Fig. 1.7. Life cycle of a single cell storm. The contours denote radar reflectivity (every 10 dBz). The bottom figure shows the reflectivity of a pulse storm: a powerful single cell. Adapted from Chisholm and Renick (1972).

We will explore the effects of vertical wind shear by considering firstly how a storm evolves in the absence of vertical wind shear. In that case, single cells or ordinary cells can be expected to form. Those storms have relatively short lifetimes. Fig. 1.6 shows the life cycle of an ordinary cell. Its life cycle can be characterized by the following steps:

Some air reaches its LFC and rises upward until it reaches the level where its buoyancy vanishes (EL)

Precipitation forms within the cloud which negatively impacts the air's buoyancy: a downdraft initiates

The precipitation falls down and its evaporation cools the unsaturated sub-cloud layer, which further reduces buoyancy

A cold pool forms beneath the convective updraft and spreads out over the earth's surface cutting off the inflow of warm air flowing into the updraft

The remaining precipitation falls out and smaller water droplets and ice particles evaporate. The storm dies.

This life cycle takes typically 30 to 50 minutes. Single cells do not frequently produce severe weather. When they do, it is often in environments of high latent instability and/or conditions favourable for very strong downdrafts. True single cells are rare as the vast majority of storms on closer inspection turn out to be associated with more than one "upward pulse". Single cells in this way form the building blocks of larger convective systems.