Winter Monsoon

[Michael]

We have been getting told day after day that there will be a sunny day in Auckland but we are getting this rain everyday-perhaps we can be given a long range forecast showers/rain till November

[NZstorm]

Winter Monsoon is a good name for it Micheal......and yes its here.
4 months of daily rain for Auckland. Although, the models do have an intense high in our vicinity later next week with a northerly.
Atleast most of the precip is just brief showers with sunny periods in between. I must be one of the few people in Auckland who like the
'winter monsoon' as it can bring very unstable weather sometimes.

1130am SW05kts, adjacent showers, 6/8cumulus, temp 17C

[NZstorm]

This cumulonimbus cloud caught my eye last week. It occured on a day of scattered heavy showers with isolated thunder in an unstable westerly flow. Cb top looks around 8000m.

Showing signs of development. Auckland in distant centre of pic.

a short time later showing signs of weakening. Single cell cumulonimbus.


As the cloud drifted into the SE, cirrus was coming off the weak anvil.


Cb looks to weaken out futher. Clumps of cirrus being formed.

[Michael]

Nice Pictures often the better ones will come in a "straight westerly" or when the SW wind is going to the south as usually in a proper SW here "the towers" are masked by the lower level cloud.

[NZ Thunderstorm Soc]

We get weak Cu development like those especially when the barometer rises up to the 1030hPa mark here in ChCh. Could be some of those, if the cold southerly Develops and the ridge kicks in, tomorrow or Wednesday !! I call it the "1030 Scenario"

JohnGaul
NZTS

[Thunder]

I don't understand John, I thought it was harder and harder for clouds to rise the higher the pressure was.

And as always Steven, good work!

Cheers
Aaron Wilkinson

[NZstorm]

Correct Aaron, you would not get high topped convective cloud in high pressure. There have been rare exceptions in the past where a summer anticyclone was under an upper cold pool. High daytime temperatures
eventually break down the low level stability. Quite rare though.

The above photos were taken on 16/06/03 around 3pm.

[TonyT]

I don't understand John, I thought it was harder and harder for clouds to rise the higher the pressure was.

Not so, although I think I know what you mean. Its the atmospheric stability which dictates how hard or easy it is for any single cloud to grow (assuming there is a forcing mechanism pushing it up to start with - eg convection (thermals), or orographic forcing (air moving over hills) etc). The stability and the forcing are independant of the actual air pressure.

What I think you are referring to is that most anticyclones contain a sinking core (which is what makes the pressures higher). This sinking core generates a temperature inversion, or at least a very stable layer, in the mid to low layers of the atmosphere, which suppresses the growth of clouds. This can happen in the middle of anticyclones whether they are at 1000hpa or 1040hpa.

Likewise, a southwest flow can be relatively unstable, and able to support cloud growth regardless of the air pressure within it.

So you are right, it is harder for clouds to grow in anticyclones, but its not to do with the absolute value of the pressure, more to do with the dynamics and the stability.

[Thunder]

Thanks Guys. This actually makes me think of the time (summer last year) that there was a big high over us and Thunderstorms were getting produced in inland Otago from what T.V ONE weather referred to as Heat Lows.

Aaron Wilkinson