Glossary

Bow Echo
A “bow echo” is a name for a pattern of a storm seen on rainfall radar as shown in the example image above. Its shape resembles a bow and therefore it’s name is bow echo. The echo shows a line of storms which bends in the direction of propagation. Bow echos are usually accompanied by severe wind gusts, these being usually the strongest at the apex of the bow.
Bears Cage

A Bears Cage is a region of a supercell thunderstorm that is located under a low-level mesocyclone and often close to a tornado. It is a term often used by stormchasers who generally try to avoid this region. This is because very strong winds and sometimes heavy rain are encountered in a Bears Cage. It is difficult to drive fast in such conditions and since a tornado can be located in the Bears Cage it is dangerous to be in such conditions close to a tornado. From outside the Bears Cage often looks like a cage formed of a ring of precipitation around the center of the rotating mesocyclone with the tornado (the bear) inside. In some cases a wedge tornado can fill the entire Bears Cage.

CAPE
CAPE is the acronym for “Convective Available Potential Energy” which is the energy available in the atmosphere to convection. This energy can’t be directly measured at the surface, but needs to be calculated from an atmospheric profile of dry bulb temperature and dew-point temperature that is obtained by the use of a radiosonde (a meteorological balloon).
Here is a bit more technical explanation of CAPE and how it can be represented. A radiosonde (meteorological balloon) obtains dense readings of temperature approximately throughout the troposphere, approximately one every 10 meters in height. An example radiosonde sounding is shown in the image above (this is called a skew-t chart). Here, the red line represents ambient temperature and the light blue line represents dew-point temperature. The black line which borders the yellow area on the right side shows the temperature and dew-point temperature (which differ below the cloud base) an air parcel would have if it started rising from the surface. The air parcel begins on the ground, having the same temperature as the ambient temperature. The parcel first rises dry adiabatically as no condensation occurs below the cloud base (temperature in the air parcel decreases by approximately 9 degrees per 1 km). This is shown by the black line that begins at the surface (the line starting at equal temperature as the ambient temperature (red line) and being parallel to the green lines (dry adiabats)). The cloud forms at a height where condensation in the parcel occurs. This is visible as a cloud base. This height can be estimated by obtaining the dew point temperature at the surface and following a constant mixing ratio line on the skew-t chart (being parallel to the pink lines which show constant mixing ratio). Where this line meets the parcel temperature condensation occurs. Above condensation and cloud base the parcel rises moist-adiabatically since latent heat is being released and its temperature trace follows one of the moist adiabats (moist adiabats shown every 5 degrees on this skew-t).  Whenever the trace of the air parcel is to the right of the red temperature trace it means the parcel is warmer than its environment and is positively buoyant. The greater the difference the greater the buoyancy. CAPE is equal to the area between the parcel and ambient temperature traces where the parcel temperature is greater than the ambient temperature (here represented by the yellow area). The units of CAPE are J/Kg (Joules per kilogram of air lifted). As an example, if the value of CAPE is 1000 J/Kg it means that an energy of 1000 Joules, or 1 kJ, is released to every kilogram of air that rises throughout this atmospheric profile.
Meteorological models can forecast values of CAPE up to two weeks ahead. The higher the value of CAPE the more energy there is for convection and the greater the strength of potential thunderstorms. However, CAPE is not the only parameter that determines the potential strength of eventual thunderstorms, but it can be used as a useful guide. As a rule of thumb:

