dyqik wrote: ↑Mon Aug 30, 2021 12:31 pm
Bird on a Fire wrote: ↑Sun Aug 29, 2021 11:04 pm
It's a whole category stormier than Katrina as well.
Could be quite bad.
Katrina is estimated to have had twice the energy of Ida, and its storm surge built while it was Category 5 over the ocean.
The category at landfall is only one factor in total risk/damage.
Useful for the denialists who are comparing the damage of Cat 2 Katrina vs Cat 4 Ida, to imply that hurricane reporting is hyped by the scientists.
In general category is not so well correlated with damage. There are three main vectors of damage - direct wind damage, storm surge, and rainfall. Often maximum windspeed, the thing that category shows, is not so important. Quite low level tropical storms are ofen disproportionately damaging to their category when they succeed in generate heavy rainfall. Ida has now created a state of emergency in New York and New Jersey because of massive rainfall there, which happened only after a long transit over land and reduction to a low level tropical storm or tropical depression. We have seen this in the past regularly in Haiti, and various parts of central America, that quite modest tropical storms can generate enormous flooding.
The damage from storm surge can be beyond anything that wind - tornadoes aside - can commonly do, even in high factor storms. Nothing left but the foundations kind of damage.
Hurricane Rita (2005), the same year as Hurricane Katrina, likewise was Cat 5 out in the Gulf of Mexico and had reduced to Cat 3 on landfall. It produced a storm surge that completely removed some coastal settlements.
Hurricane Michael (2018) landed on the Florida panhandle as a Cat 5, which did indeed cause very extensive wind damage in the locality of the landfall. But the
complete removal of a large part of Mexico Beach resort was due to storm surge. The
remarkable survival of one isolated building, deliberately built to survive both winds and storm surge, shows that you can build to that standard if you want to. Whether it makes economic sense to build the entire gulf coast to that standard, as opposed to accepting the occasional localised destruction of some buildings from storms, is another issue.
Category is merely the highest wind speed, but that can be focused on a small area. Large, high energy storms have difficulty maintaining maximum wind speed because that situation often promotes an
eyewall replacement cycle. Even that wikipedia article unfortunately refers* to an erc "weakening" the storm, when what it means is "reducing the maximum windspeed at the centre". This results in the extension of high windspeeds to a much larger area. So before the erc the maximum windspeed might be 70m/s (Cat 4/5 boundary), but the 32 m/s (TS/Cat 1 boundary speed) might have a diameter of only 150km. But afterwards the max windspeed might be only the 45 m/s, but the 32 m/s boundary could have spread to 500km diameter. The total energy of the storm continues growing. The reason ercs happen is precisely because outer areas of the storm are gaining in energy and "competing" with the centre. An erc is far from good news for the people in line to be hit by the storm, especially those some distance from the centre landing point.
You get a similar thing with earthquakes - RIchter scale number is not perfectly correlated with damage potential. The
2011 Christchurch Earthquake was 6.2, a level which often passes almost unremarked in relatively well-organised countries accustomed to regular large earthquakes like NZ, Japan, Chile, Iceland, etc. The very largest earthquakes have all produced terrible tsunamis, but go a bit further down the list and it is much more variable whether the resulting tsunami is large and terrible or not.
*Another terminological issue I'd like the meteorological community to reconsider is describing conditions as being "favourable" for a hurricane. Such conditions are not at all favourable in the broader sense.
