• Height contours 1000 hPa and 500 hPa:
  
  • In contrast to front related convection, development can take place within a low and upper level ridge
  • Weak pressure gradient
  • No large scale upward motion or forcing within the ridge
• Thermal front parameter (TFP) and equivalent thickness:
  
  • initiation of convection in clear weather is not related to a significant gradient of equivalent thickness nor any distinct Thermal Front Parameter
• Equivalent potential Temperature at 850 hPa:
  
  • ridge and/or a maximum of Theta_E 850 hPa
• Showalter Index:
  
  • unstable stratification over the area of interest indicated by the Showalter Index 500/850 hPa
  • the diurnal variation of the instability indicated by the Showalter index is a result of the diurnal cycle of the available insolation.

Height contours 1000 hPa and 500 hPa

Thermal front parameter (TFP) and equivalent thickness

Equivalent potential Temperature at 850 hPa

Showalter Index

Meso-Scale Parameters

ALADIN model forecast fields:

ALADIN model fields are computed with a higher spatial resolution (about 15 km horizontal Grid) than ECMWF model fields (average distance between grid points is about 39km). Therefore, meso scale dynamics of convection can be shown in more detail.

Fields of divergence and vertical motion show a diurnal life cycle:

• Divergence
  No significant convergence or weak divergence at low levels and weak convergence at higher levels in the morning hours. With beginning of convection, there is distinct convergence at lower levels and divergence at higher levels.

On 16 May 06.00 UTC there is distinct divergence over the Alps and convergence in the region of the WV eddy over S. Germany at lower levels (925 hPa). At higher levels only weak indications can be observed. At 12.00 UTC the convection has already started and is clearly indicated by distinct convergence at 925 hPa and divergence at 700 hPa associated with the bright white convective cells seen in the WV image.

• Vertical motion
  A strong upward motion at lower levels (for instance 850 hPa) indicates convection.

At 06.00 UTC weak vertical motion is seen near the WV eddy over S. Germany and distinct descent over the Alpine region.
At 12.00 UTC strong ascent can be observed over the Alps especially in the region of the convective clouds over Austria. Distinct downward motion is indicated in the region the black stripe associated with the WV vortex.

The above model fields show a good correspondence with the convective activity observed in the satellite images at 12.00 UTC. However, there is no indication as to when convection might start in the model fields at 06.00 UTC. The initiation of convection still seems best indicated by the WV features described above (see Cloud structure in satellite images).