Storm Australian Severe Weather Forum

Severe Weather Discussion => General Weather - all topics not current severe weather. => Topic started by: Michael Bath on 09 January 2009, 02:52:00 PM

Title: Understanding Aerological Diagrams / Soundings
Post by: Michael Bath on 09 January 2009, 02:52:00 PM
This thread has been started to help members read and understand aerological diagrams - or soundings - the atmospheric measurements taken by weather balloons. These provide a cross section of the temperature, dew point, wind speed and wind direction from the surface to the troposphere.

Some of this information will be pretty basic to many forum readers. To others it may be new. I encourage anyone to ask questions and also provide your own examples and answers. Use the image attach function as necessary.

Soundings are often included in the severe thunderstorm threads as they can provide a very clear picture of atmospheric conditions associated with the event. They are also extremely useful in non-alpine snow events too.

You can then try and identity similar features or patterns on other days to help with forecasting or nowcasting.

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

Archived soundings are available from this website:
http://soundings.bsch.au.com/index.html


(http://australiasevereweather.com/storm_news/2009/soundings/sounding1.png)

The ground is at the bottom of the chart. Working your way up the left margin is the height in hectopascals (hPa). The equivalent height in km and feet is also shown slightly to the right of the hPa scale.

The temperature scale in degrees Celsius is marked with diagonal lines connecting equal values in the bottom, top and right hand side margins.

There are two red trace lines. The one on the right is temperature, the one on the left is the dew point. You can follow the diagonal temperature scale lines and where they cross the trace it is the temperature at that height (hPa). In this example above, the 500hPa temperature is about minus 12C.  The closer the left and right lines are together the moister the atmosphere is at that level, and of course the further apart they are the drier it is.

There are also two blue trace lines. These show you the values from the previous sounding for comparison.

The time the weather balloon was released is shown at the bottom of the diagram in red. The previous sounding time/date is in blue.

Along the right hand side of the diagram are wind speed and direction barbs. They point towards the way the wind is heading but are referred to by the direction the wind is coming from. In this example all the winds in the top half of the sounding are from the west. One line on the end of barb equals 10 knots, a half barb is 5 knows, and a solid triangle barb is 50 knots. You add all the barbs together to get the totals winds speed in knots. At 200hPa, the wind is a 100knot westerly (jetstream). At 700hPa there is a 20 knot SW wind.

You now have one chart that gives you a complete picture of the temperature, moisture, wind speed and direction.

Next we have to look at some of the other lines and variables marked, then how to work out good conditions for thunderstorms, snow, rain, heat or other weather.
Title: Re: Understanding Aerological Diagrams / Soundings
Post by: Dave Nelson on 10 January 2009, 02:17:33 PM
Thanks Michael for following up on my suggestion  :)

 Even the new bit of info like .... "The closer the left and right lines are together the moister the atmosphere
 is at that level, and of course the further apart they are the drier it is."  .....    added to my knowledge,  something I
never knew before

As time goes by and you add to this teaching session looking at identifying things like   Windshear, CAP's,
  how moisture at a particular level determines if you would get low or high based storm cells,  etc,  will be great

cheers
Dave N
Title: Re: Understanding Aerological Diagrams / Soundings
Post by: Harley Pearman on 10 January 2009, 02:31:47 PM
Michael

Thank you. I am still learning how to use these. I have one question to ask. On many of these sounding charts, I see a grey line curving usually to the left. I do not understand what the grey curve means or what it is telling me. Can you please explain how the grey curve line function.

I see this grey curve on the standard NCEP Soundings or NOMAD soundings on the Stormcasts charts. Thank you.

Harley Pearman
Title: Re: Understanding Aerological Diagrams / Soundings
Post by: Jason(pato) on 10 January 2009, 05:27:22 PM
Gday Harley,

That grey line you refer to is known as the Theoretical Air Parcel Plot line (TAPP). Its generally understood that the area between this grey line and the temp trace line indicates the amount of Convective Available Potential Energy (CAPE). So if there is a greater area between the TAPP and temp trace lines then the atmosphere is more unstable but a smaller or no area between those two lines would generally mean the atmosphere is more stable. There are many other variables that come into play though, and this area should not always be taken as gospel that storms are on the cards. Hope this helps, I'm still learning about all the little nuances that Aerological Diagrams (or Skew-T diagrams) throw up which either help or hinder a potential storm day. Looking forward to more input from our more experienced weather watchers, and feel free to add anything that might expand on my brief explanation.