-If CAPE is less than 100 J/Kg thunderstorms are very unlikely (unless they are supported by fast-moving cold fronts mainly in the winter half of the year)
-If CAPE reaches 100 – 300 J/Kg then there is a good chance of at least some lightning if convective showers develop, but the electric activity is generally short lived and sporadic
If CAPE reaches 300 – 1000 J/Kg then there is a high chance that thunderstorms will form somewhere in the area. If they form there will likely be decent lightning activity and these thunderstorms can usually manage to persist for several hours (if multi-cellular)
If CAPE reaches 1000 – 2500 J/Kg then unless there is a strong convective inhibition thunderstorms are very likely. These thunderstorms can be long-lived and can organize into Mesoscale Convective Systems. Thunderstorms can be accompanied by hail larger than 1 cm in diameter and by frequent lightning. Values of 2000 J/Kg or more are rare in the UK but when such values are present they are nearly always accompanied by strong thunderstorms or large Mesoscale Convective Systems.
-Values of CAPE over 2500 J/Kg are very rare in the UK and do not occur every year. However, such values occur much more often in other countries in central and western Europe (e.g. France, Germany). If thunderstorms form in such a strong CAPE environment they will nearly always be accompanied by frequent lightning and very often by at least some hail. The strength of the thunderstorms then very strongly depends on the wind shear.
-Values over 3000 J/Kg are rare even in central Europe, but occur several times per year over the great plains of the US. Such values do not need much wind shear for thunderstorms to organize and if supercell thunderstorms form in such a high CAPE environment they can produce hail greater than 5 cm in diameter.
-Values over 4000 J/Kg are very rare anywhere in Europe, but occur at least once a year in the US. Thunderstorms are always very strong in such extreme CAPE conditions and can produce hail over 5 cm in diameter if mesocyclones/supercells occur.
-Values over 5000 J/Kg are rare even in the US and there is only a handful of cases of such a strong CAPE recorded in Europe. In the US, the highest values of CAPE ever recorded were approaching 10.000 J/Kg and it’s likely that such extreme values also occur over northern India south of the Andes.
As already mentioned the intensity and degree of organization of storms also strongly depends on values of wind shear (how wind speed and direction changes with height). If there was a profile with 4000 J/Kg CAPE but zero wind shear (no wind between the ground and 6 km above the ground) then thunderstorms can be very strong but they will likely not persist for long or they will be disorganized and will weaken with time. During the initiation stage such storms may produce hail of 3-4 cm in diameter, but if there is no wind shear the hail and precipitation would fall into the updraft of the storm, cutting it off eventually.
Hook Echo
A Hook echo is a name given to a pattern on rainfall radar which resembles a hook. An example is shown on my iPad case in the image above. A hook echo is one of many signs of a potential supercell thunderstorm and is normally located on the southwest side of the thunderstorm. A supercell contains a mesocyclone which is a region of rotation in the storm. This rotation also usually occurs at the southern end of the storm and usually rotates in the anticlockwise direction in the northern hemisphere. Hook echoes are dangerous parts of the storm as they can be accompanied by tornadoes (usually in the southern end of the hook) and very large hail (usually in the north or northwest part of the hook/rotation). There is also a “clear notch”, which is an area of very weak or zero rainfall echo to the right of the hook (area with no rainfall). This is because the strong updraft that rotates in the anticlockwise direction brings all the rain and hail away from this area whereas it brings the heavy rain and largest hailstones back south on the left side of the rotation. This very large hail and rain then forms the hook echo.
Lapse Rate

A lapse rate is a rate of change of temperature with height. For a rising air parcel in the atmosphere there are two types of lapse rates, a dry adiabatic lapse rate and a moist adiabatic lapse rate. If an air parcel rises through the atmosphere (such as in an updraft of a thunderstorm) its temperature reduces purely due to decreasing pressure acting on that parcel with height. If the parcel rises dry adiabatically which means that no condensation occurs within the parcel (a parcel of hot air rises, but no cloud forms) the rate of change of its temperature is approximately 0.9 degrees per 100 meters (or 9 degrees per kilometer). However, if condensation of water vapor does occur in this air parcel, latent heat due to condensation is released and heats up that air parcel which causes the decrease in its temperature to be slower, approximately 0.6 degrees per 100 meter (or 6 degrees to kilometer).

The ambient atmospheric temperature (temperature surrounding the parcel) also normally drops with height (determined by the environmental lapse rate), but the decrease varies across different atmospheric environments. If the decrease in ambient temperature is lower than 6 degrees per kilometer then the rising air parcel in a cloud which rises moist-adiabatically and is cooled by 6 degrees per kilometer will begin to be cooler than its surroundings at some point. As long as a parcel is warmer than its surroundings it has a positive buoyancy and tends to rise upwards. As soon as it becomes cooler than its surrounding it acquires a negative buoyancy and tends to sink. If however the ambient atmospheric temperature decrease is greater than 6 degrees per kilometer the air parcel would continue to rise since a moist-adiabatically rising air parcel cools at 6 degrees per kilometer and hence will remain warmer than its surroundings. Eventually, in an unstable atmosphere the air parcel will reach a point where its surroundings become warmer again, normally at the tropopause. 

We can therefore conclude that if the environmental lapse rate is greater than the moist adiabatic lapse rate (rising air parcel in a cloud cools slower than its ambient temperature), parcels of air in clouds tend to rise until they reach some point where they become colder again which is either at some inversion (a temporary rise in temperature with height) or at the tropopause where temperature naturally again rises with height. Since the moist adiabatic lapse rate is 6 degrees per kilometer, environmental lapse rates greater than 6 degrees per kilometer tend to create an unstable atmosphere – an atmosphere susceptible to the formation of deep moist convection (deep moist convection are convective clouds that form showers and thunderstorms).

