Destructive Effects

Storm Surge

• Greatest killer• Extremely costly• Preferred on right side of storm:

-onshore flow-wind and wave vectors generally

reinforce each other

Storm SurgeSome influencing factors (specific values courtesy Steve Lyons):• Wind speed of storm (“intensity”)• Size of storm, especially radius of maximum wind (+/- 60% for storms of

same category)• Angle, or storm heading relative to the coastline (+/- 40% for storms of

same category)• Forward speed of storm (+/- 30% for storms of same category)• Location- impacts of bathymetry (+/- 25% for storms of same category)• Pressure of storm• Shape of coastline• Astronomical tide• Intensity trends (weakening or steady-state probably more surge, due to

expansion wind field expansion and probability of already built up sea state)

• Direction of wind relative to the coastline• Quadrant

Katrina

Storm Surge• Can be modeled from the SLOSH (Sea, Lake, and

Overland Surges from Hurricanes).• SLOSH useful for watches and warnings and also

climatological purposes, which, in turn, influence insurance industry and so forth.

• In the right quadrants of a major hurricane, where onshore flow and onshore wave propagation generally combine, surge values of 20 feet or greater and penetration of 20 miles or more inland are not uncommon. Maximum surge is often near or slightly beyond the radius of maximum wind in the right quadrants.

Wilma – SLOSH model

Observed surge for Ophelia

WavesWave height dependent upon:• Wind speed• Fetch (distance over which wind blows)• Duration. In a hurricane, the limiting factor is typically

fetch. The increase in fetch of weakening or extratropical transitioning systems due to enlarged wind fields may build the seas higher even though the maximum sustained wind speed is decreasing.

WavesExtensive foam production from waves at high wind

speeds may reduce the drag coefficient over the oceans, which may possibly make it more energetically easy for hurricanes to maintain high surface wind speeds than otherwise.

Especially in the open sea, waves of 50 feet or greater are common in major hurricanes. They are most common in large storms in the right quadrants, where relatively parallel wind vectors reinforce the wave propagation vectors to increase height.

Coastlines with deep water nearby, which typically have little surge potential, may have greater potential for very large waves.

Isabel

WavesWave setup and size (specific values from S. Lyons)

influenced by:-Intensity.For storms of the same Saffir-Simpson category,

following attributes can cause great variability:-Size (+/- 500%)-Track and speed (+/- 300%)-Location, importance of bathymetry (+/- 300%)-Refraction, as headlands focus wave energy while it is

directed away from bays (+/- 200%)-Angle, or heading relative coast (+/- 50%)

Wind• In major hurricanes, category-3 or greater or 96

kt+ maximum sustained surface winds, hurricane wind (64 kt or greater) radii may be less than 20 mi for very small storms to 150 mi for extremely large storms.

• Tropical storm wind radii (34 – 63 kt) often extend to 125-175 mi radius for major hurricanes, though some storms may be very small (<50 mi) and a few very large (>300 mi).

• Size often a function of latitude and age, as seen in earlier topics, and environmental conditions (shear, SST, moisture) as well as size of initiating disturbance (for example, ITCZ rollups often generate very small wind fields, initially).

Wind

• Wind sustained for much longer time scales than midlatitude severe thunderstorm episodes. Duration of high wind increases damage potential.

• If eye passes near or around you, wind direction will change considerably with time. This results in high winds blown from various directions, which also enhances damage potential from wind.

Wind• Wind defines hurricane category.• 1/5 of all TCs hitting US are major hurricanes (cat

3, 4, and 5), but they account for over 4/5 damage.

• Cat-4 and 5 landfalls in US are rare, only once a decade or so on average (less for cat-5’s).

• S Florida, SE Louisiana, and E North Carolina have greatest frequency of hurricane and major hurricane landfalls in U.S.; the parts of FL and LA mentioned as well as N and Central TX have the highest frequency of cat-4 & 5 hurricane landfalls.

Wind

• Hurricane watches are for a relatively high likelihood of hurricane strength winds within 36 h; warnings are for expected hurricane strength winds within 24 h.

• Typically larger warned area to right of storm, where wind vector and forward motion vector of storm combines to increase wind speed near and above ground.

Hazel (1954)

Wilma

Heavy Rainfall• Ptot = 100/s where: Ptot= rainfall amounts in heavy

band along storm track (in inches) s = forward speed (in kt)

Heavy rainfall often results in freshwater flooding, which is one of the leading causes of death for TCs which hit relatively prosperous countries.

Amounts in excess of 5” over swaths of tens of miles wide by hundreds of miles long are frequent.

WilmaRelatively dry: -fast forward speed-midlevel dry air-strong UL winds

Heavy RainfallFactors include:

Storm speedTerrainSizeRecurvature or loops in trackBaroclinic zone enhancement

Time of day? Intensity (only a weak correlation for this last

one)

Effects of Ivan, in God-forsaken Ohio

Tornadoes• Large low level shear in TCs when they make

landfall. Friction over land is larger and results in an enhanced veering PBL profile relative to the ocean.

• Frictional convergence along coastline due to friction difference often helps increase tornadogenesis.

• Relatively low CAPE.• Low BRN.

Tornadoes• Helicity typically enhanced in right quadrants,

particularly right front quadrant.• TCs that have a northward or northeast

motion typically produce more tornadoes. May be due to greater baroclinicity expected with the large-scale, often midlatitude induced, flow in such steering patterns (often accompanied by mid level dry air which may enhance supercell and tornado formation).

• Typically small and shallow mesocyclones.

Tornadoes

• Tilting/tipping (usually larger, highlighted in white) and continuity terms in vorticity equation often help produce TC tornadoes:

• -[(δw/δx)(δv/δz) - (δw/δy)(δu/δz)] + η ·V A B C D• A, B = horizontal gradient of vertical velocity• B, D = VWS• Many landfalling TCs produce no tornadoes;

some (though less than 20%) may produce dozens.

Waterspouts in Lili… they are not exactly the same as tornadoes, particularly in cases like this photograph

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