Matt's Convective Thread 1                                                                                                  

This is part of 1 of 4 convective threads dedicated to the discussion of convective variables and how they are used within forecasting and where you can get the information from.

Thread 1 covers the following specific variables that are used not only in convective forecasting but other areas of meteorology as well;

Level of Free Convection (LFC): >640ft (>750ft)
Surface Dew Point (Td): >+13C (>+18C) **summer situ**
Wet Bulb Zero Height (WBZ): >7000ft (9000ft)
Cloud Tops: >18,000ft (>25,000ft)
Cloud TT's: -18C (>-30C)


• Level of Free Convection (LFC)

The LFC is the first area/level within the atmosphere where the temperature of a parcel of air raised from the surface is larger than that of the surrounding air. This means that the parcel of air (because its warmer than the surrounding air) is free to continue to rise without the need for any further additional energy sources. The below shows what the LFC looks like on an atmospheric sounding;

http://img92.imageshack....83/brosoundinglfc9bk.gif

The height of the LFC can be an indicator as to the strength of instability within the atmosphere. Lower LFC heights are generally conducive to greater instability because it means a larger portion of the atmosphere can be unstable and also low LFC values can also signal the threat of tornadic development. Below is a quick table showing this indication;

Height of LFC Convective potential
600 mb - 640 mb Weak
640 mb - 745 mb Moderate
745 mb - 850 mb Strong


• Surface Dew Point (Td): >+13C (>+18C) **summer situation**

While convective activity can occur all year around, dew points are of great importance during the Spring and especially Summer periods. The dew point is an indicator of how much moisture there is in the air, thunderstorms and convective development thrive on lots of heat and moisture within the atmosphere, hence dew points are often high during times of active convection and thunderstorm activity. To an extent dew point temperatures can be more important than surface temperatures when it comes to convective development and thunderstorm activity.

Dew points of 13C and above are reasonable for convective activity, but values above 15C are more favourable for significant convective activity and dew point values of 18C and above are really 'juicy' indeed and bring a risk of significant convective activity and thunderstorm development.


• Wet Bulb Zero Height (WBZ): >7000ft (9000ft)

The Wet Bulb Zero Height is essentially that, the height at which the wet bulb zero isotherm is located within the atmosphere. This values is not only used within convective situations but in winter as well and when snow forecasting.

However in a summer situation the WBZ height is directly related to the development of hail and also strong convective wind gusts. The WBZ height has to be above 7000ft and below 9000ft (roughly) to bring a risk of significant hail development. This altitude is optimal for the development of hail and allows for a limited time period of melting from when the hail falls from the cloud to the surface.


• Cloud Tops: >18,000ft (>25,000ft)

Cloud top heights are obviously directly related to the intensity of convective activity within the atmosphere. In general and as a rule of thumb CB (Cumulonimbus) clouds need to grow to a height of at least 18,000ft for thunderstorm activity to occur, anything less and the cloud hasn't the available ingredients to do so. A CB cloud that has a cloud top in excess of 25,000ft would signal an unstable atmosphere and a well developed cloud and a solid risk of thunderstorm activity. We have all read the reports of cloud tops to 40 and 50,000ft over in the states and obviously when these level of cloud heights are forecast you know the end result is some serious amounts of convective activity.


• Cloud TT's: -18C (>-30C)

Cloud Top Temperatures (TT's) are directly related to the height of the cloud. The greater the height of the cloud the colder the cloud top and hence the more well developed the cloud. In essence the colder the cloud top the better for signalling significant convection and a thunderstorm risk. Cloud top temperatures of -30C are needed generally to produce a significant risk of thunderstorm activity. -40C to -50C cloud top temperatures signal extreme convective/cumulonimbus development.