Cheers Jason
Title: Re: Understanding Aerological Diagrams / Soundings
Post by: Michael Bath on 12 January 2009, 09:54:42 AM
All thunderstorms require low level moisture, instability and a trigger. Severe thunderstorms also require decent wind shear - speed and/or directional change of the winds with height.

The sounding immediately shows the moisture profile. The better thunderstorm setups will have plenty of moisture in the lowest few km of the atmosphere with dry air aloft. But not too much low level moisture - you want to see the temperature and dew point lines with a few degrees of separation like in the first diagram posted above. If there is too much moisture it will be cloudy and/or raining. The temp/DP lines will show very close together or the same values in those cases.

You can also see the complete wind shear profile at a glance. Wind speeds will typically increase the higher up you go. The better thunderstorm setups will have winds backing (anticlockwise) with height in the lower few km. For example N-NE at the surface, N at 925hPa, NW at 850hpa and W by 700hPa is a typical scenario along the east coast. This turning helps establish good updraft/downdraft separation and hopefully a supercell. Of course it is not that simple.

Instability has been touched on by some comments already. We are now going to look at working out instability on analysis soundings retrieved from the BSCH archives. The data for these is actually archived BoM data stored at the University of Wyoming (US). It is presented in much the same way as the BoM diagrams but also include some extra variables. The forecast soundings available from BSCH are drawn in the same way but are based on the GFS forecast of conditions, not the analysis of actual conditions.


(http://australiasevereweather.com/storm_news/2009/soundings/sounding2.png)


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). The left grey line is a forecast surface dew point (DS), the value of which is also shown amongst the variables on the top right of the diagram. The right grey line is a forecast surface temperature (TS). These two lines will follow the angles of the other two light green dashed lines.

The LCL will be where cloud bases will initially form - ie. the first cumulus clouds. As more and more convection occurs the air will mix - and cloud bases will develop lower down.

How do we know what the right values for DS and TS should be? There are a few calculations used with the BoM using one that differs from BSCH. With the BSCH website, you can also put in your own values. Eg. Look at observations from towns near the sounding location and enter in the actual temp and dew point.

Numerous thunderstorm indices can be determined from the calculations. The most common are Lifted Index (LI) and Convective Available Potential Energy (CAPE). The high the CAPE and lower the LI the better.


(http://australiasevereweather.com/storm_news/2009/soundings/sounding3.png)


The CAP (or inversion) is a relatively warm layer of air that may delay or completely suppress the formation of thunderstorms. On the sounding it can be seen where the temperature stays the same or often increases with height. You will regularly see this showing above 900hPa. The first sounding posted in this thread has a CAP starting just below 900hPa. The 2nd sounding has none and the one just above this paragraph has quite a strong cap around 900hPa.

Inversions will also occur higher up - the most obvious one is at the tropopause where thunderstorm anvils occur. You may also see inversions in the mid levels. The 2nd sounding shown in this thread has an inversion at 550hPa. These do not necessarily stop thunderstorms but may slow updraft strength.


Lastly (for now), the trigger. This is one part of thunderstorm forecasting that you cannot see on a single sounding. Thunderstorm triggers can be from a surface trough or low pressure system, an upper trough or upper low, cold front, surface convergence, or just surface heating during the course of the afternoon.  None of these can be seen on the sounding, however you can use more than one sounding and compare upper level temperatures to help determine if an upper trough may be approaching.

Title: Re: Understanding Aerological Diagrams / Soundings
Post by: Dave Nelson on 12 January 2009, 03:57:14 PM
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
Title: Re: Understanding Aerological Diagrams / Soundings
Post by: Michael Bath on 13 January 2009, 04:25:05 AM
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
Title: Re: Understanding Aerological Diagrams / Soundings
Post by: Dave Nelson on 13 January 2009, 03:18:20 PM
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
Title: Re: Understanding Aerological Diagrams / Soundings
Post by: cloudfairy on 14 January 2009, 06:27:07 AM
soundings (http://soundings.bsch.au.com/index.html?mode=forecast) 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.
Title: Re: Understanding Aerological Diagrams / Soundings
Post by: Michael Bath on 14 January 2009, 09:39:53 AM
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.

