winds exceed 20 knots, become a tropical storm when winds exceed 35 knots, and finally then qualify as a hurricane when winds exceed 64 knots.

Figure 16.6


Erratic Paths of Hurricanes

Figure 16.8


Historical charts of hurricane location may reveal erratic, and hard to predict, patterns of movement.

As this figure shows, hurricanes may occasionally double back.

Further, when removed from the ocean and without a moisture source to supply energy, they may still continue an inland journey.

In the North Atlantic, on average 3 storms per year move inland and bring damaging winds and rain.


North Atlantic Hurricanes

Composite infrared imagery of Hurricane Georges reveals the pattern of a seasonal threat for Central and North America coastlines.

Tropical cyclones at the same latitude survive longer in the Atlantic than Pacific Ocean because of warmer Atlantic Ocean waters.

Figure 16.9


Hurricane Damage & Warning

Figure 16.11

Figure 16.10

Hurricanes have their highest wind speeds on the side where storm pushing winds amplify cyclonic, or counterclockwise, rotational winds. In coastal areas, flooding is aggravated by the hurricane low pressure triggering higher tides and Ekman transport piling up water.


Hurricane Watch & Warning

Figure 16.5A

The National Hurricane Center in Florida issues a hurricane watch 24 to 48 hours before a threatening storm arrives, and if it appears that the storm will strike within 24 hours, a hurricane warning is issued.

While some consider the warning area too large, causing unneeded evacuation, such evacuations have saved many lives.

Hurricane Hugo, with peak winds near 174 knots, caused tremendous damage.


Hurricane Saffir-Simpson Winds

Figure 16.14

Figure 16.13A

In 1989 Hugo caused nearly $7 billion in damages in the U.S., killing 49 in the Caribbean and United States.

Current classification of hurricanes is based on their wind speed, however, and not on human or property damage.

Hurricanes range from category 1 to 5, with winds of 64 to more than 135 knots.


Hurricane Names and Cost

Category 5 Hurricane Andrew (1992) was the costliest US storm, but it ranks as less intense than 1935 and 1969 hurricanes.

Hurricane names are chosen from an alphabetical list of male and female names for the Atlantic and Pacific, some of which are retired if the storm was especially damaging.

Figure 16.14


Hurricane Andrew Devastation in Homestead, Florida August 24, 1992

Figure 16.15


Likelihood for Landfall

Between 1900 and 1999, only two category 5 hurricanes have made landfall along the Gulf or Atlantic.

Numerous category 1, and less damaging storms, that do make landfall may not cause much damage, but bring needed rainfall.

Figure 16.16


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