Guide to Convective Instability and Thermodynamic Variables:

Convection is one of the three methods in which heat is transferred (conduction and radiation being the other two). Convection occurs due to buoyancy. Lighter materials rise and warm air is less dense than cold air and will therefore rise in a given medium. Therefore, during a hot summer afternoon solar insolation will cause rising air currents. The same thing happens when we boil water on the stove. The flame heats the pan by contact (conduction) and the bottom of the pan heats the water by contact (conduction) and the warmer, lighter water convectively rises up (the bubbles you see).

Thermodynamic variables aid meteorologists in numerous ways. When using Bufkit or any Skew-T many of these parameters will help us in diagnosing the stability of the air and the potential for severe weather outbreaks. If you are very fluent with skew-T diagrams you can figure these out yourself with a bit of arithmetic and graphical analysis but many SkewT’s plot certain values on the side for convenience. Bufkit provides us with an excellent Skew-T with a lot of thermodynamic variables listed numerically or plotted graphically on the sounding itself. They can be plotted graphically in the time lined overview mode as well under the Convection panel. Clicking heights brings up LCL, EL, and LFC.

Below is a screen shot of the profile mode indices:

Cap or Inversion – A cap or inversion is a lid or an area of stability that traps convectively rising air. In the troposphere air normally cools adiabatically as it rises by an average of 6.5 Celsius per kilometer. Inversions present exceptions to the rule and are easily identified on a Skew-T. The temperature will generally increase with height for that layer but there are several different types of inversions (see below).  Also, isothermal temperatures in a given layer of significant thickness also indicate stability.

Bufkit makes identifying inversions easy because you can click a button and plot inversions on your skew-T under the Controls panel.  Caps can cause headaches for meteorologists by causing forecasts to bust. Whether the cap can be broken via dynamic uplift and solar heating will determine whether convective storms can occur. The conditions may be favorable for convective precipitation but if an inversion is too strong precipitation will not occur. In addition to the strength of the cap the time when it breaks is also important, as greater instability exists in the afternoon when solar insolation is at a maximum.  A cap can also serve to “store” energy which, if broken, can be violently released as convective thunderstorms. When a moderate cap is present all the suppressed convective motion is builds up just as if you press down on a spring it stores the energy until it is released in a fury.

Inversion layers are important in forecasting wintry precipitation. If snow falls through an inversion layer it may melt completely. If the layer next to the surface is cold the melted snow may freeze into ice and fall as sleet or it may freeze on contact with the ground (freezing rain).

Caps or Inversions are important in urban areas as well as they are used in determining air quality. If there is a strong cap in place then the surface air from rush hour traffic or the morning commute will not mix throughout the troposphere but will instead be trapped near the surface by an overlying stable layer. Inversions will cause more severe photochemical smog in places such as Los Angeles and other busy urban environments.  The height of the inversion is also important as there are different types of inversions. Subsidence inversions occur aloft and will allow greater room for surface air to mix in the troposphere whereas a low inversion can leave an urban area smothered in pollutants. See below for a description of the different types of inversions. Inversions can also lead to large vertical gradients in things such as wind shear.

Types of Inversions:

Subsidence: usually found in areas of high pressure sinking motion or subsidence predominates. Sinking air will adiabatically increase in temperature creating a warmer layer. It tends to show up aloft, in the sounding, as a very dry inversion, noted by a large separation of the dewpoint and temperature lines.

Radiational – this is a low level inversion produced on nights with 1) little to no cloud cover and 2) calm winds where radiational cooling causes the lowest layer of air to be substantially cooler than the overlying air . The lack of clouds allows the radiation to escape back into space. This inversion can be detected easily on a Skew-T or in Bufkit since it is low level (usually in the first 100-150 mb) and moist.  Dew or frost can appear in these same conditions.

Frontal —this is a temperature inversion encountered upon vertical ascent through a frontal zone.  The height of the inversion is proportional to the distance the station is from the surface front.  A frontal inversion can be identified since it is characteristically moist in the inversion layer and dries out in the warmer, overlying air.

Advectional – warm air advection aloft can lead to inversions for obvious reasons. These types of inversions are common in the great plains. Elevation increases as you move E to W. So if you have warm, moist, tropical air in the East and Hot Continental air in the West moving easterly you will end up with WAA (warm air advection) aloft as the high elevated surface air from the west will advect at over the lower elevated surface air in the east (see picture below).

As we have seen, inversions can useful in forecasting air quality, convective precipitation and thunderstorms and the type of wintry precipitation an area will receive.