(http://australiasevereweather.com/storm_news/2009/soundings/sounding4.png)

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.

(http://australiasevereweather.com/storm_news/2009/soundings/sounding5.png)


Title: Re: Understanding Aerological Diagrams / Soundings
Post by: Shaun Galman on 14 January 2009, 12:14:53 PM
Thanks for starting this thread idea Dave N. I too have had a few head-scratching times looking at soundings lol.

MB: Thanks for explaining it so well too! It's very easy to grasp and follow and I will definitely put it to the test asap! I usually have a quick glance at the CAPE and LI numbers then make a decision from those but now you have showed us what the lines ie; temperature/dew point traces are and how they actually work it will help tremendously!

It's hard sometimes to correlate between soundings when they are so far away from a particular area (such as us out here) when we can be receiving severe weather from the West and at the same time the Moree sounding is looking rather stale and unimpressive? I'm now looking forward to comparing the Cobar and Moree soundings the next time we have storm development taking place!

I think this thread will become very popular!
Kindest regards,
Shauno 
Title: Re: Understanding Aerological Diagrams / Soundings
Post by: David C on 14 January 2009, 01:12:24 PM
I have one question to ask. On many of these sounding charts, I see a grey line curving usually to the left. I do not understand what the grey curve means or what it is telling me. Can you please explain how the grey curve line function.

I see this grey curve on the standard NCEP Soundings or NOMAD soundings on the Stormcasts charts. Thank you.

Harley Pearman
Gday Harley,

That grey line you refer to is known as the Theoretical Air Parcel Plot line (TAPP).

You are probably referring to the TAPP as Jason suggested above, Harley.  You might also be talking about the Adiabats, and since Michael introduced the concept of lapse rates above, it is probably timely to locate the adiabats on the above skew-ts. (Cue for someone to do the photoshop!).
I think are grey in some cases hence my reply to Harley -- on the BoM soundings they are the two sets of curved, teal green of whatever they are (can damn well barely see them on this 11 inch monitor!), dashed lines in increments of 10Kelvin (hence large values on the x and y axes). 

Dry adiabats are lines of constant potential temperature. (From wikipedia - "The potential temperature of a parcel of air at pressure P is the temperature that the parcel would acquire if adiabatically brought to a standard reference pressure P0, usually 1000 millibars). In otherwords, the dry adiabats show the rate of change in temperature with height of a parcel of dry air. The slope of dry adiabats is 9.8C -- the dry adiabatic lapse rate.

Moist adiabats on a skew-t represent lines of constant equivalent potential temperature (theta-e). (many chasers who chase the US know that there we have available and often use theta-e as an indicator of convective energy in the pre-storm environment, since it is easily computed from surface data and reflects both the temperature and moisture content and therefore theta-e ridges demarcate regions where surface instability is at a maximum).

From wikipedia - "The equivalent potential temperature is the temperature a parcel of air would reach if all the water vapor in the parcel were to condense, releasing its latent heat, and the parcel was brought adiabatically to a pressure of 1000 millibars.

In otherwords, moist adiabats show the rate of change in temperature with height of a parcel of saturated air. The slope of the moist adiabats does vary since the moist adiabatic lapse rate is not constant. eg. From one of the skew-ts above, you will note that the moist adiabats have smaller slopes than dry adiabats because rising, saturated air cools more slowly than dry air (as the water vapor condenses the latent energy absorbed during evaporation is released). When lifting warm and humid air since more latent heat is released but as the parcel ascends, less condensation and therefore less latent heat release occurs -- near the top of the plot at the lowest pressures and temperatures, the slope of the moist adiabats is more or less the same as the dry adiabats.

Title: Re: Understanding Aerological Diagrams / Soundings
Post by: Harley Pearman on 14 January 2009, 02:33:13 PM
David, Jason and Michael, thank you for explaining this and of course this thread. This thread will come in useful as I have started using these when I go out. Even though I had limited grasp of how to use it properly, I have realized that I have done 4 storm chases this season using the charts and have managed to score storms of varying degree of intensity on all four occasions.