Matt's Convective Thread 2                                                                                                            

Find below further information and the uses of the main variables that are analysed in association with convective activity. Thread 2 covers the following;

SBCAPE: >500j/kg (>1000j/kg)
Lifted Index: -2C (>-5C)
Showalter Index: <3 (>-4)
Thompson Index: >30 (>34)
Jefferson Index: >29 (>31)
K Index: >25 (>35)
Total Totals: >45 (>55)
SWEAT Index: >300 (>500)
CAP Strength: <2C (<1C) **Highly variable on surface T/Td**
Boyden Index: >94 (>98)
S Index: >40 (>45)



• SBCAPE: >500j/kg (>1000j/kg)

SBCAPE stands for Surface Based Convective Available Potential Energy. CAPE represents the amount of buoyant energy available to accelerate a parcel vertically in the atmosphere. Essentially the higher the CAPE value the more energy there is available to aid in convective activity a thunderstorm activity. CAPE is especially important when an air parcel is able to reach the LFC (Lifted Condensation Level - discussed in Thread 1). From a UK's perspect a SBCAPE value at or above 500j/kg would signify a reasonably unstable atmosphere, anything above 1000j/kg again from a UK's point of view would be classed as extreme instability. SBCAPE is the CAPE that is calculated on the wetterzentrale charts for example, there are other types of CAPE including MLCAPE and DCAPE. The following images shows which part on a sounding CAPE represents;

http://img92.imageshack....6/brosoundingcape0ob.gif


• Lifted Index: -2C (>-5C)

The Lifted Index or the LI as it is better termed is quite a complex variable but is essentially derived in conjunction with the amount of low level moisture, it often accompanies CAPE on an atmospheric sounding. A negative LI values indicates the potential of convective activity, in the UK a value of -2C and below is quite significant instability, anything below -5C would represent extreme instability.


• Showalter Index: <3 (>-4)

SWI is useful when shallow, cool air below 850mb conceals grater convective potential aloft. SWI is useful when tornadic potential is high, because the moist layer almost always extends above 850mb during violent outbreaks. A value generally less than +2 or +3 indicates a thunderstorm risk but another trigger may well be needed, values below -2 indicates a significant risk of thunderstorm activity.


• Thompson Index: >30 (>34)

The Thompson Index is an index that isn't used as often as some of the other variables and the data is hard to come by. However a value of more than 30 indicates a convective risk and anything above 34/35 would indicate a high risk of convective potential.


• Jefferson Index: >29 (>31)

The Jefferson Index is again another index that you don't see too often and can only be found from indepth forecast and actual atmospheric soundings. Values greater than 29 bring a risk of convective activity and anything greater than 31 a more serious and significant risk.


• K Index: >25 (>35)

The K Index is quite a complete variable in relation to who it is computed. The K Index takes into account vertical temperature lapse rate, moisture content of the lower atmosphere, and the vertical extent of the moist layer. Values greater than 25 are needed to signal a general convective risk, anything above 35 signals a high/significant risk.


• Total Totals: >45 (>55)

The Total Totals or TT as it is better know is computed from the following data; TT is: T850mb + Td850mb - 2(T500mb). This is the temperature and dew point at the 850mb level added together minus the temperature at the 500mb level times 2. The TT does not require the computation of dry or moist adiabatic lapse rates or humidity. TT is good if the moisture extends up to the 850mb level. If the moisture is just below the 850mb level, TT will be much lower in value. Values greater than 45 signify a reasoanble risk of convective activity, but values above 55 are prefered.


• SWEAT Index: >300 (>500)

SWEAT (Severe Weather Threat Index) is used to determine more specifically the threat of severe convective weather and also the risk of tornadic activity, hence the SWEAT index is an important variable to analyse. It's a complicated one as well and is derived from by examining low-level moisture, convective instability, jet maxima, and warm advection. Values at or above 300 indicate the a moderate risk of some severe thunderstorm activity, values at or above 500 a strong risk of severe thunderstorm activity with an isolated risk of tornadoes. The optimum range is between 600 and 800, if the SWEAT is within that range that convective activity will be extreme with a solid risk of tornadic outbreaks.


• CAP Strength: <2C (<1C) **Highly variable on surface T/Td**

The CAP (Capping Inversion) strength is a very important variable in relation to the potential for the sudden release of a build-up of heat and moisture (energy) within the lowest layers of the atmosphere. The CAP is essentially that, it CAPS the lowest layers of the atmosphere. The middle and upper part of the atmosphere may well be unstable, but the CAP within the lowest layers is prohibiting the air from rising to the LFC. The greater value the CAP strength the less likely air will rise, hence values of less than 2C is preferred by a value of less than 1C signals an atmosphere that has great potential for a sudden release of stored energy which then is transfered vertically up within the atmosphere creating rapid convective activity and development. This process in where heat and moisture is stored beneath the CAP and then the CAP is 'broke' late in the afternoon is known as the 'loaded gun', all the days heat and moisture is suddenly released to allow for explosive convective activity. A detailed diagram showing where the CAP is and what it looks like can be seen at the below URL;

http://www.stormtrack.or...ibrary/forecast/cap1.jpg


• Boyden Index: >94 (>98)

