Doesn't the friction over land kill tropical cyclones?

[gatorcane]

The answer may not be what you think. We saw this with Wilma as even though it had CAT 2 winds there were CAT 3 "bursts." Note Andrew is mentioned as it weakened dramatically as it moved over the Everglades. Katrina, however, seemed to get better organized, although she weakened some to strong T.S. I wonder where her "heat source" was that was lacking with Andrew?

Answer:

"No. During landfall, the increased friction over land acts - somewhat contradictory - to both decrease the sustained winds and also to increase the gusts felt at the surface (Powell and Houston 1996). The sustained (1 min or longer average) winds are reduced because of the dampening effect of larger roughness over land (i.e. bushes, trees and houses over land versus a relatively smooth ocean). The gusts are stronger because turbulence increases and acts to bring faster winds down to the surface in short (a few seconds) bursts.

However, after just a few hours, a tropical cyclone over land will begin to weaken rapidly - not because of friction - but because the storm lacks the the moisture and heat sources that the ocean provided. This depletion of moisture and heat hurts the tropical cyclone's ability to produce thunderstorms near the storm center. Without this convection,the storm rapidly fills.

An early numerical simulation (Tuleya and Kurihara 1978) had shown that a hurricane making landfall over a very moist region (i.e. mainly swamp) so that surface evaporation is unchanged, intensification may result. However, a more recent study (Tuleya 1994) that has a more realistic treatment of surface conditions found that even over a swampy area a hurricane would weaken because of limited heat sources. Indeed, nature conducted this experiment during Andrew as the hurricane traversed the very wet Everglades, Big Cypress and Corkscrew Swamp areas of southwest Florida. Andrew weakened dramatically: peak winds decreased about 33% and the sea level pressure in the eye filled 19 mb (Powell and Houston 1996)."

Courtesy: NOAA Hurricane Research Division

It is the offseason, so I thought, "why not a hurricane trick question?"

[Tampa Bay Hurricane]

Excellent Information

[StormScanWx]

This is a GREAT read!

[gatorcane]

This is a GREAT read!

Thanks, I thought it was rather interesting and something for us to chew on in the off season...

[benny]

The lack of heat source is the best guess right now... not friction. Friction could theoretically help to spin the cyclone up a little by increasing low-level convergence. However the boundary layer of the tropical cyclone just gets too stable over land and that is what starts the spindown.

[Ixolib]

The lack of heat source is the best guess right now... not friction. Friction could theoretically help to spin the cyclone up a little by increasing low-level convergence. However the boundary layer of the tropical cyclone just gets too stable over land and that is what starts the spindown.

Interesting concept. I've never been much of a believer in the friction theory. Stability of the atmosphere (all things being relative) and lack of warm, 100% moisture sounds much more plausable.

[HurricaneHunter914]

Thanks for the article I now alot more about hurricane landfalls.

[boca]

good read boca

[Aslkahuna]

The gust factor is why we use a 1.4-1.6 ratio for wind gusts after landfall-a figure that was determined by JTWC.

Steve

[TSmith274]

Yeah really, good read. I've always wondered the same thing... especially being in SE Louisiana... mostly marsh. Thanks for posting it.

[Cookiely]

Can you tell me what would happen to a hurricane that slowly follows the coast with say part of the eye in the water and part on land. Would the surge be less, would the winds be diminished? Has this ever happened before? Is this even possible?

[Normandy]

This means a cat5 (165 mph winds) could potentially produce a 230 mph gust overland using the ratio.

If Wilma had struck land at her peak 9185 mph), then she could have produced a 260 mph gust overland.

Scary stuff.

[Derek Ortt]

Katrina did not weaken as much because it being a weaker storm, did not require as much energy to sustain itself as did Andrew

[Extremeweatherguy]

This means a cat5 (165 mph winds) could potentially produce a 230 mph gust overland using the ratio.

If Wilma had struck land at her peak 9185 mph), then she could have produced a 260 mph gust overland.

Scary stuff.

This is also why the Houston area is so vulnerable. If a Cat. 4 with, let's say, 145mph winds hit the Galveston area and then curved back up through Houston..and then had 100mph sustained winds in Houston...gusts could still reach 120+mph (the same power as an F2 tornado).

[Aslkahuna]

Remember that surface roughness will decrease the sustained winds as mentioned above so the sustained winds would not have been 165 mph over land but probably closer to 130-140 mph with gusts of 195-210 mph using a 1.5 ratio (splitting the difference). A situation where you have the eye of a storm moving parallell to the coast is usually not the worst surge potential, though in the case of the East Coast of the US it could cause a big set up with the onshore winds, depending upon the configuration of the coast line, etc. a perpendicular strike where the surge is driven directly onshore is the worst-this is why Katrina was so bad-large intense storm with a big surge buildup driving in perpendicular to a coastal region prone to extreme storm surges. Katrina was, in effect the same as a moderate sized major typhoon.

Steve

[Normandy]

Remember that surface roughness will decrease the sustained winds as mentioned above so the sustained winds would not have been 165 mph over land but probably closer to 130-140 mph with gusts of 195-210 mph using a 1.5 ratio (splitting the difference). A situation where you have the eye of a storm moving parallell to the coast is usually not the worst surge potential, though in the case of the East Coast of the US it could cause a big set up with the onshore winds, depending upon the configuration of the coast line, etc. a perpendicular strike where the surge is driven directly onshore is the worst-this is why Katrina was so bad-large intense storm with a big surge buildup driving in perpendicular to a coastal region prone to extreme storm surges. Katrina was, in effect the same as a moderate sized major typhoon.

Steve

Thats true, but areas immediately near the coast (within 5 miles) would experience some of the worst winds (Like say Miami Beach). Thats what I was taking into account.

[Aslkahuna]

The immediate coastal areas with onshore winds will get the overwater winds which for a 165 mph storm which would mean gusts 0f 190-210 mph also. Overwater winds are about 1.25 for the gust ratio.

Steve

[Hybridstorm_November2001]

There are certain cases in which storms can also be Barocliniclly enhanced in the Mid-Latitudes, due to interaction with troughs that off set the effects of an unfavorable environment to some degree. In such cases they weaken more slowly over land.

[Aslkahuna]

Yes, that's the case and in the case where former Tropical systems come onshore they can be stronger than the original Tropical system-case in point, the Columbus Day Storm of 1962 in the Pac NW.

Steve

[gatorcane]

There are certain cases in which storms can also be Barocliniclly enhanced in the Mid-Latitudes, due to interaction with troughs that off set the effects of an unfavorable environment to some degree. In such cases they weaken more slowly over land

Yes I agree. Do you think Wilma was a good example of this? There was a powerful trough digging down into FL that I think baroclinically enhanced her. Also the friction with land caused CAT 3 "bursts" judging by isolated severe cases of damage here.

Katrina did not weaken as much because it being a weaker storm, did not require as much energy to sustain itself as did Andrew

Thanks, that is what I thought also.