Topic: Tropical Lows - what are they?

[Richary]

Last night the tropical low that formed in the Gulf of Carpenteria crossed Cape York Peninsula. The low is now in the Coral Sea, although it is not a "tropical low" at this stage,

Time for me to be confused then. It's a low pressure system that was a tropical low, but now isn't. Even though it is still in the tropics. So a tropical low is what? Something that has the potential to become a cyclone, or some special version of a low? Time to learn a bit more

[Paul D]

It should have to do with the structure of the low / how organised it is / amount of rotation, along with windspeeds.
now who has the proper Aust definition...


PD
 

[Colin Maitland]

You need a dictionary to understand all the the terminology of the process of a low forming into a cyclone.  


The simplest way I try to look at the difference is: A low pressure system will develop in the tropics and as it deepens and the process of cyclogenesis begins (the development or strengthening of cyclonic circulation in the atmosphere, or in the low pressure system) it will form into a tropical low, usually you will notice the Barometer drop below 1000 as it strengthens as well. As the cyclogenesis continues due to the favourable conditions, it will then form into a cyclone. The pressure readings will quickly drop, the lower they become the more intense and dangerous the cyclone will tend to be.

Each of the systems is defined by pressure, circulation and wind speed etc.

To me I always looked at is a 3 step process, a low, forms into a tropical low and then cyclone.

There is definitely a lot more to it than that when you research it, but this is a very very simple way of looking at it.

[Paul D]

I guess the question of what point at which a normal low changes to a tropical low is a slightly variable one, but is there any sort of cut off or defining attributes which can be assigned to say why a low is one or the other?
eg if one low shows potential for self amplification, ie determined that it is likely to encounter an area where it may self amplify is this still a normal low?
Vs another low which starts showing the initial signs that it has encountered the right conditions / starts showing initial signs of development.
Also what are the signs used to define the two? Does it come down to the measurable observations or visible observations such as structure/organisation, or just simply convection process which are starting to differ from normal patterns in non-tropical low?.

PD

[Richary]

I think I'm still confused but that's probably a normal state of affairs.

Given what is stated so far a "tropical low" differs from a normal one in terms of rotation etc - which isn't something that is reflected in the name. So a "tropical low" could in theory form outside the tropics, I wonder if some of the deep lows that pass Sydney moving down the coast some winters bringing freezing rain and winds could fit into the category. And other lows in the tropics may not exhibit those characteristics.

If those assumptions are right then perhaps it needs a name change :-)

[Colin Maitland]

Basically a low will have a cut off point in statstics and calculations that will define it from being a low or a tropical low. You also have the monsoonal lows in the equation that can deepen and become topical lows or rain depressions.

There are so many different classifications of lows that form.

With the lows that form of the southern coast they could be classified as sub tropical or polar lows, as well as other low pressure systems, which can in effect cause as much damage as a tropical cyclone given the right conditions.

It does become very deep and mathematically it appears to be a nightmare looking at all the formulas involved in classifying lows and tropical lows etc. John Allen would be good at technically explaining this.

A good link to get started with is http://www.islandnet.com/~see/weather/almanac/arc2006/alm06jan.htm

As you read you can expand and google technicalities.

Hopefully this helped. There is a lot in it and becomes very deep and interesting.

Cheers
Col

[Richary]

Thanks Col, an interesting read on that one. Doesn't quite explain when a sub-tropical low changes into no longer being one, or quite explain the sometimes quite intense lows experienced here in winter, where a series of low pressure zones will travel down the NSW coast bringing strong winds and sometimes heavy rain.

I guess in the first point it might be that in crossing land is has lost it's circulation pattern so no longer fits the model, though when it moves back over the ocean it might re-intensify and redevelop that pattern?

[Colin Maitland]

The term that BOM uses for these lows that form in the southern half of Australia are called "East Coast Lows".

This probably is the sight that best explains these lows http://www.bom.gov.au/weather/nsw/sevwx/facts/ecl.shtml

BOM explains the difference between these lows and a tropical cyclone as well as some significant weather events that have occurred since recording detail data from 1973.


An extract from BOMS sight states:

East Coast Lows (ECL) are intense low-pressure systems which occur on average several times each year off the eastern coast of Australia, in particular southern Queensland, NSW and eastern Victoria. Although they can occur at any time of the year, they are more common during Autumn and Winter with a maximum frequency in June. East Coast Lows will often intensify rapidly overnight making them one of the more dangerous weather systems to affect the NSW coast. East coast lows are also observed off the coast of Africa and America and are sometimes known as east coast cyclones.

How do they form?

East Coast Lows may form in a variety of weather situations. In summer they can be ex-tropical cyclones. At other times of the year, they will most often develop rapidly just offshore within a pre-existing trough of low pressure due to favourable conditions in the upper atmosphere. ECL's may also develop in the wake of a cold front moving across from Victoria into the Tasman Sea. The sea surface temperature gradients associated with the warm eddies of the East Australian Current also contribute to the development of the lows.

The gales and heavy rain occur on and near the coast south of the low centre, while to the north of the low there can be clear skies. The challenge for forecasters is to accurately predict the location and movement of the centre of the low.



Cheers

Col


 

[Paul D]

Tropical Low: A localised region of convergence and uplift, characterised by active deep convection (so very tall thunderstorms) with a low-level circulation centre (an obvious stationary point within the rotating mass of cloud). It also has a deep warm core, (IE elevated temperatures within the circulation centre). These are precursors to tropical cyclones. Also requires relatively warm SSTs to occurs (thresholds are relative to air temperature).

Also

Initiation by tropical planetary wave: This is what happens over the Atlantic, When a low-level circulation centre is identifiable and sufficient and sustained deep convection exists the system is identified as a tropical low/depression and produces surface winds in excess of 25 knots it is classified as a tropical low.

John for the Atlantic systems you state a defining set of characteristics for a developing Tropical Low:- ID'd LLC, sustained deep Conv., and a classified Tropical Low surf winds > 25 knts etc
From this for the Aust classification would it be correct to say the main difference, seeings many characteristics can be shared, e.g. deep convection, is the depth of the warm core and in a developing (tropical) system the rate of LLC?
I gather part of the confusion lies within the reasoning that in Aust term Depression seems reserved for decaying cyclones which have drooped below cyclone intensity.

To the south of Aust you sometimes hear of lows which approach/reach cyclone intensity. I gather these fall into the mid latitude systems you mention John.

tropical cyclones are not as distinct as they appear, transition occurs between the various classes regularly

What are the different classes where transition occurs, how does their structure and formation differ from the Tropical warm SST variety?
Also with ECL's John you mentioned they "characterised by an Upper-Level Cold Air Mass" and

East Coast Lows.. often develop rapidly just offshore within a pre-existing trough of low pressure due to favourable conditions in the upper atmosphere

Speer M et al 2009 mentions 5 types of non tropical cyclone related lows and states their explosive development is related to "influence of a strong sub tropical jet or polar front jet above"
I gather that is what you suggested Col by favourable conditions? or the same as "upper level cold air mass"?   Also how does the influence of the STJ or PJF cause the further development (explosive development) of the ECL's and how does this differ in life cycle to other mid latitude "cyclone" development?

Cheers
Paul

Hope thats not too many questions.. feel free to throw in refs if any of the explanations are too in depth to be able to post easily.

[Richary]

Interesting answers from the experts, thanks guys. I suspect it will take a couple of re reads to get it all sorted in the head!

Will enjoy reading the BOM definition when I have more time as well.