Jason, thanks for explaining the TAPP line to me as I had no idea what that was until your explanation.

Harley Pearman
Title: Re: Understanding Aerological Diagrams / Soundings
Post by: Dave Nelson on 14 January 2009, 03:41:26 PM
Thanks Michael
                    and to the input of the others as well   :)

  Some of the best teaching I have seen on the forum for ages !!!

ok  here's my next question ....

  I have noticed that in all the above soundings that the DP trace goes into wild variations in the ~ 600 - 400hPa
area.  This seems to happen to varying extents regardless of levels of stability/ instability indicated to the right
side of the sounding....

  What is happening in that region of the atmosphere that is always causing the DP to vary so wildly ?

EDIT

Additional Q I have been meaning to ask...  where does the name Skew-T come from ?

cheers
Dave N
Title: Re: Understanding Aerological Diagrams / Soundings
Post by: cloudfairy on 15 January 2009, 05:18:26 AM
The dewpoint temp isn't always varying like that. Look at this one for example.

(http://australiasevereweather.com/storm_news/2009/soundings/sounding6.png)

The Dewpoint temp is an indicator for how dry or moist the air is. So, if the slope is not steady you can see the different air layers.
The air isn't mixing as if you would stir it permanently.

Well, maybe I should leave it to Michael. I don't find the right words.

Renate
Title: Re: Understanding Aerological Diagrams / Soundings
Post by: Michael Thomas on 15 January 2009, 05:58:55 AM
Just to further that last point, I wouldn't worry too much about the amount of moisture above say 700 hPa as it has no direct effect on the amount of CAPE. That is not to say that it is not important. Too much moisture in the mid/upper troposphere is often associated with clouds which of course block out the sun and keep surface temperatures lower. This results in less CAPE and potential problems in breaking the cap. Furthermore, when the mid/upper troposphere is extremely moist the mid-level lapse rate is often quite poor. Assuming though that there is no problems with cloud cover and that mid-level lapse rates are steep, the amount of moisture above 700 hPa will affect storm characteristics, however, these are reasonably minor. For example, with dry air in the mid troposphere, dry air may be entrained in the storm's downdraft resulting in greater evaporative cooling. This makes the downdraft more negatively bouyent resulting in a stronger downdraft. This may actually in some instances have a negative impact due to the downdraft undercutting the storm's inflow region (I believe).

Michael
Title: Re: Understanding Aerological Diagrams / Soundings
Post by: Dave Nelson on 15 January 2009, 07:01:33 AM
For example, with dry air in the mid troposphere, dry air may be entrained in the storm's downdraft resulting in greater evaporative cooling. This makes the downdraft more negatively bouyent resulting in a stronger downdraft. Michael

interesting comment Michael, would this be a source of microbursts, like those that have downed aircraft ?

just a small digression from the main theme :)

Dave N
Title: Re: Understanding Aerological Diagrams / Soundings
Post by: Mike on 15 January 2009, 08:11:38 AM
Finally after all these years a thread for something that has perplexed many of us !!!  Thanks Michael et al - I'm reading feverishly and no doubt havea question or two later.
Title: Re: Understanding Aerological Diagrams / Soundings
Post by: Mike on 15 January 2009, 09:07:53 AM
With regards to the CAP and CIN:  If  the CIN number is below 20 then that is what I understand to be good as far as no great inhibition for storms to break through?  What about the CAP values?  Sometimes I see the sounding and it has a minus value next to it, so which is more preferable - a minus or without and what do they tell me?

Can the sounding tell you at what height convection will be maximized? ie: storm tower height - the colder the better?
Title: Re: Understanding Aerological Diagrams / Soundings
Post by: Michael Thomas on 15 January 2009, 11:28:17 AM
So many questions :) I will try my best. First of with capping and CIN. As far as I am aware, the cap is more of a slang term for convection inhibition (CIN). Think of it as a cap of a bottle keeping its contents in place, the cap keeps the warm moist air from rising. Since the individual letters do not stand for anything, upper case is not needed (cap vs CAP). A minor point and a little too picky maybe. I cannot not tell you what values to look at for CIN unfortunately but I can say it does vary. Sometimes a strong cap may be in place but storms will still form with strong forcing. In such situation, I strong cap can actually keep storms isolated rather than forming a squall line. This is hugely important in US where if storms remain as descrete supercells the risk of tornadoes is much higher than if too many storms form and merge to form a squall line. On the otherhand, the cap will need to be weaker if little forcing is present to intiate convection. If capping is too weak however, storms will most likely form early before peak heating. Also, numerous storms may form and interfer with each (rather than a large supercell with only warm moist air in its path).

