Cb and MCS are the result of strong convective processes in the air column. These processes and the paramters which describe them are discussed in detail in the Basic chapter "Numerical parameters for small scale convective cloud systems" (see Basics ).

The conceptual models summarized below have been derived from a series of radar observations as they describe either small-scale single storms, or substructures of MCSs which are also of an order of magnitude comparable to Single Cell Storms. Satellite images cannot normally resolve such a structure.

Mesoscale conceptual models for single cell storms

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

• Developing stage
• Mature stage
• Dissipating stage

Developing stage

The developing stage lasts 5 to 10 minutes and is characterised by a distinct single updraft. The diameter of the cell is between 2 and 8 km. The process of entrainment at the cloud edges is essential for the further development of the Cb. The entrainment results in a smaller water vapour content at the cloud edge compared to the centre of the Cb and, consequently, in the evaporation of water droplets. This causes a cooling within the boundary area of the whole cell with a consequent reduction of buoyant energy. Only a supply of sufficient humidity from surface levels will support further growth of the developing cell.

Mature stage

The mature stage of the single cell body (which of course can be embedded within a group of cells) typically lasts between 25 and 30 minutes. During this stage downdrafts develop associated with the falling of ice (hail stones) which are no longer kept aloft by the updraft of the cell. The downdraft is accelerated as a consequence of cooling by the evaporation of cloud droplets. Simultaneously the updraft weakens because rising warm humid air is then removed by cool air spreading horizontally at the base of the cell. The downdraft initiates successive developments of new cells which can be observed in satellite imagery as pronounced gust fronts. The end of the mature stage is reached when the outflowing dry air completely cuts off the incoming supply of moist air. During the mature stage of the Cb rain or hail are most intense, whereas the lightning density shows a maximum at the end of the developing stage (see Weather events).

Dissipating stage

The last stage, the dissipating stage, of a Cb is reached when the updraft weakens and increasing downdrafts of dry cold air spread at lower levels. The supply of warm moist air from the lower levels is then interrupted and the Cb dissipates.

The role of vertical wind shear

The life time and intensity (related to weather events) of a Cb and MCS depend upon the vertical wind shear (speed and direction), also the differences in strength and direction are responsible for convective development and type of convection (single cells, super cells, squall lines, organised convection). If there was no wind shear the updraft would soon be equalised by the downdraft. Cells that develop in an environment with strong vertical wind shear show a longer life time and more severe weather. The most intensive thunderstorms occur in an environment with vertical wind shear showing changes in both velocity and direction.

Mesoscale conceptual models for multi-cell storms

An ordinary single cell produces a gust front which may lift the surrounding air along the periphery of the cell to its level of free convection (LFC). Now new, secondary cells can form adjacent to the old cell (see the schematics below). These new cells are the so-called "daughter cells", which mostly develop in the right leading part (so called "right mover"), but sometimes they also develop on the left side (so called "left mover"). Both kinds of newly developed cells can be distinguished according to their characteristics in the vertical wind shear. These new cells have a diameter between 3 and 5 km, and the distance to the centre of the thunderstorm is approximately 30 km. The daughter cells develop very rapidly and after a short time become the new centre of the Multi-Cell Storm (mother cell). This rapid development takes place because the daughter cells develop immediately in front of the mother cell; therefore (as described in the Single Cell Storm) no kinetic energy is taken away from the cloud. Although the older cells dissolve to the rear of the cloud complex, the Multi-Cell Storm is still active due to the continuous new development of daughter cells. If the sequence of cells is short the Multi-Cell Storm can change to a Super Cell Storm. The diagram above (adapted from Browning et al., 1976; Greyshades represent radar reflectivities of 35, 45 and 50 dBZ) shows a typical cross section through a Multi-Cell Storm, and can be interpreted in two ways. On one hand, as a space cross section which shows four different cells in different stages of development. On the other hand, as a time cross section which shows the life time of one single cell during a Multi-Cell Storm. The diagram shows that the daughter cell (n) develops from the so-called shelf cloud (n+1). This lasts approximately 15 minutes. After an additional 15 minutes the daughter cell becomes the centre of the Multi-Cell Storm. The centre of the storm is shown with the cell (n-1). The cell is now in its mature stage and strong updrafts and downdrafts can be found. 15 minutes later the cell has finished its mature stage and dissolves at the rear part of the Multi-Cell Storm (n-2).

Observations and verification by radar

See the chapter on key parameters (see Key parameters ) for typical radar products. Constant Altitude Plan Position Indication (CAPPI) measurements show some of the features mentioned above.

A typical characteristic of an MCS is a boomerang-shaped (or hook or bow-shaped) strong (> 40 dBz) echo area seen in a CAPPI display. It is also typical to have even stronger echos within a moderately strong echo area, surrounded by weak echos within the hook echo. Typically, the hook echo lasts for a few hours, beginning as a straight line, developing into a bow shape, finally into a Comma shape. The region near the centre of the bow is typically ahead of strong surface winds, thought to be associated with a rear inflow jet entering the MCS. The Comma head is often associated with a mid-level mesoscale cyclone. The image below shows a hook echo in a CAPPI display.

A gust front may occur ahead of the leading edge of the reflectivity pattern. Even when it is invisible to the human eye, it can often be detected with radar, in apparently clear air, because of debris and insects. An example of a gust front can be seen in the image below. (Below) Side-lobe echoes are often seen in radar displays of storms producing hail. The "tail" running parallel to the distance marker (from the radar position) is, in fact, a reflection from a strong cumulonimbus, created by side-lobes of the antenna beam.