(Potential) Instability

Radio Sonde soundings are an effective tool to obtain information about the stability of the atmosphere, the lapse rate of the temperature, and the detection of the moist layers. Cross sections of conservative parameters such as potential temperature and wet bulb potential temperature (ThetaE or ThetaW) are appropriate to detect the height of the unstable layer over a larger area.

Instability index

In this manual the Showalter Index is used. The relevant area indicates an area with high probability for CB development; Showalter index values below 3 units represent high probabilities of development of thunderstorms.
Another measure of instability is the Boyden index. Values above 94 give an increased chance of developing thunderstorms.

Lightning reports and Radar images

Thunderstorm reports from surface observations cannot cover an entire cluster area, therefore lightning reports from a detection system are more useful.
Radar images are essential in diagnosing the strength of a CB Clusters in terms of precipitation amount and dangerous weather events like wind gusts, heavy hail or downbursts (see radar in Cumulonimbus (Cb) and Mesoscale Convective System (MCS) - Key Parameters ).

Equivalent thickness

CB Clusters in Warm Air often develop in the ridge or within a zone of high gradients of equivalent thickness (see Cumulonimbus (Cb) and Mesoscale Convective System (MCS) - Key Parameters ).

Potential Wet Bulb Temperature at 850 hPa

CB clusters in Warm Air also develop in a tongue of high values of ThetaW; The clusters develop in the area of the maximum values.

Convergence

Mainly in lower layers due to differential heating and/or orography.

Troughs in the surface pressure may set up convergence zones (see Meteorological physical background).

Numerical model parameters in vertical cross section

This is described in more detail in the chapter on the vertical cross sections (see Typical appearance in the vertical cross sections). Special attention is paid to:

• Behaviour of isentropes Convergence

Cloud top height

The height of the cloud top indicates the potential activity of the cell. Cloud top temperatures measured from IR images combined with radio sonde soundings give a good estimation of the top height. The estimated top height is computed using the IR temperature and the temperature height profile derived from the nearest radiosonde sounding.
In the diagram shown below, the average cloud top height is 275 hPa (dispersion around 100 hPa) lying just below the mean value of the tropopause height.