severe stroms

8/7/2019 severe stroms

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Severe Storms:

Tropical Cyclones (Hurricanes)We start severe storms (and focus on) tropical

cyclones (hurricanes):

� ³Tropical cyclone,´ ³hurricane,´ ³typhoon´

� Wind, rain (flooding), and storm surge

� About 15% of global population is threatened

� Perhaps 6,000/year fatalities (about 60K during

1992-01)� $10b annual damages



Themes

� Monitoring, warning, evacuationsystems

� Hazard risk assessment� Physical protection, & ways to decide

how to deploy protection

� Building and land use mitigation



Causes

� Tropical lows form over 26 C (82F) oceansurfaces, often in ITCZ.

� If conditions are right, the rising warm air over low-latitude, solar-warmed waters sets off apositive-feedback:

� Ascent causes convergence� Condensation, release of latent heat causes

increased buoyancy, thus increased ascent, and

increased convergence� Area of convergence enlarges, more water

vapor to draw on



Causes� Spin imparted by Force, increases convergence,

ascent, and buoyancy, so winds speed up

� centrifugal and centripetal forces come intobalance (also creating the infamous eye wall)

� Input and output balance: low level inflow to lowpressure, upper level outflow (from highpressure) and storm can reach a steady-state

� Decay: Loss of energy (water vapor); loss of

upper air divergence; friction of land. All thesecan weaken it by up-setting the balance of forces.



The mature storm is drawing in warm, moist air from a large area, into the tighteningcounter-clockwise spiral, then up in the wall cloud and out with upper level divergenceand clockwise outflow. The eye is distinct as the one area of concentrated sinking air inthe system (sinking warms and dries the air, thus eye is often clear). Like a spinningskater, the fastest spin (winds) are right near the center where the angular distance

they must travel to circulate around the low are shortest.



Magnitude

� Wind speed: 33 m/s (74 mph) arbitrarythreshold

� Central pressure: commonly 28.00 in or less (30 is normal sea level pressure)

� Storm surge: height (1-8 m; 3-20+ ftabove mean or high tide;

� Also: Wave heights; total rainfall andrainfall rates; inland flood heights



Visible satellite image: spiral bands of convection, wall cloud aroundsomewhat cloudy eye, plus outflow cirrus clouds.



Infrared image shows cloud top temperatures: higher are colder, colored red;higher clouds is sign of stronger storm. Outflow cirrus especially vis to north.



Katrina¶s eye wall from Hurricane recon aircraft



Hazardous geographies:low-lying, densely population coastal

areas:� Bangladesh: low gradient deltaic area

with little refuge, subject to cyclones in

Bay of Bengal (only about 5/year)

� 1970: 300K deaths, $75 million, duemostly to storm surge (3-9 m)

� 1991: another strike, this time 139Kdeaths in 6 m surge



Hazardous geographies:

� Islands:

±Philippines; Taiwan; smaller Pacificand Hawaiian islands; CaribbeanIslands:

� Urbanized coasts of large land

masses:±Atlantic and Gulf of USA; China,

Japan, Australia.



Main damage

causes are:

Storm surge:



And:

wind:



Great Galveston Hurricane, 1900: storm surge was the main

killer here, and still the most lethal part of hurricanes.



Katrina surge near Gulfport,

MS. Not many photos of storm

surge as difficult for storm

chasers to stay in front of it.



But plenty of pictures of

the effects of storm surge:

Hurricane Camille, 1969

Gulfport,MS



Hurricane Camille,

1969

Gulfport,MS



And plenty of developedcoastline at

risk from stormsurge, thoughmodernbuildings maybe constructedwith somemitigation (e.g.,parking onground levelallows surge topass under building). Thisis near

PompanoBeach, FL



Surge Risk is Key. Riskassessment needed for:

� Long-term hazard assessment / preparation:

± Evacuation zones

± Land use planning

� Short-term forecast and warning

� SLOSH (Sea, Lake and Overland Surges fromHurricanes) Model: run for planning and real

time forecast, see an example here:� http://www.nhc.noaa.gov/HAW2/english/surge/slosh2.gif



Surge factors:� Pressure: low pressure is higher surge� Wind speed: high speed, higher surge� Wind direction: blowing right angle onto land from water � Wind fetch: distance wind has blown in relatively straight

line across open water²allows it to drag more water upagainst the shore

� Wind duration: longer time wind blows onto shore morewater can pile up, more chance of surge and high tideoccurring simultaneously.

