Quick Guide on Thunderstorms

Most of us have observed at least one of these in our lifetime, but what exactly makes up a thunderstorm?
Here is a quick, but useful, guide to understanding a little more about our day to day thunderstorms.
Thunderstorm (Defn.):

     1. One or more sudden electrical discharges occur.

• Lightning Flash
• Sharp rumbling - thunder

     2. The cumulonimbus base is lower than 0˚C isotherm.
 
     3. The cloud top temperature is less than -20˚C.

• Glaciated Tops
• Precipitation 

The life cycle of a single cell can be separated into three stages:

• Cumulus stage
• Mature stage
• Dissipating stage

    

1. Cumulus Stage


Updraft:

• Everywhere in the Cell
• Varies in space and time
• Maximal speed occurs at higher altitudes later in the period.
• There is Horizontal Convergence at all levels.

There is an entrainment of drier environmental air.

Temperature

•  The cell temperature exceeds the environmental temperature at each level.
• As well, there are temperature anomalies in the updraft. Maximum usually occurs at end of the cumulus stage.                                                                

Precipitation

• Greatest concentration is above the freezing level.
• However, no surface precipitation.

2. Mature Stage

Updrafts continue...


Here Downdrafts are introduced:

• Initiated at mid-levels of the storm
• They increase in space and time
• Maximum downdraft winds in falling precipitation (rain, hail)
• Strong horizontal divergence at the surface ( Gusts can exceed 60 km/h)

Cloud grows physically

• Reaches maximum height in the mature stage

Maximum turbulence is present in regions of greatest vertical motion.                            

← Storm Motion 

Temperature

• Cloud tops < -40°C
• Cold anomalies in downdrafts (Max at low levels)
• Warm anomalies in the updrafts

Precipitation

• Ice crystals present at the cloud top
• Low level precipitation (liquid rain and solid (hail) possible)
• The precipitation horizontal boundary marks the downdraft boundary                                                              
• Hail possible in the downdrafts

    3. Dissipating Stage 


Collapsing Cell

Downdraft

• Spreads throughout the entire cell
• Speeds are less than in the mature stage
• Surface wind divergence rapidly decreases
• The turbulence intensity diminishes in the cell

Temperature

• Cell temperature falls below the environmental temperature at each level and eventually equalizes 

Precipitation

• The cirrus clouds blow off the top
• Shower activity becomes light and decreases in intensity

Severe Thunderstorm (Defn.):

Simply put it is a meteorological event consisting of a severe weather occurrence produced by a very large cumulonimbus cloud (CB).

The diagram above (which is adapted from Browning et al., 1976; The different greyshades represent radar reflectivities of 35, 45 and 50 dBz) shows a typical cross section through a Multi-Cell Thunderstorm. There are four successive cells in different stages of development, each of which takes ~15 minutes:

• The shelf cloud n+1 has a, crisp, flat base which indicates an active updraft ahead of the cell n.
• Cell n is a daughter cell which is in a developing stage.
• Cell n-1 is in its mature stage and forms at the center of the storm.
• Finally, Cell n-2 is in its dissipating stage.  

Severe weather occurrence: If one or more of these occurs,

• Wind Gusts   → 90 km/h or more
• Hail  → 2 cm in diameter or greater
• Rainfall  → 50 mm in an hour or less
• Tornado

Super Cell (Mature Stage)

• Becomes very well organized with the surrounding environment
• The dissipating stage is in this case postponed 
• Updraft and downdraft work independently and lead to longer storm life

Updraft 

• It is tilted
• Precipitation particles carried out which initiate a separate downdraft