The Boyden Index is a direct measure of the mean thermodynamic stability in a layer beneath the 700 mb level. The Boyden Index is computed by I-Z-T-200, where I is the Boyden index, Z is the 1000–700- mb thickness in dam, and T is the 700-mb temperature in °C. Values greater than 94 signal a threat of convective activity while values greater than 98 to 100 are prefered.


• S Index: >40 (>45)

Another variable that isn't generally used that frequently. However where applicable values of greater than 40 are required but values greater than 45 are more useful.






Matt's Convective Thread  3                                                                                           

Part 3 of 4 covering the following topics;

Bulk Richardson No.(BRN): >20 (>40)
Energy Helicity Index (EHI): >1.5 (>3)
Vorticiy Generation Param. VGP: >0.15 (>3)
Storm Relative Helicity (srH)0-1KM: >100m2/s2 (>150m2/s2)
Storm Relative Helicity (srH)0-3KM: >250m2/s2 (>300m2/s2)

The majority of these variables are all used in the analysis for the potential of more severe convective weather and tornadic activity, as a result while still important they aren't focused on just as much as some of the more 'common' convective variables.


• Bulk Richardson No.(BRN):

Bulk Richardson Number or as it is better known BRN is a variable that combines wind shear and bouyancy and as a result allows for a guide as to the storm type, whether it be supercell, multicell or single celled feature etc. The formula for BRN is;

BRN = CAPE / (U2/2)

CAPE is Convective Available Potential Energy which is discussed in earlier threads, U is the measure of the vertical wind shear. Please note while not shown above because of font issues, the first '2' after the U is actually meant to symbolise a squared value.

In general values below 10 signal a low risk of any severe convective weather and single celled features in general. Values between 11 and 49 signal a greater risk of more severe weather and organised multicells/supercells. Values in excess of 50 signal a strong risk of multicell/supercell activity.


• Energy Helicity Index (EHI):

The Energy Helicity Index or EHI as it is abbreviated/better known combines two indexes. Standing alone the EHI is no doubt the best single variable for predicting tornadic activity because it combines CAPE with helicity.

Helicity is a tricky parameter to understand, but in its simplest forms (not that it has one!), but helicity is the product of low level shearing (streamwise vorticity) and storm inflow directly into the streamwise vorticity. So helicity is the calculation of storm inflow interacting with low level shearing. The end result is a value which represents the amount of 'spin' within the atmosphere, greater 'spin' (helicity) and greater the threat of tornadic activity.

In general EHI less than 1 signal a risk of supercells, values between 1 to 5 signal a moderate risk of tornadic activity and values in excess of 5 signal a high risk of some serious tornadic activity. The scaling is directly related to the Fujita scale within the US so hard to correlate to the the TORRO one off hand, but a EHI index value of greater than 5 signals a risk of F4 and F5 tornadoes within the US. Hence see an EHI value of 5 and its time to get a little worried!.


• Vorticity Generation Param. VGP:

Vorticity Generation Parameter or VGP as it is better known is a variable that estimates the rate of tilting and stretching of a horizontal vorticity region in conjunction with a thunderstorm updraft. Values greater than 0.2 signal a risk of tornadic activity.


• Storm Relative Helicity (srH)0-1KM & 0-3KM:

Storm Relative Helicity or srH as it is better known is generally measured within the lowest 0 to 1KM of the atmosphere and also within the a 0 to 3KM layer. The storm relative helicity (as discussed earlier) is a result of the verticle wind structure and storm motion. The storm relative helicity is highly dependant on three variables;

shear vorticity + storm motion + strength of storm inflow

Storm relative helicity is measured by m2/s2 which stands for 'meters squared per second squared' and is the unit of measure directly related to J/kg (Jules per kilogram). Remember from earlier how the EHI uses helicity and CAPE, CAPE is measured in J/kg and hence the close connection within the EHI index.

