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#1414124 - 22/03/2017 09:49 Rain moving in intersecting directions
David Findlay Offline
Weather Freak

Registered: 23/12/2001
Posts: 865
Loc: Woody Point, Qld
On this morning's Brisbane radar there's what's clearly convective cells coming in from the NE from offshore. There's also a band of more difuse looking rain(I'd suspect from stratus style clouds) coming from the north west. Yesterday this happened as well.

My understanding of this is that the rain forming clouds are at different levels moving in different steering and this can produce significant heavy rainfall.

I remember someone calling it "upslide". Could someone give me a better explanation of this? My wife asked me to explain it and I realised I couldn't.
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#1414167 - 22/03/2017 12:08 Re: Rain moving in intersecting directions [Re: David Findlay]
Snapper22lb Offline
Weather Freak

Registered: 17/02/2015
Posts: 148
Loc: Golden Beach
Can clearly see the interaction on Stapy 128 Radar behind Caloundra now. It is Causing increased precip along a clear line/front.

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#1414169 - 22/03/2017 12:14 Re: Rain moving in intersecting directions [Re: Snapper22lb]
David Findlay Offline
Weather Freak

Registered: 23/12/2001
Posts: 865
Loc: Woody Point, Qld
I'll capture a gif for the thread later. Can anyone explain how teh interaction actually works? I've got a vague idea, but can't really explain it.
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#1414173 - 22/03/2017 12:29 Re: Rain moving in intersecting directions [Re: David Findlay]
retired weather man Offline
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Registered: 01/07/2007
Posts: 4237
Loc: Wynnum
Is also a form of convergence with 2 air streams meeting. Can lead to good falls in the convergence area, particularly if hills/ranges are involved.
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#1414175 - 22/03/2017 12:34 Re: Rain moving in intersecting directions [Re: David Findlay]
Ken Kato Offline
Weatherzone Addict

Registered: 05/03/2012
Posts: 4395
Yeah that was probably me.

The simplest way to explain it is to use an extreme/classic example where you have say a surface low.
On the eastern side of the low, the low level northerly winds drag warm air down (warm air advection) across a thermal gradient where the warm air runs into cooler air to the south then slides up and over it. This causes upslide and if there's enough moisture, cloud and precip.

I've drawn a diagram below that shows the low level winds (black arrows) on the eastern side of the low and the thermal gradient shown by the coloured lines. If you took a sounding in that area, there'll be northerly winds in the low levels backing around to a more westerly flow higher up. This is also the basis of the crossed winds rule (that dates back to the old days) in identifying if there's a low and inclement weather upstream of you if you're in the midlatitudes.

The opposite happens on the other side of the low where you get downslide.

This is a textbook example but you can also get upslide without a surface low as long as the flow is essentially running across a warm to cool thermal gradient.

Jeff Callaghan, one of the most experienced severe weather gurus in the Bureau (now retired) and Samantha (can't remember her last name offhand.. maybe Taylor) were one of the pioneers in the research into this phenomenon and quantified the correlation between the amount of upslide/backing winds with height profiles and past heavy rain events on the east coast. The phenomenon is also sometimes known by its affectionate nickname of snake-oil.

Nowadays, there's dedicated forecast products from models such as the EC which specifically show the amount of forecast upslide/warm air advection.

My explanation above is a bit of an oversimplification though. It's more correct to say that isentropic upslide occurs because air parcels moving horizontally tend to follow the same surfaces of potential temperature at different heights of the atmosphere... so if a potential temp surface is sloping upwards, the air parcel will tend to follow it upwards as well.

So to put it in simple terms, if you "face" towards the direction from where low level precip or winds are coming from, and higher level precip or winds are coming in at an angle from your left (not right), it's often a sign of backing winds with height which in turn is often a sign of isentropic upslide/warm air advection which in turn can indicate the potential for enhanced cloud/precip. Of course, if there isn't enough moisture, you won't get unsettled weather and not all cases of backing winds with height lead to lots of cloud/precip depending on the setup.





Edited by Ken Kato (22/03/2017 12:38)
Edit Reason: added stuff

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#1414184 - 22/03/2017 13:02 Re: Rain moving in intersecting directions [Re: David Findlay]
David Findlay Offline
Weather Freak

Registered: 23/12/2001
Posts: 865
Loc: Woody Point, Qld
Thanks. So why is there rain at two levels? Why high level rain, what sort of clouds are producing it? The low level rain is coming from convection produced by the instability on that eastern side of the low? So the warm air rising over the cooler air to the south produces instability or is it just forcing?
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#1414208 - 22/03/2017 14:38 Re: Rain moving in intersecting directions [Re: David Findlay]
Mike Hauber Offline
Weatherzone Addict

Registered: 13/07/2007
Posts: 2506
Loc: Buderim
I believe it is warm air underneath cooler air that is rising.

The rain from NW is kind of like anvil rain, with the rain from NE lower based cumulus/convective showers. The range will help push the clouds/rain into higher levels where the winds are more westerly/north westerly.

In a more violent uplift situation the change in wind direction also causes rotation of the updraft, which can lead to supercells and possibly tornadoes. IT can also knock the updraft on its side which can move water droplets up and down several times through the freezing layer so that large hail can grow.