Regarding storm height, this is easily read form a sounding. The level at which the Theoretical Air Parcel Plot line (TAPP) crosses the temperature trace is known as the Equilibrium level (EL). This is the level where storms anvil out. Note though, a strong thunderstorm can overshoot this level resulting is storm tops above the EL. For this reason, on any given day, the taller storms are most likely to be the strongest but storm height is largely dependent on the height of the EL. As you can see in the soundings below from Darwin the EL is very high compared to those of Brisbane.This would result in higher thunderstorm but not necessarily stronger storms (so many other factors are important, such as wind shear, mid-level lapse rates, surface boundaries etc). Interesting to know, the temperature of the tropopause is actually colder in the tropic than the poles and mid latitudes. This can be seen with the soundings below. Take the darwin soundings, the temperature is around -85 at 100 hPa whereas in Brisbane on the 8th of October the temperature of the tropopause is around -66.

Michael
Title: Re: Understanding Aerological Diagrams / Soundings
Post by: David C on 15 January 2009, 11:40:13 AM
With regards to the CAP and CIN:  If  the CIN number is below 20 then that is what I understand to be good as far as no great inhibition for storms to break through?  What about the CAP values?  Sometimes I see the sounding and it has a minus value next to it, so which is more preferable - a minus or without and what do they tell me?

Can the sounding tell you at what height convection will be maximized? ie: storm tower height - the colder the better?

CAPE and CIN are both obtained from a measurement (by integrating) of the area between the TAPP and the temperature trace (ELR) on Michael's skew-t's 1 and 3 above. Where the TAPP is to the right of the ELR - this is positive buoyancy = CAPE -- since the parcel (depicted by the TAPP) is warmer and therefore less dense than its surrounds and will want to rise. Where the TAPP is the the left of the ELR that is negative buoyancy = CIN (that is the parcel is cooler and therefore denser than it surrounds so it will sink). Both are usually expressed as J/kg (Cin being -ve).

Looking at the 2nd skew-t above, we can get an idea as to the height of the storm anvil's upper levels by looking at the equilibirum level, which is where TAPP and ELR again meet. In this case somewhere above the 200mb line ~13km. Of course the actual storm tops can be momentarily or, in the case of an intense, sustained updraft (mesocyclone) can be persistently higher than this equilibrium level. This is because when there is sufficient upward momentum then the parcels overshoot the equilibirum level by several km or more even though they are more dense at that point. Evenutally, they run out of puff and as storm chasers we often see this as an anvil dome or overshooting top.

CIN (SIN) is used to determine the strength of a CAP.
Title: Re: Understanding Aerological Diagrams / Soundings
Post by: Michael Bath on 16 January 2009, 03:55:43 AM
Dave: I found this one - have a search for the origins:
http://en.wikipedia.org/wiki/Skew-T_log-P_diagram

Michael T: I think CAP gets used instead of just cap so it stands out in the descriptive text.

I don't look at values for CIN - probably because that and CAPE are not even displayed on the BoM soundings for some strange reason. I guess you get to know whether the CAP will break from past experience and the setup.

MB
Title: Re: Understanding Aerological Diagrams / Soundings
Post by: Mike on 16 January 2009, 05:28:03 AM
And calculating the possible height of any forecasted storms/CUs can be had by adding one's current DP and Temp from the sounding during replotting it and multiply them by 220 ?

After talking to the Met officer at BoM he said that for here re the soundings I should be replotting the skew-t using the 50mb range as our DPs change so much that our storms get much of their energy from just about ground level!  For CAPE he indicated, that whilst there's lots of CAPE through the entire trace, if I want to look for decent strong to severe storms then cape in the lower portion of the trace is the key.  CAPE on 15th Jan on the sounding is nudging 4000 but when the storms hit town it was 5000 - hence we had two big storms and downpours in the city.  This indicates really good updraft strength in the lowers for storms rather than the 'overall' CAPE in the atmosphere.   