THERMODYNAMIC INDICES and their OPERATIONAL SIGNIFICANCE

CIN of CINH is the Convective Inhibition Index. It is a numerical measure of the strength of a cap or inversion over a certain region. CIN is the amount of energy required to overcome the negatively buoyant energy the environment exerts on an air parcel. It is the energy opposing convective motion. It is the antithesis or arch-villain of CAPE though storm enthusiasts want a moderate cap to store up energy which if broken leads to stronger thunderstorms. For deep convection to occur the capping layer needs to be broken. Otherwise planetary boundary layer based lifting is unable to move into the +CAPE region of a sounding.

CIN values are reduced by 1) low level WAA with higher dew points, 2)  low level convergence, 3) solar insolation, 4) synoptic scale vertical forcing. 

CIN values relevant only to planetary boundary layer convection (PBL)  when a CAP is PRESENT and are meaningless if positive CAPE is not present. 

1. •0-50 represents a weak cap

2. •51-199 a moderate cap 

3. •200+ a strong cap

Bufkit will plot inversions when CIN is listed at 0 so do not be confused about what CIN actually is or what Bufkit is doing. Bufkit defines an inversion in the following way:

•temperature is increasing with height greater than a threshold (0.5oC per layer)

•nearly isothermal (dT <   0.5oC per layer) and the dewpoints rapidly decreasing with height.

CIN or CAPE will not be plotted on Bufkit if a Level of Free Convection is not present (LFC). See below for a description of the LFC.

Lifting Condensation Level (LCL). The height at which an air parcel, when rising dry adiabatically first reaches an RH of 100%. A low LCL is good in regions of high Cape for Tornadic activity. Draw a line parallel to the dry adiabatic lapse rate starting from the temperature that is 50 mb above the surface. Draw a line parallel to the mixing ratio lines starting from the dewpoint that is 50 mb above the surface. The intersection of these two lines is the LCL. Some meteorologists prefer to start at the surface pressure and not 50mb above.

The LCL tells us the base height of clouds (RH = 100%) when air is mechanically forced up (e.g. frontal lifting, low level convergence, etc.). We should not use the LCL in cases when the rising is due to positive buoyancy alone, in those cases it is the CCL.

The convection condensation Level (CCL) is the height to which a parcel of air, if heated sufficiently from below, will rise adiabatically until it is just saturated (condensation starts) (RH = 100%).  In the most common case, it is the height of the base of cumuliform clouds.  This level can be found by starting at the mean mixing ratio of the surface moist layer (or lowest 150 mb., whichever is most representative) and ascending this constant mixing ratio line to its intersection with the temperature sounding.

Mechanically lifted air reaches RH = 100% at the LCL but buoyantly lifted air due to convection has RH=100% at the CCL. Bufkit plots both of these features. The CCL will generally be higher than the LCL and drier the PBL the higher the loud bases will be. So use the CCL when air is lifted convectively due to solar insolation and the LCL when air is forced up mechanically.

Level of Free Convection (LFC). This is the pressure level in which an air parcel first becomes equal to the temperature of the surrounding environment. This occurs at the bottom of the most significant CAPE region on a sounding. A LFC will only be present when instability exists in the troposphere.  A very low LFC is indicative of potential for Tornadic activity in severe thunderstorm environments. Since an LFC only exists when instability exists then if Bufkit plots an LFC forecasters know that instability exists in the troposphere.

Equilibrium Level (EL) is similar to the LFC with the exception that on a Skew-T is occurs at the upper boundary of the most significant CAPE region. Buoyantly rising air equals the surrounding air at this point. The significance of the EL is that a very high EL allows for deep convection and maximal growth of convective storms. 

All these features (LCL, EL, LFC, etc.) can be plotted in the overview panel of Bufkit as well.

Convective Available Potential Energy (CAPE).  This is an important parameter. It is the value of the area in between the curve between the LCL and EL on a Skew-T profile. When finding LFC and EL, use a surface based lifted parcel. Find the LCL. From the LCL, parallel the wet adiabatic until it has intersected the temperature curve twice. The first intersection is the LFC. The second intersection is the EL. CAPE and instability exist between the LFC and EL. The region where the theoretical parcel temperature is warmer than the actual temperature at each pressure level in the troposphere. High cape means storms will build quickly and is indicative of rapid vertical motion which also can lead to the development of super cells, tornadoes, strong downdrafts, hail ( usually > 2500J/kg) potential, lightning and explosive storms. High cape does not necessitate severe weather. Storms will only form is the low level CAP is broken. CAPE values rise and fall rapidly across time and space. Values from 1-1000 indicate marginal instability. 1000-2500 indicate moderate instability and storm potential.  Anything greater than 2500 indicates very unstable conditions and above 3500 conditions are extremely unstable.

