The Southern Alps effect on Chch rainfall

[Rich]

Hi all, I was wondering if anyone could possibly explain what you think the likely effect on Christchurch's climate would be if their was no mountain range to the west? greatly increased rainfall?

While for the most part I enjoy living in Christchurch I've always idealized the 1200 + mm's of rain that Wellington and Auckland seem to get. what do you think would it take for christchurch to get similar annual average rainfall as WLG or AKL?

We seem to get cheated out of the vast majority of decent heavy rainfall here unfortunately. For me personally Christchurch would have the near perfect climate if we could have a consistent (bare) minimum of 500mm preferably 650 to 700mm of additional rainfall annually.

Any thoughts?

[Manukau heads obs]

not easy to answer
because there is no where in the south island to use as a comparison,...i.e a place not affected by the southern helps or other ranges re rainfall
only place would be maybe the sothern end of stewart island but even that has a rain shadow affect I would say in a northerly from Fiordland
if you really are looking for an answer then I would say the annual rain total would be higher , due to the higher rain totals from the fronts that cross the SI than the NI

[NZstorm]

if anyone could possibly explain what you think the likely effect on Christchurch's climate would be if their was no mountain range to the west? greatly increased rainfall?

Its summer rainfall in particular would be higher due to more convection. And of course there would be more frontal rain from the west. Such a climate could be quite a wet one.

[Nev]

Sounds a little like a typical school homework assignment question?

MetService's online 'Learning Centre' once had some great info. Shame that after they upgraded their website a few years ago, it now appears to be only a shadow of its former self.

Yes, the Southern Alps tend to block rain from prevailing westerlies (see Southern Alps/Rainfall grahpic). Probably one of the most extreme examples being in Nov 2006, when over 300mm fell in a single 24-hour period at Cropp Valley on the western side of the Southern Alps. However, due to the fohn (or foehn) effect, that same wind brought a dry, mild spring day to Chch, with not a drop of rain.

[RWood]

While for the most part I enjoy living in Christchurch I've always idealized the 1200 + mm's of rain that Wellington and Auckland seem to get. what do you think would it take for christchurch to get similar annual average rainfall as WLG or AKL?

We seem to get cheated out of the vast majority of decent heavy rainfall here unfortunately. For me personally Christchurch would have the near perfect climate if we could have a consistent (bare) minimum of 500mm preferably 650 to 700mm of additional rainfall annually.

Any thoughts?

Well, you do average about 650mm, a lot more than Alexandra's 345mm or so. I live in a 1200+mm climate - and unfortunately it's been considerably higher several times in the last 6-7 years. I'm envious of lower totals! I reckon I've seen enough rain to last me for about 3 lifetimes.

[spwill]

Give consideration to how the rain falls, eg. 3mm of drizzle with 8C or 30mm of rain with 18C.

Christchurch's climate

Without the Alps rethink NewZealand climate. Christchurch would be a lot wetter

[Michael]

It would be like Wanganui's rain,due to no lift as theres no ranges,not excessive but enough when westerly,though still get easterly weather(Wanganui sheltered by the plateau and ranges) and still get southery drizzle/showers.
Obviously incidently westcoast rain wouldnt be extreme either being quite high relative but only more than Chch probably 100-200mm more a year.Also the south island be windier more times than its currently.

[Razor]

I reckon without the mountains we would have a slightly warmer version of Stewart Island's climate- ie 25 seasons each day

[Richard]

I would imagine it would be more like the type of weather that say Ireland would get where there rainfall is more evenly spread,but also in the South Island's case, the West Coast would be dryer due to not having that blocking effect from mountain ranges.
Take today for an example,instead of the showers only just clipping Banks Peninsular like they were,all of Canterbury would be effected by them

[Orion]

Give consideration to how the rain falls, eg. 3mm of drizzle with 8C or 30mm of rain with 18C.

Having lived with both, I think the warmer option would be preferable!

[spwill]

Warmer air holds more moisture, the eastern side of the SouthIsland tends to get the rain on the cooler side of weather systems due to those Alps.

[Rich]

Thanks for all the interesting replies everyone

It seems the consensus is it'd be considerably wetter just as I suspected !


Hopefully I won't dumb down the forum since I'm something of a weather novice so probably a bit out of my depth on here. To put this in context I wasn't particularly interested in weather until I really got into gardening a few years ago and found out how limiting the Christchurch climate is when one wants to grow plants that are really only suited to growing in coastal areas north of about 38 degrees lat-


nev- lol yes I suppose I wrote that without really thinking it through but I can understand why it might've sounded like a 6th form geography essay question.


rwood- But Alexandra has an appalling climate , I could handle the winter cold...kind of, but the lack of rain would just be completely unacceptable. Once again as an avid gardener I dislike "dry" vehemently, anyway, it looks as though you're a resident of wellington which is a truly splendid city with an abundance of rainfall & magnificent, lush, perpetually verdant hills on all sides...I on the other hand live south of the Christchurch CBD and all I get here to look at are the hideous parched tussock covered port hills but anyway I digress (I'm suffering from a serious case of climate envy)

[Michael]

West coast would be much drier,more showers(than it is currently) winds would be westerly over the whole island rather than N or NE on the westcoast prefrontal,and Chch wouldnt be NE as it have no lee troughs unless a true NE,It be westerly too given the same situation.