You can see by our sounding when replotted with the averaged obs at the time of replotting that CAPE at the 600 was very high, this is what caused the large storms to come through in the late afternoon.  Over 60+mm fell over an hour or so.  I asked him about the big slot of dry air in the mids and I always thought that this would create too much drying, but to the contrary.  MB and he confirmed that this dry air is actually beneficial to disperse the moisture and watching the storms in the afternoon it was clear to see it do just that and strong winds aloft also were aiding the spread of anvil wash which when formed, they really did fan out quickly and thin.

(http://australiasevereweather.com/storm_news/2009/soundings/sounding7.png)

As far as triggers are concerned...we talked about that also.  We put some scenarios back and forth and he agreed that good triggers for storms are troughs, boundaries and the like.  They may not be visible on maps or charts as they are not real time, but sea breezes pushing inland can cause a boundary (as in the Humpty Doo tornado) and seeing convection form in a line is a visible trigger.  He said the trigger is the $64,000 question.  It's something that chasers take more note of when searching for storms rather than forecasters and it's only when they see condition/satpics/radar change for pending severe weather that they can formulate the prognosis prior and during the storms lifespan.  Of course they have the benefit of real time imagery at their disposal - so it's a bit of an oxymoron!

This particular thread topic has helped indeed in understanding the soundings better and now at least I can decipher them for possible storms and afterward to see how the atmosphere changed when storms fired up.
Title: Re: Understanding Aerological Diagrams / Soundings
Post by: Michael Bath on 28 January 2009, 06:27:53 AM

As a side note ....  Michael B.   if you are reading this....  how much credence can you place on a sounding that is already more than 4 hours old at the time of writing this note  (1115Hrs EDT)

Hi Dave - some, or a lot :)

It really depends on how quickly overall weather patterns are moving or changing since the sounding. For example - you need to forecast whether upper level temps may have cooled, warm, or stayed about the same. Then consider whether the surface conditions have changed by using real-time obs.

There is no set rule - you consider the sounding then factor in all things that could change using model output and observations.
Title: Re: Understanding Aerological Diagrams / Soundings
Post by: Dave Nelson on 31 January 2009, 03:08:55 PM
Hi Michael,
                   thanks  :)    interestingly enough,  my comments in that other thread about the probable
lack of storms for the Sydney basin for last sat 24th held true.  and although the sounding for Sydney
showed good CAPE etc  I just had that feeling from previous experience that the storms were more likely
to fire up somewhat nth of the city.  and that they did.

as to all the other info you have presented in this thread ... I havent finished with the Q's  ;)
jst been busy with other activities and I ... when time permits... will sit down and lay out what you have
presented ...digest it ... and then get into more refined questions.

thanks so far

Dave N
Title: Re: Understanding Aerological Diagrams / Soundings
Post by: Colin Maitland on 23 February 2009, 10:46:38 AM
Thanks to everyone regarding aerological diagrams. I have printed it out and study it as much as possible.

I was wondering though, how to forecast snow on these charts. Each year, my wife and 2 daughters do a trip to Stanthorpe and then where ever we dare to venture. 2006 we ended up travelling right down to Tamworth, missed the snow by a week, (small town just north, had a good snow fall the week prior,). the day we arrived, it was approx high teens, even the receptionist from New Zealand could not believe how it had warmed up.

2007 was very cold but we missed the snow in Tenterfield by approx 4 days.

Last year, we found snow at Glen Innis. Earlier in the morning we had climbed Bald Rock, (QLD/NSW National Park) and at the peak I had a good view of the cloud formation. The wind chill factor was freezing. We drove down to Glen Innis arriving about 3.00pm on July the 9Th 2008. Light snow was falling, but we know, from talking to the locals, there were heavy falls all day at Armidale. Some of the cars coming through had a good build up of on them. We run out of time and had to head back. If I had of known I would have bypassed Stanthorpe last year and headed straight to Armidale. We have to try and play it around school holidays in QLD. (Wife grew up in Stanthorpe, so that is why we go every year. We use it is home base.)

