Monday 13 February 2023

13/2/2023: Gabrielle

It's quite clear that I don't write very personal stuffs here very least explicitly, but sometimes it's hard not to do so.

Auckland has experienced the most extreme weather that is not seen since I was there, but the same sentence applies to everyone living there with no exception -- the recent weather has been the most extreme ever recorded. Janurary 2023 turned out to be a record wet month even without the storm on the 27th. Then there comes the storm with 200 mm+ in 6 hours (actually maybe in 3-4 hours).

When I first arrived the country and started reading the forecast, I always came up with the question: why do you guys call a rain at 6 mm/h heavy? And then when it actually rained I questioned: why is it like raining shit outside and you guys call that a 5 mm/h? 

Well I guess we know the answer somehow after such storm. Some cities are simply not built to encounter such amount of water because economically that does not make sense (also possible that the government doesn't make sense...or both), but when such rare flood happens it backfires heavily. 10 mm/h of rain isn't much to tropic cities, but is already a significant amount to cause water dashing down alongside the roads of Auckland. 

Imagine cities like that, but we get 200 mm instead.

What we got is 2m of flood everywhere in the city: domain became domain lake (which is was before the Brits decided to transformed that into a cricket field), Victoria Park New World turned to be wise building its supermarket a floor above because otherwise the ground floor car park is completely submerged, and I am sure Mt Eden becomes a giant bowl of water except that I couldn't find pictures anywhere on the Internet.

Record breaking wet month followed by a torrential storm, but January ended with another hazardous wet day on the 31st where they started to call that atmospheric river descending onto the ground.

Well, at least we enjoyed a few good days in February. A bit too hot and humid, but a humid day is still better than a rainy day at this point.

Then there comes the monstrous cyclone.

As someone who's used to NW-Pacific typhoons(cyclones), experiencing an ex-tropical cyclone is definitely something new to me -- in NW-Pacific those are usually riding on Westerlies roaming at 60 km/h and is gone in a blink of an eye. 

Cyclone Gabrielle is different though. It edges near Northland slowly, even dipped towards Auckland to maximize damage. Although it was downgraded from cat 2 as it isn't a tropical one anymore, it is still a 55 knots cyclone at worst with indicators like 966hPa at center showing that it's far from just a loose "strong tropical cyclone" that we classify at T3.5 or so. 

Wind could be high but it doesn't seem to be the most troublesome factor right now. Kiwis may not used to downpours, but they are definitely familiar with strong winds. Consistent 30 kts wind with gusts up to 55 knots in the city, or 50 knots up to 80 knots gust in rural areas is uncommon but surely not rare. It is the other two hazards that cyclone brings that causes all the havoc: precipitation and surges.

Precipitation -- I don't think I need further explanations, but another 200-400 mm of rain would only cause more trouble to the still-loose ground. 

Surge on the other hand, is often overlooked but when things aligns could cause much greater problem. Simply speaking, the low pressure together with strong wind pushes water back to the shores piling up the sea level. If the flood should have been 1m and a surge of 0.5m arrives that would be a flood of 1.5m. While it looks like simple addition, any extra sea level is going to cause exponentially more flood damage both in terms of money and personal safety. As of now there are stations reporting a record breaking surge of 0.7m. Not as much in other places, but any of that would only worsen the flood.

As a math guy it's always interesting to discuss those "once in N years" statements, but I don't think this is the right time to do so. All I would say is to stay safe if you are affected/to-be-affected by the storm, and may God Defend New Zealand.

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