Physical Background
Stability indices are a group of indices that have been developed to illustrate the potential for convection within an air mass based upon a radiosonde or model derived sounding. In most cases stability indices are derived from the temperature and humidity data of the sounding at certain fixed levels. Some stability indices even use information from all observed levels of the sounding (such as CAPE). Other indices even contain a contribution of wind information.
All indices, however, eventually give only one numerical value (which may be dimensionless). This value represents the potential for convection at a certain fixed location. The value of the stability index is compared to some statistical threshold value differentiating e.g. non-thunderstorm and thunder storm cases.
For air mass classification purposes index values are plotted and analysed on a weather map, preferably with a suitable satellite underlay image. This latter method is particularly effective for model derived stability indices, since model derived soundings can be calculated at each model grid point for each model time step.
In meteorology, the use of convection indices varies from country to country, reflecting different methods, but also the different climatologies. A full list of indices used would contain tens of different indices and would go beyond the scope of this physical background. For this reason only a small selection of stability indices are presented which are commonly used in EUMeTrain.
The Warm Sector
The theory of convection starting in the warm sector has been often referred to in this case study. The theory behind it is explained the following SatManu abstract:
Warm Sector - Physical background
In the above document several conditions are discussed that also are seen in this case over Ukrain.
- Ahead of the cold front high values of ThetaE are analysed by the model
- A pronounced thickness ridge.
- WA in the lower layers of the troposphere is analysed
- Unstable situation with Showlater Index below 0
There are some additional features associated with the Cold front, obviously pre-frontal convergence that could be seen in the set of satellite images, enabled the development of MCS formation later in the evening of 9 July. This was enhanced by Cold advection at middle layers, which further de-stabilised the atmosphere ahead of the Cold front.