lecture 320

Chap. 8: Severe Storms:Tropical Cyclones (Hurricanes): 

Chap. 8: Severe Storms: Tropical Cyclones (Hurricanes) We start severe storms (and focus on) tropical cyclones (aka hurricanes): “Tropical cyclone,” “hurricane,” “typhoon” Wind, rain (flooding), and storm surge (and embedded tornadoes) About 15% of global population is threatened Perhaps 6,000/year fatalities (about 60K during 1992-01) $10b annual damages (1995 dollars)

Themes: 

Themes Monitoring, warning, evacuation systems Hazard risk assessment Physical protection, & ways to decide how to deploy protection Building and land use mitigation

Causes: 

Causes Tropical lows form over 26 C (82F) ocean surfaces, often in ITCZ. If conditions are right, the rising warm air over low-latitude, solar-warmed waters sets off a positive-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: 

Causes Spin imparted by Corriolis Force, increases convergence, ascent, and buoyancy, so winds speed up centrifugal and centripetal forces come into balance (also creating the infamous eye wall) Input and output balance: low level inflow to low pressure, upper level outflow (from high pressure) and storm can reach a steady-state Decay: Loss of energy (water vapor); loss of upper air divergence; friction of land. All these can weaken it by up-setting the balance of forces.

Slide6: 

The mature storm is drawing in warm, moist air from a large area, into the tightening counter-clockwise spiral, then up in the wall cloud and out with upper level divergence and clockwise outflow. The eye is distinct as the one area of concentrated sinking air in the system (sinking warms and dries the air, thus eye is often clear). Like a spinning skater, the fastest spin (winds) are right near the center where the angular distance they must travel to circulate around the low are shortest.

Magnitude: 

Magnitude Wind speed: 33 m/s (74 mph) arbitrary threshold Central pressure: commonly 28.00 in or less (30 is normal sea level pressure) Storm surge: height (1-8 m; 3-20+ ft above mean or high tide; Also: Wave heights; total rainfall and rainfall rates; inland flood heights

Slide8: 

The S-S scale puts all the magnitude measures together into five categories, a practice common in many hazards now (from earthquakes to snowstorms) but one that does give up some detail and specificity of magnitude.

Slide9: 

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

Slide10: 

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

Slide11: 

Radar bounces off of precipitation, which is most intense in eyewall and feeder spiral bands. dBZ is measure of radar beam reflectance.

Slide12: 

Katrina’s eye wall from Hurricane recon aircraft

Hazardous geographies: low-lying, densely population coastal areas:: 

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, due mostly to storm surge (3-9 m) 1991: another strike, this time 139K deaths in 6 m surge

Hazardous geographies:: 

Hazardous geographies: Islands: Philippines; Taiwan; smaller Pacific and Hawaiian islands; Caribbean Islands: Urbanized coasts of large land masses: Atlantic and Gulf of USA; China, Japan, Australia.

Slide15: 

Main damage causes are: Storm surge:

Slide16: 

And: wind:

Slide17: 

Great Galveston Hurricane, 1900: storm surge was the main killer here, and still the most lethal part of hurricanes.

Slide18: 

Katrina surge near Gulfport, MS. Not many photos of storm surge as difficult for storm chasers to stay in front of it.

Slide19: 

But plenty of pictures of the effects of storm surge: Hurricane Camille, 1969 Gulfport, MS

Slide20: 

Hurricane Camille, 1969 Gulfport, MS

Slide21: 

And plenty of developed coastline at risk from storm surge, though modern buildings may be constructed with some mitigation (e.g., parking on ground level allows surge to pass under building). This is near Pompano Beach, FL

Surge Risk is Key. Risk assessment needed for:: 

Surge Risk is Key. Risk assessment needed for: Long-term hazard assessment / preparation: Evacuation zones Land use planning Short-term forecast and warning SLOSH (Sea, Lake and Overland Surges from Hurricanes) Model: run for planning and real time forecast, see an example here: http://www.nhc.noaa.gov/HAW2/english/surge/slosh2.gif

Surge factors:: 

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 water—allows it to drag more water up against the shore Wind duration: longer time wind blows onto shore more water can pile up, more chance of surge and high tide occurring simultaneously. Shoaling: shallow bottom stretching far off-shore cause more surge than quick deepening as you go off-shore (Atlantic coast has steeper shoaling than Gulf, where surges are higher) Shape of coastline: embayments and other concave shapes “focus” or concentrate the water for high surges; headlands and convex shapes shed the water for lower height of surge.

Slide24: 

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

Slide25: 

Speed, direction and fetch all mean surge is highest on right quadrant of a landfalling hurricane. The coastline illustrated here is also concave in shape.

Slide26: 

Surge risk maps show area inundated by different Safir-Simpson scale hurricanes. Maps assume that each spot is just right of eye at landfall. Of course, not all areas can have that surge in a single storm, but forecasting uncertainty means larger areas warned than actually affected. These zones can then be used to plan, decide-on and organized an evacuation.

Slide27: 

Some areas place storm surge markers to raise awareness—maybe especially need in tourist areas where visitors may be unaware.

Slide28: 

Forecast of surge for an actual storm is segmented into different areas with different heights—always a range of heights.

Slide29: 

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.

Slide30: 

Other physical protections include “beach nourishment” which adds sand, is expensive, and only marginally effective but also adds recreational space at least until it re-erodes..

Slide31: 

Other physical protections include beach “armoring” with “rip-rap” or large rocks . Doesn’t always do much about surge (left) but can resist or slow erosion.

Slide32: 

Inflation-adjusted and Normalized by property/wealth at risk, based on property values along coast. 1926 Miami hurricane worse than Andrew. Inflation-adjusted, actual losses. Hurricane losses in US are increasing, roughly proportional to development on coast at risk.

Slide33: 

A neat NOAA web calculator lets you see population growth in your county and hurricane history. Issue is that many residents have not experienced a hurricane, which may affect how they respond to warnings and evacuation notices. Note clump of palm beach strikes in 1920-1945, then little action while pop grew. You can create this graph for different places at: http://maps.csc.noaa.gov/hurricanes/pop.jsp

Hurricane Hazard Mitigation: 

Hurricane Hazard Mitigation Disaster aid and insurance: like other hazards, though surge (biggest loss) is not covered by commercial insurance, only thru 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 codes—widespread 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)

Slide35: 

Simple, cheap ties keep roofs on. But shingles and air conditioning units might still come off!

Slide36: 

Wind damage varies greatly depending on construction, wind direction, and effects of missiles form nearby houses. This is Andrew damage near Homestead, FL. We can 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.