I've never been able to understand whether the backing with height wind causes additional factors beyond cool air above warm air to enhance rain. With a severe thunderstorm the result is obviously very different if winds are at the same direction surface and with height even if the temperature gradients and thermal instability are the same. If we could get warm air at ground and cold air above with winds in the same direction would that lead to rain just as heavy as the classical backing with height scenario? Or is it just that high thermal instability and a moist NE ground flow is much more likely to happen in a backing with height wind profile - on the NE quadrant of an upper low?

Also upper level divergence also helps rainfall. Upper level NW on NE quadrant of upper level low are likely to be diverging from the more general upper level W flow to the north.

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#1414214 - 22/03/2017 15:06 Re: Rain moving in intersecting directions [Re: David Findlay]
Ken Kato Offline
Weatherzone Addict

Registered: 05/03/2012
Posts: 4395
It's the conveyor belt of warm air that slides up an isentropic surface due to running into the warm->cool gradient that isentropic upslide is all about, not cool air rising up above warmer air.
It'll always be cooler aloft than down low if you just look at the profile over any given point of the earth's surface but it's the *horizontal* advection of the airflow into a region where temps aren't the same at that particular height which causes the air parcels to start sliding up or down those heights (specifically referring to upslide or downslide). Think of it as what happens when warm air slides up over colder air below along a warm front.
Of course a warm moist unstable airmass with NE'lies will help but that upslide
mechanism is pretty powerful when the setup's right and can cause a lot more precip than otherwise would be the case.

It's also why the max upslide is typically found on the SE quadrant of lows where the rain and winds are often the strongest because the low level flow is the most perpendicular to the thermal gradient its forced to cross.
Thats also why that area of max upmotion stands out like dogs b**ls on upmotion diagnostics and analysis maps.

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#1414218 - 22/03/2017 15:14 Re: Rain moving in intersecting directions [Re: David Findlay]
David Findlay Offline
Weather Freak

Registered: 23/12/2001
Posts: 865
Loc: Woody Point, Qld
So that explains the strong convection that's coming in from the north east. Why is there also wider bands of rains coming from the north west? I understand that the wind higher is more from the west. Is the rain there formed as part of the same mechanism or does it just happen to be there?
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#1414242 - 22/03/2017 15:54 Re: Rain moving in intersecting directions [Re: David Findlay]
Mike Hauber Offline
Weatherzone Addict

Registered: 13/07/2007
Posts: 2506
Loc: Buderim
So a parcel of air at 925hp level may be at 20 deg celcius, with temps at 500hp at -6 degrees. This parcel moves 500k south due to warm air avection, and is now under 500hp air at - 8 degrees.

Will this air parcel rise more than a similar parcel of air at 20 degrees that stays at the same location while 500hp temps cool from -6 to -8 degrees?

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#1414266 - 22/03/2017 16:47 Re: Rain moving in intersecting directions [Re: David Findlay]
Ken Kato Offline
Weatherzone Addict

Registered: 05/03/2012
Posts: 4395
Originally Posted By: David Findlay
So that explains the strong convection that's coming in from the north east. Why is there also wider bands of rains coming from the north west? I understand that the wind higher is more from the west. Is the rain there formed as part of the same mechanism or does it just happen to be there?

I suspect a lot of the precip from the higher levels is from the plume of moist NW'lies in the mid and upper levels being lifted by the sharp mid/upper trough stretching down from the Coral Sea and over inland eastern Australia... and some from the upslide aloft.
There's a lot of moisture in the mid/upper flow atm. I haven't had a look at any specific upslide diagnostics for today but judging from the soundings, the WAA doesn't look TOO strong here atm although there is some present.

Originally Posted By: Mike Hauber
So a parcel of air at 925hp level may be at 20 deg celcius, with temps at 500hp at -6 degrees. This parcel moves 500k south due to warm air avection, and is now under 500hp air at - 8 degrees.

Will this air parcel rise more than a similar parcel of air at 20 degrees that stays at the same location while 500hp temps cool from -6 to -8 degrees?

Basically yes, if that southward advecting first parcel of air encounters an upward sloping potential temperature surface especially if that surface keeps sloping up past the height at which the latter parcel of air would lose its buoyancy at even with the same environmental lapse rate i.e. the upward sloping isentropic surface gives it a big helping hand. Obviously in reality, you have to factor in things like the time it takes for the first parcel to advect, how strong the WAA is, how much heat/moisture the parcel loses on its way up, etc but isentropic upslide is a fundamental influence that's been used extensively by many weather agencies around the world for a number of years as an important factor in diagnosing areas where there's potential for elevated embedded convection, enhanced rainfall/snowfall, etc in the favourable quadrant of significant storm systems.
By the way, when cold air flowing horizontally encounters warm air at the same height, it'll never want to naturally rise up over the warmer air since it's more dense - even forced ascent of the cold air will be quickly counteracted by the fact that it's "heavier" than the warm air e.g. cold air invading a warm airmass will undercut the warm air as it plows into it which is what happens at a cold front. Likewise, warm air flowing horizontally and encountering colder air at the same height will never naturally want to sink underneath it - it'll naturally want to slide up over the top of it... as what happens with a warm front.

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#1419306 - 30/03/2017 07:05 Re: Rain moving in intersecting directions [Re: David Findlay]
explorer Offline
Weather Freak

Registered: 29/01/2011
Posts: 522
Loc: Coolum Beach, Qld
Received about 65mm overnight at Yandina Creek, about 6km in from Coolum Beach. It has been an on/off pattern of heavy rain all night .... Still going ... good soaking ... Not sure if this is the correct thread to report this ...


Edited by explorer (30/03/2017 07:07)

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