Now we can describe what environments are typical for different environmental lapse rates. Environmental lapse rates of less than 6 degrees often occur in areas of high pressure or some form of warm advection (where warmer air is advected (transported from elsewhere) at higher levels). Such environmental lapse rate makes the atmosphere stable and the formation of convective showers or thunderstorms is unlikely. Typically, the environmental lapse rate over the UK varies between 5 and 7 degrees per kilometer on average throughout the whole troposphere. Environmental lapse rates approaching 9 degrees per kilometer (dry adiabatic lapse rates) are rare. Under such conditions air parcels would rise even if condensation (formation of clouds) does not occur. This is called dry convection. Such steep envirnmental lapse rates can form over dry and hot regions such as deserts or dry plateaus such as in Iberia. In these regions the air is very dry so formation of clouds in not possible until the air cools substantially (down to its dew-point which is very low in deserts). However, very strong solar radiation makes the ground very hot which heats up the near-ground layer of air. This very hot air begins to rise and cool at the dry adiabatic lapse rate (since condensation does not occur in this very dry air). Because the solar radiation lasts long and heats the surface layer to very high temperatures these air parcels rise to substantial heights (up to several kilometers). Because they rise dry adiabatically at 9 degrees per kilometer they eventually mix the lowest approximately two kilometers of the atmosphere in such a way that the environmental lapse rate in this several kilometer thick layer is 9 degrees per kilometer (e.g. 40 degrees near the surface, 31 degrees at 1 km height, 22 degrees at 2 km height, etc.). If this environment of such steep lapse rates is advected elsewhere at higher levels it is called an “Elevated Mixed Layer”. In Europe we often see this “Elevated Mixed Layer” being advected either from the Sahara Dessert or from Iberia (called a “Spanish Plume). If such “Elevated Mixed Layer” is transported over a very moist and warm airmass near the ground it can cause a very unstable atmosphere since the warm and moist air would then form clouds where the updrafts would rise at a moist adiabatic lapse (6 degrees per kilometer) rate in an environment characterized by 9 degrees per kilometer dry lapse rates. However, Elevated Mixed Layers can form in other areas as well, often over high mountain ranges which are also strongly heated in the summer but where the supply of moisture is restricted (blocked) by the mountains. An example are the Alps or the Pyrenees (although its often difficult to distinguish an EML (Elevated Mixed Layer) originating from Iberia from that originating from the Pyrenees). A lapse rate greater than 9 degrees per kilometer is extremely rare. This is because if such an environment develops the air becomes very unstable in the layer of such a lapse rate and convection (either dry or moist) immediately begins, trying to mix out such a lapse rate. An example of how this occurs is on hot summer days when the solar radiation is very strong. If there is a dark surface which strongly absorbs solar radiation it can heat up the surrounding near surface air at a faster rate than what the lower atmosphere manages to transport upwards. Under such conditions dry convection often develops resulting in dust devils which again try to equalize the very steep lapse rate. Another example is if a very cold air is advected over much warmer air, such as if an arctic air is brought over a relatively warm sea. In such conditions steam devils can form over the warm water surface.

 

Supercell Parameter

A “Supercell Parameter” is a severe thunderstorm index (like e.g. CAPE) that is determined by a combination of several convective parameters including CAPE, Storm Relative Helicity, etc. It is calculated by several equations which I will not describe here in detail. If anyone is interested in the deep maths and more details you can Google it or email me at keblecollege@gmail.com

Here, I will give a simplified explanation. A Supercell Parameter depends strongly on CAPE since for supercells to form there needs to be instability in the atmosphere for thunderstorms to develop. Another very important variable is 0-3 km Storm Relative Helicity. This depends on the wind speed and direction between the ground and the height of 3 km above the ground. An example of strong Storm Relative Helicity is if the near surface wind direction is from the southeast, 500 meters above the ground from the south, at 1km above ground from the south-southwest, at 1.5km from the southwest, at 2km from the west-southwest, at 2.5km from the west and at 3km from the northwest. The greater the wind speed the higher the values of Storm Relative Helicity and the higher the values of the Supercell Parameter. The higher the Supercell Parameter is the higher the chance of the formation of supercell thunderstorms and the greater the chance of strong and long-lived supercells. However, a Supercell Parameter is only one of many parameters that need to be studied when producing a forecast and including the probability of supercell thunderstorms. Based on my experience, Supercell Parameter is a very useful variable when forecasting supercells over the Great Plains of the US. In Europe, however, I have seen situations with very high values of the Supercell Parameter, but no supercells developed in the end. The problem in Europe is often related to weak values of convective inhibition and topography, which lead to a high number of thunderstorms. These interact with each other, disrupting the general environment that is favourable for supercells. Normally, supercells like to stay isolated if they are to be long-lived. On the other hand, I have also been surprised by a few nice supercells in Europe where the Supercell Parameter was not forecasting storms to be as good as the situation verified. As a rule of thumb, values of the Supercell Parameter less than 1 mean that the chance of supercell formation is very low. Values between 1-4 means that there is a higher chance of a supercell, but conditions are not yet good enough to produce a strong or long-lived supercell so these are usually just transitioning supercellular structures. If the value is 4-8 then probability of supercells is quite high. If it goes above 8 then conditions are normally very good for supercell formation and if any storm manages to remain isolated for some time it will very likely become a supercell. Values of Supercell Parameter rise quadratically with improving conditions (hence 2,4,8,16, etc.). I have not seen values greater than 16 in Europe, but have seen such values in the US and such situations were ​​often associated with strong supercells accompanied by giant hail and tornadoes.