[Nev]

Hi Rich and welcome to the forum.


Sorry, nothing personal about that comment - just my suspicious nature I guess, as it wouldn't be the first time we've been asked to answer direct homework questions. Interesting discussion though and we're normally more than happy to try and answer any weather-related querries and welcome anyone with even the slightest interest in weather.

Without the Alps rethink NewZealand climate.

Well put!
Imagine Wgtn without the Southern Alps to deflect and enhance winds? Just wouldn't be Wgtn, lol

[David]

Imagine Wgtn without the Southern Alps to deflect and enhance winds? Just wouldn't be Wgtn, lol

Isn't it mainly the Cook Strait and the Tararua ranges that create Wellington's strong winds?

[Richard]

Isn't it mainly the Cook Strait and the Tararua ranges that create Wellington's strong winds?

Yep but also the Kaikoura ranges would help increase the wind flow also

[Twizel Dave]

Hello! Since the end of March we have had no more than half a dozen days that weren't fine in Twizel. None of the predicted snow arrived here either although Lake Tekapo saw a light dusting that had melted away well before lunchtime. Often you can see that the weather is rubbishy over on the west coast side of the Alps but it usually stays lovely on this side. High country autumns are usually lovely and this year has been no exception, my estimate of the number of completely clear days since the end of March being around 50 - give or take a couple at most.

[snowstormwatcher]

Hello! Since the end of March we have had no more than half a dozen days that weren't fine in Twizel. None of the predicted snow arrived here either although Lake Tekapo saw a light dusting that had melted away well before lunchtime. Often you can see that the weather is rubbishy over on the west coast side of the Alps but it usually stays lovely on this side. High country autumns are usually lovely and this year has been no exception, my estimate of the number of completely clear days since the end of March being around 50 - give or take a couple at most.

I think the Hunter hills and other eastern ranges are more responsible for the great climate up in the Mackenzie basin. While its been good weather here it certainly hasn't been anywhere near as good as Tekapo, I was up there most days last month and it was very mild.

Getting back the the topic without the alps I think The whole climate of the east coast would be cooler wetter and windier. Would be interesting how much wetter it would be As Christchurch would still have the Banks Peninsula Sheltering effect during South/Southeast events.

[spwill]

. Would be interesting how much wetter it would be As Christchurch would still have the Banks Peninsula Sheltering effect during South/Southeast events.

The Alps reduce the dewpoint so reduce instability. A lot of rain would come from the west esp in Summer with a lot more thunderstorms. I reckon Canterbury would be a hot spot for Summer thunderstorms.

[NZstorm]

I think the mean temperature would be higher due to slightly milder winters and higher night time minimums all year round.

[snowstormwatcher]

I wonder how much more snow the east coast would get without the alps? Obviously it would be more exposed to very cold west/Southwest airflow's, But maybe not as much snow from the south/southeast as there wouldn't be the cooling effect from the southern alps. Probably less risk of heavy warm advection snow like 2006?

[Michael]

If there were No ranges whatsover in N.Z we'd be like Englands climate without the continental influence of europe but just a tad warmer than England in winter especially the S.I.

Imagine Wgtn without the Southern Alps to deflect and enhance winds? Just wouldn't be Wgtn, lol

Isn't it mainly the Cook Strait and the Tararua ranges that create Wellington's strong winds?

[Cook]

"Hi all, I was wondering if anyone could possibly explain what you think the likely effect on Christchurch's climate would be if their was no mountain range to the west? greatly increased rainfall?"

Rainfall would increase in westerlies but it might actually be less in an easterly rainfall event. Even though the Alps are quite a distance from CH they enhance upward motion and therefore rain in easterlies. On the whole rainfall would certainly be more in CH without the Alps since rain from the west is more common.

Other effects would be higher relative humidity on average, colder mean temperatures and more cloud. Westerlies across the South Island are common and they the bring warm, dry, sunny and windy conditions to CH. Without the Alps it would be wet, cool, cloudy and perhaps not quite so windy.

[NZstorm]

Hokitika Airport and Christchurch Airport have similar annual mean temperatures, so the net affect of the Alps on the annual mean temperature is possibly small.

[Michael]

Christchuch probably wouldnt get NE unless true flow,the wind directions would be more varied on the whole and have no shelter any direction though less showers than Holkiatika in westerlies.