So what are the important factors to look for, if any one can help, I will truly appreciate it.

Thanks

Colin.

I tried looking at the archive soundings but not sure which station and time to try and see what data shows the event.
Title: Re: Understanding Aerological Diagrams / Soundings
Post by: Michael Bath on 24 February 2009, 03:27:05 AM
Hi Col,

I've got a comprehensive guide to snow forecasting - have a look here:
http://australiasevereweather.com/storm_news/2007/docs/snow_chase_guide.htm

There are sounding examples included for some recent events. If you have some specific questions still not answered please then ask them here.

regards, Michael
Title: Re: Understanding Aerological Diagrams / Soundings
Post by: Colin Maitland on 24 February 2009, 05:06:07 AM
Thanks very much, I have hours of reading there, it will be of a great asset for us in our limited yearly snow trek. If this year pans out to be cold as predicted (by some forecasters) we might extend the girls holiday.

Thanks again

Colin

PS
I think this is the Aero-diagram for the 9th of July snow fall from the Moree station. I was forgetting about adding the 9 hours ( i think) for the UTC time. 
Title: Re: Understanding Aerological Diagrams / Soundings
Post by: Mike on 16 January 2010, 12:27:25 PM
I have a question or two...

Is it correct in using the previous day's DP's and the following day's DP's to calculate whether this will enhance the CAPE?  I know that often non-real time CAPE numbers are actually factored in, in the morning sounding here from the previous day/night due to high levels of DP which are higher than the actual temps.  So if one was pondering the question of okay, the DP's overnight were fairly high compared to the temp, should i factor this in when dissecting the sounding for the previous day in determining CAPE valoues for storm initiation and would potential storms be using this for formation?

On the sounding is this true when looking...CAPE values from the SFC to 500hpa are more likely to initiate mid-level storms and CAPE below 500, if good, are surface based?  If so on the sounding how can one see that updrafts reach 20C or colder to initiate electrification (lightning)?  Is it a good sloping lapse rate to this level where one should be looking?  So looking at the skew-T at what point does one pen to locate this?

Thanks guys..
Title: Re: Understanding Aerological Diagrams / Soundings
Post by: Simon McCombe on 09 December 2010, 03:32:46 PM
Hey everyone. Thanks so much for that,about trying to decipher soundings. Yes,I've been scratching my head also about what everything means. Some i have worked out myself and from finding info to help me,i will admit i am a novice. There is a couple of questions i would like to ask,the 8 green dashed lines going from the bottom (ground level) up and to the right to the 700 hPa height,is that dewpoint ?,and on the upper right-hand side where the values are,i'm gathering that the CAP value is in Celsius yes ? Again,thanks for explaining how to read the charts,starting to sink in now !
Title: Re: Understanding Aerological Diagrams / Soundings
Post by: Michael Thomas on 09 December 2010, 05:25:54 PM
"the 8 green dashed lines going from the bottom (ground level) up and to the right to the 700 hPa height, is that dewpoint?

Yes, that is the dew point lapse rate. As air rises, the dewpoint will decrease slightly even thought the amount of moisture remains unchanged.

"on the upper right-hand side where the values are,i'm gathering that the CAP value is in Celsius yes?

Yes
Title: Re: Understanding Aerological Diagrams / Soundings
Post by: Simon McCombe on 10 December 2010, 12:32:21 PM
Thanks for confirming that Michael. I have another question. In the sounding image below of where i am,the dewpoint line appears to cross over the temp. at the 200 hPa height. Is this due to the increase in windspeed?,and is this something to watch out for at that height or is it too high in the atmosphere. Is there something else i should be looking at ? Funny,its ended up being 4 questions !   
Title: Re: Understanding Aerological Diagrams / Soundings
Post by: Michael Bath on 11 December 2010, 01:00:37 AM
Hi Simon,

It would not be crossing, just 'touching' at that point. The temp will nearly always be greater than the dew point trace, but they can be equal at some points. In the example it would indicate cirrostratus clouds.

Title: Re: Understanding Aerological Diagrams / Soundings
Post by: Simon McCombe on 19 December 2010, 05:39:12 AM
Thanks Michael,sorry its taken me a little while to reply. I'm getting to know what to look for now,especially whats been happening in the last few days.