Bufkit lists CAPE values on the profile screen under “Indices”. You can also see CAPE graphically on the Skew-T. It is the area enclosed between the yellow and Red Temperature profile line in between the LCL and El in the picture below:

In overview mode there is an entire tab devoted to and aptly named “Convection”. This is what you will see when plotting CAPE. The Y axis is a measure of the value of cape and the table I pasted in the graph tells you what the different color lines indicate in regards to CIN values.

Helicity (hel) is related to wind shear changes with height (speed and directional). It measures the amount of rotation present in a storm’s updrafts. Helicity leads to the formation of super cells and possibly tornadic activity. Helicity does not give any indications of moisture content, CAPE or CIN (is there a stable inversion layer that needs to be broken?) so it must be used in conjunction with other parameters. Values from 150-300 indicate potential for super cell. Values from 300-400 indicate super cell formation is favorable. Above 400 tornados are possible.

Energy Helicity Index (EHI) is a combination of CAPE and HEL. You multiply the CAPE and HEL values together and divide the total by 160,000. It is a good indicator of thunderstorm activities and tornadoes since it combines two thermodynamic variables. EHI > 1 = supercell potential. EHI from 1-5  means F2 and F3 tornadoes are possible and anything greater than 5 indicates F4 and F5 tornadoes are possible. Tornadoes can still form even if EHI is low as long as either CAPE or HEL is very large, especially when CAPE is low and HEL is high. EHI is highly variable and you have to make sure the cap will break (CIN isn’t too strong).

Showalter Stability Index (SSI):  Numerical measure of local static stability. Calculated by raising a parcel from 850 mb dry-adiabatically to the LCL and then moist adiabatically to the 500-mb level. The temperature of the parcel is compared to that of the environment, with the magnitude of the index being the difference between the two temperatures. If the parcel is colder than the environment, the index is positive; if warmer, the index is negative. Cumulonimbus phenomena usually fail to develop for index values greater than +4, whereas showers and thunderstorms become increasingly evident as index values decrease from +4.

Lifted Index (LI):  Measures the potential instability present in the atmosphere.  It is computed by lifting the mean moisture in the lower 3000 feet of the atmosphere moist adiabatically to 500 mb and subtracting the temperature at this point from the reported 500 mb free-air temperature. Positive values (on the order of +2) indicate stable air while negative values indicate unstable air.  Convective activity usually can be expected when values go to or below –2. LI is also plotted on Fous text products.

K Index (KI): Calculated from the following formula: [ K value = T850  -T 500  +Td 850 -DD700 ] A K value in the 30’s is highly correlated to thunderstorm activity With a 40 or more almost assuring 100% chance of thunderstorms. Values in the 20’s are usually more indicative of showers.  The K index is used most often in the summer and is more geared to after-noon heating type thunderstorms. Other parameters must be considered in conjunction with KI.

Total Totals index (TT). Calculated as:   [ (T850 - T500) + (Td850 - T500) ] Index values will vary with location and season. Is generally valid for flat, low elevated regions. Does not assess CIN, CAPE, moisture, and the index will be too stable if the layer of moisture is just under the 850mb level.  

1. ➡  < 44 convection is unlikely. 

2. ➡  44-50 thunderstorms are likely 

3. ➡  51-52 isolated severe storms can form

4. ➡  53-53 widely scattered severe storms. 

5. ➡  56+ scattered severe storms.

Bulk Richardson Number (BRN) measures the balance between CAPE And wind Shear. Cape determines updraft strength and Shear  the storm type (super cell, multi-cell storms or pulse storms).

1. ➡  >45 then CAPE >>> SHEAR—pulse storms possible w/ weak-moderate CAPE 

2. ➡  < 45 supercells are possible. 

3. ➡  Teen Values are optimal for supercells 

4. ➡  < 10 means Shear is much higher than cape. 

PULSE Storms – high cape and low sheer

Supercells—Cape and Shear in balance

Multi-cell Storm – good spead shear but low level directional shear is weak.

Convection Temperature (Tc):  The convection temperature is the surface temperature that must be reached to start the formation of convection clouds by solar heating of the surface-air layer.  This is a very useful parameter in predicting the onset of convection.  It can be determined by taking the dry adiabat that intersects the CCL down to the surface and reading the corresponding temperature. BUFKIT will plot the convective temperature in Celsius on the horizontal temperature scale near the surface. It will not be under the Indices tab.

That should give you an understanding of how to use the convective parameters that Bufkit features.

HAPPY FORECASTING!!!