Severe Weather Discussion > General Weather - all topics not current severe weather.

Understanding Aerological Diagrams / Soundings

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Dave Nelson:
This great info Michael  :) 

  Quote..."As mentioned by Jason, the Theoretical Air Parcel Plot (TAPP) is a representation of how a parcel of air may rise from near the surface. Where the grey line is to the right of the temperature trace it is unstable. You will see that the TAPP starts at the bottom of the diagram as an inverted "V". Those two lines meet at the Lifted Condensation Level (LCL) and the single line continues upwards following the alignment of the light green dashed curve (shown on the diagram from the surface to 200hPa). " unquote

So presumably  the further to the right of the Temp trace the grey line is  the more instability there is ?
and if the grey line is to the left of the temp trace then things are relatively stable ?

in the 2nd sounding in the thread the grey line pretty much follows the temp line where in the 3rd sounding it arcs out far to the right of the sounding and doesnt even cross the temp trace  as in the 2 earlier soundings.

So I am concluding that you can determine an area of instability in the atmosphere between where the grey line crosses out from the temp trace at low levels and where it may cross back across the temp trace at a higher level ??

or if it doesnt cross back across the temp trace then the instability extends far up into the atmosphere ?

hope that query makes sense   ::)

cheers
Dave N

Michael Bath:
Hi Dave - yes that's correct. Though you have to keep in mind that the position of the grey TAPP line is a forecast based on conditions at the time of the sounding. In reality the late afternoon period a few hours after the sounding was taken can be quite different in many setups. Eg. if the surface layer moisture increases or decreases more than expected. Increasing or decreasing the dew point makes a much bigger difference to overall instability than a changing surface temperature. The best way to see this in action is to load a sounding via BSCH then type in varying values for dew point and temperature (custom temp/DP from the dropdown list below the diagram) and click replot.

The third diagram in this thread is an extremely unstable environment (though I have entered a custom temp/DP). A temp of 35 and dew point of 21 was observed at Casino (NE NSW) at 3pm and it was just enough to break the CAP and provide CAPE of about 4000 - check the severe weather thread for 30th Dec 2008 to see what happened!

The 2nd diagram in this thread does not indicate a lot of instability but in reality it was probably a fair bit more as severe storms were occurring in that Charleville region of QLD during that afternoon.

Michael

Dave Nelson:
Michael,

   OK,  your last reply leads me to the next question .....  these soundings are carried out in a limited number of
places ( I dont have a list of where)  I assume the main metro centres and a few other places.

The question...   pick on a sounding for say... for Brisbane ....  what sort of area/region is that applicable for ?

  Can you use it over a 200km radius with some sort of reliability .... more ?  less ?    I appreciate  that they may
only give a rough indication and that you use soundings from other areas to get a pattern.   But heck Sydney is a
long way from Brisbane, conditions could be VERY different.  Where do I find a list of accessable sounding locations
for Oz ?  Are they closer together than Bris - Syd. ?

so many Q's    :D

Dave N

cloudfairy:
soundings for Australia

try these :)

I think soundings are an indicator and as you said if you are in a place with no sounding station near by I don't think it makes sense to rely on one 200km away.

But what I do for Darwin (yep, we do have one) is, I have a look on the surrounding soundings. Storms usually don't form OVER Darwin. So Depending on the wind I have a look on Goves or Katherines Sounding, to get an impression if there may form storms, that could move towards me.

The size of area for which it is usable is depending on the topography, I would say and on the wind conditions.

Michael Bath:
Dave - following on from Renate's post, you can use also use the direct link to the BoM soundings:

http://www.bom.gov.au/products/reg/IDS65024/
(enter bomw0007" for userid and "aviation" for password)

You'll see a map of all available sites. You often have to use one a couple of hundred ks away - eg. Wagga. For me, I use Brisbane and Moree all the time. You can use conditions above the surface layer to help determine what you'll get downstream a few hours later. It really depends on the weather pattern at the time. For a system progressing fairly quickly across the state using the Moree sounding may give a better idea of conditions later in the day than Brisbane will. Other setups will be fairly slow moving and using Brisbane gives a much better feel for things than Moree.

The surface or boundary layer can often be quite different between the sounding site and where you live, or nearby mountains, and working this out can be a key to picking areas for storms. For example, the dew points in the Northern Rivers are often a few or several degrees higher than what you'll have at Moree or Brisbane.

Another thing to look for on soundings is the lapse rate, or how quickly the temperature falls with height. The steeper the lapse rate the stronger updrafts will be. Take a look at the following two examples.

The first is taken at Darwin. There is a steady fall in temperature with height, in fact the example below (from this morning) is not too bad for Darwin, quite often you will see warmer temperatures throughout. Note also there does not appear to be a tropopause - storms in that region at this time of year are some of the tallest thunderstorms in the world and the tropopause is "off the chart". The winds are nearly always chaotic (compared to mid-latitudes) like in this example are a reason why organised storm cells such as supercells are rare.



This next example illustrates a steep lapse rate. The green dotted lines aligned from the bottom right of the chart towards the top left show you the maximum lapse rates possible. So from 950hPa to 480hPa is about as good as it gets. Very powerful updrafts and storms develop in this environment. Given also the awesome windshear, plenty of moisture in the lower levels and dry air aloft, supercells are the likely outcome. Is that a CAP at 940hPa? Convection (clouds) would start above that level (850ish) so there is no CAP and you would expect strong storms to fire early in this setup - which is what happened. The tropopause is around 200hPa.

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