Storm Surge • Greatest killer • Extremely costly • Preferred on right side of storm: -onshore flow -wind and wave vectors generally reinforce each other
Storm Surge Some influencing factors: • Wind speed of storm (“intensity”) • Size of storm, especially radius of maximum wind • Pressure of storm • Shape of coastline • Astronomical tide • Intensity trends (weakening or steady-state probably more surge) • Slope and structure of bottom (bathymetry) • Storm heading relative to the coastline • Direction of wind relative to the coastline • Quadrant • Forward speed of storm
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 combines, 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
Waves Wave 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.
Waves Extensive 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.
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.
Wilma Relatively dry: -fast forward speed -midlevel dry air -strong UL winds
Heavy Rainfall Factors include: Storm speed Terrain Size Recurvature or loops in track Baroclinic zone enhancement Time of day? Intensity (only a weak correlation for this last one)
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
End Presentation • 1 more presentation to go!!