Synoptic Situation: Satellite images and derived numerical parameters
Basic parameters like surface pressure (height of 1000 hPa) and upper level height (here at 300 hPa) do not reveal the whole physical background of a situation and can therefore not explain details seen in the cloud features of satellite images as well as in the different conceptual models.
Therefore, in order to understand the processes that lead to certain cloud configurations, it is usually necessary to consult different derived physical parameters.
The Manual of synoptic satellite meteorology describes the following keyparameters as being important to study convergence lines and prefrontal convergence. They are:
- Wind vectors: The wind field shows confluence up to a height of 850 hPa; above there is a uniform wind regime. Often there are easterly winds to the east and southerly winds to the west of the convective Convergence Line resulting in a discontinuity in the low level wind field.
- Convergence is found in the lower levels, sometimes only at 925 hPa, sometimes up to 800 hPa
- Upward motion can be observed to a slightly higher level.
For completion, besides convergence and wind at different levels also a selection of other derived parameters will be presented and described in this chapter.
Meteosat 8 IR10.8 and Divergence/Convergence 925 hPa: time sequence
Using a LAM model (Aladin) the converging movements near the surface, in this case 925 hPa, can be numerically clarified.
Meteosat 8 IR10.8 and Divergence/Convergence 700 hPa: time sequence
In the previous chapter the prefrontal convergence on a cold front was nicely pictured by the various channels and artificial composites of Meteosat 8. In the next set of image the convergence is again described but now is numerically reflected by the divergence at 700 hPa
Meteosat 8 IR10.8 and Thermal Front Parameter + Relative Topography: time sequence
In this chapter the large frontal zone over Western Europe is presented using the Thermal Front Parameter. Along with this parameter also the relative topography is shown.
Meteosat 8 IR10.8 and Temperature Advection 700 hPa: time sequence
Since thermal advection is analysed on a more synoptic scale it is not likely to play a direct role in the local development of thunderstorms. The horizontal distribution of warm air advection and identifying its maxima is important since it can slightly contribute to upward motion and convection. In this chapter the Thermal Advection at 700 hPa is described in relation to the prefrontal convergence. It is overlayed on Meteosat 8 IR10.8 imagery and presented in 6-hour sequences.
Meteosat 8 WV6.2 and height of PV=2: time sequence
In the following sequence the existence of several areas where deepening observed is numerically being reflected. The fields of the height of PV=2 are displayed in a 6-hourly sequence.
Meteosat 8 IR10.8 and Showalter Index: time sequence
One way of characterising the stability of the atmosphere is by making use of the Showalterindex. Values under 0 are a sign of a lable atmosphere. Values under -3 are very unstable and are related to thunderstorms. In this chapter the Showalterindex is presented, overlayed on Meteosat 8 IR10.8 imagery, in 6 hour sequence.
Meteosat 8 IR10.8 and ThetaE 850 hPa: time sequence
Second way of representing the characteristics of the stability of the atmosphere is by making use of ThetaE 850. In this chapter ThetaE 850hPa is presented, overlayed on Meteosat 8 IR10.8 imagery, in 6 hour sequence.
Global Instability Index (GII): time sequence
Global instability index (GII) is an airmass parameter indicating the stability of the clear atmosphere.
The GII product should serve as a nowcasting tool to identify the potential of convection and possibly of severe
storms in still preconvective conditions. The applied retrieval method makes use of six MSG SEVIRI thermal bands,
and together with the a priori information of forecast profiles, the scheme infers an updated
atmospheric profile for each MSG pixel, from which instability indices can be computed. The instability index used in this case is the K-index. The images are presented in 1 hour sequence.