Values between 150-299 generally signal a weak srH within the atmosphere and hence a low risk of severe weather and/or tornadic activity. Values between 300 and 449 signal a more significant risk of severe weather/tornadoes and values in excess of 450 signal a high risk of severe weather and tornadic activity.

Matts Convective Thread 4                                                                                                 

The final thread covers the following topics;

850mb Windspeed: >25KT (>35KT)
500mb Windspeed: >35KT (>50KT)
200mb Windspeed: >55KT (>85KT)

Deep Layer Shear (0-6KM) >40KT (>60KT)
Low Level Shear (0-1KM) >25KT (>40KT


• 850mb Windspeed, 500mb Windspeed & 200mb Windspeed:

Windspeed within the atmosphere is highly important to the severity of any convective activity. An atmosphere with light winds will overall have a lower risk of producing severe convective weather. Windspeed aids in the sustainability of a thunderstorm or other convective feature, by allowing the updraught and downdraught to co-exist. In an environment where windspeeds are light the downdraught will eventually cut off the updraught, as a result the storm/cell dies.

Windspeeds at the 850mb level, 500mb and 200mb level especially are good indicators as to the potential severity of any convective weather. At the 850mb level windspeeds of greater than 25KT are suitable for more organised convective activity, 35KT winds at the 500mb level and 55KT winds are the 200mb level would all signal a more significant risk of organised convective activity.


• Deep Layer Shear (0-6KM) & (0-1KM)

As with the above specific windspeeds and specific altitudes, another very useful forecasting tool is Deep Layer Shear. Instead of taking just one windspeed reading at one level deep layer shear gives a general mean speed throughout a section of the atmosphere. The two sections used are 0-6KM and the 0-1KM. 0-6KM DSL values at or above 40KT are conducive to more organised convective activity, possibly signalling the development of multicells or even possibly supercells depending on numerous other ingredients. The 0-1KM DLS is often used along with other variables to judge the risk of tornadic activity. Values in excess of 25KT are generally required.

A good webpage giving more information on the usefullness of windspeed/shear within the atmosphere can be found at the below URL;

http://www.theweatherprediction.com/habyhints/275/


Hope the 4 various threads have and will continue to be of use over the coming summer months. 


Soundings                                                                        

Given the convective season is well underway and personally one of the best forecasting tools to use is forecast soundings, I thought I'd just quickly put together some information on what soundings are and how they can be used;

The following link should take you to a site where I have just overlayed some information;

http://oi53.tinypic.com/2lav51k.jpg

Despite variations in soundings given their initial production, the end results are often the same.  So using the image at the above link you can see that;

1) The blue line is the dew point temperature through the atmosphere obviously starting at surface level and finishing above 250mb which is approx 35,000ft

2) The red is the environment temperature or simply the standard temperature through the atmosphere again starting at the surface and continuing up to 35,000ft

3) The dotted line is very important and simply it shows us where the cloud base is and also whether the clouds will be able to rise or not.  You will notice that after a short period of time the dotted line changes angle and i've highlighted this with the blue arrow.  This is known as the LCL or the Lifted Condensation Level - the point where saturation occurs and clouds are able to develop.

4) The extremely important point to remember is that;

IF the dotted line is to the RIGHT of the environment (red) temperature line then this signals clouds can continue to rise to great heights. 

IF the dotted line is the LEFT of the environment (red) temperature line then the atmosphere is stable and clouds are essentially unable to build and develop.

What this particular sounding shows us is a perfect example of an unstable atmosphere.  With a surface dew point of near 10C and a temperature of near 15C, the Lifted Condensation Level is near 900mb. It is from the LCL that the clouds can then develop and because the dotted line is to the RIGHT of the environmental temperature line then clouds can rise all the way past 400mb, which is over 25,000ft up in the air!

The other feature of interest is that we often use CAPE charts to show us instability. Well the CAPE is actually based around the area between the dotted line and the solid red environment temperature line. In this instance there is approx 661 j/kg of CAPE available (shown at the top of the graph).

Anyway just thought this may be of use, as when it comes to thunderstorms and convection as I mentioned right at the start the best tool in my opinion is the atmospheric sounding and if these basic rules can be understood then you can have a great footing in terms of knowing if and when thunderstorms and/or convection is likely.

Regards, Matt.