� Shoaling: shallow bottom stretching far off-shore causemore surge than quick deepening as you go off-shore

(Atlantic coast has steeper shoaling than Gulf, wheresurges are higher)

� Shape of coastline: embayments and other concaveshapes ³focus´ or concentrate the water for high surges;headlands and convex shapes shed the water for lower

height of surge.



Surge is modeled above mean sea level, but in real-time forecasting it is added totide expected at time of max surge. Waves are not part of surge height, but addedas additional hazard.



Surge risk maps showarea inundated bydifferent Safir-Simpsonscale hurricanes. Maps

assume that each spot isjust right of eye atlandfall. Of course, not allareas can have thatsurge in a single storm,but forecasting

uncertainty means larger areas warned thanactually affected.

These zones can then beused to plan, decide-onand organized anevacuation.



Some areas place storm surge markers to raise awareness²maybeespecially need in tourist areas where visitors may be unaware.



Forecast of surge for an actual storm is segmented into different areas withdifferent heights²always a range of heights.



Structural protection can

make a difference in surge

impact. Here¶Galveston¶s sea

wall, built after 1900 storm:

Left: at 1905 construction;

bottom left in March 2007 with

my neighbor for scale; and

bottom right: touched by a

not-unusually high tide due to

beach erosion.



Other physical protectionsinclude ³beach nourishment´which adds sand, isexpensive, and only

marginally effective but alsoadds recreational space atleast until it re-erodes..



Other physical protectionsinclude beach ³armoring´ with³rip-rap´ or large rocks .Doesn¶t always do muchabout surge (left) but canresist or slow erosion.



Inflation-adjusted andNormalized byproperty/wealth at risk,based on property values

along coast. 1926 Miamihurricane worse thanAndrew.

Inflation-adjusted, actuallosses.

Hurricane losses in US are increasing, roughly proportional to developmenton coast at risk.



A neat NOAA webcalculator lets you seepopulation growth inyour county and

hurricane history. Issueis that many residentshave not experienced ahurricane, which mayaffect how theyrespond to warnings

and evacuation notices.Note clump of palmbeach strikes in 1920-1945, then little actionwhile pop grew.

You can create thisgraph for differentplaces at:

http://maps.csc.noaa.gov/hurricanes/pop.jsp



Hurricane Hazard Mitigation� Disaster aid and insurance: like other hazards, though surge

(biggest loss) is not covered by commercial insurance, onlythru federal flood insurance.

� Hazard resistant design:

� Shelters (1600 cyclone shelters in Bangladesh); after 1992,all new schools in FL are designed as shelters (wall and

window strength; doors; interior space; raised floors; roof engineering; facilities; etc.).

� Building codes²widespread and effective!:

± Roof construction (shingles, cladding, gutters) attached

with hurricane clips± Roof firmly attached to walls

± Windows resistant missile impact (So FL: 4 kg timber striking at 15 m/s (33 mph)

± Shingles/tiles tested at 49 m/s (110 mph)



Simple, cheap tieskeep roofs on.

But shingles and air conditioning unitsmight still come off!



Wind damage varies greatly depending on construction, wind direction, and effectsof missiles form nearby houses. This is Andrew damage near Homestead, FL. Wecan surmise wind from right to left: 1st house collapses, sends impacts to next house,

but pile of debris finally gets large enough to partially protect 4th house.

severe stroms

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