• Isentropes :
  In vertical cross sections from the ECMWF model the isentropes of equivalent potential temperature show a superadiabatic stratification in the mid levels of the troposphere in all stages of the life cycle of convection. This is clearly a good indicator for strong convection.
• Relative humidity:
  As convection appears near the boundary between moist and dry WV features, two indicators are of interest in vertical cross sections of the relative humidity:
  • the representation of the dry and moist zones in WV images
  • the development of the CBs
  The dry stripe is accompanied by a relative minimum of relative humidity. As soon as CBs develop, the WV and IR pixel values show distinct maxima associated with areas of convection. However, the corresponding humidity maxima are mostly small.

Isentropes

Relative humidity

The MCS over S. Austria is well resolved in the IR and WV signal, but the ECMWF vertical distribution of relative humidity shows neither the dry, nor the wet areas satisfactorily.
Therefore, the use of information from a high resolution Local Area Model (LAM) NWP model would be a useful alternative.
The ALADIN LACE NWP model offers data at a higher spatial horizontal resolution and an acceptable vertical resolution when compared to the ECMWF model output. Note, relative humidity is presented as vertical distribution at 3 gridpoints: Linz (LIN), Aigen (AIG), and Graz (GRA). The WV image above shows the position of a curl over Eastern Austria with the positions of the pseudo - TEMPS superimposed. A small feature at the southern transition zone between "wet" and "dry" air, and lying between Aigen and Graz, indicates a growing convective cell complex over Southern Central Austria (see Cloud structure in satellite images).

The ALADIN pseudo - TEMP of relative humidity shows a distinct signal demonstrating the physical concept of convective initiation under fair weather conditions.

The ALADIN output displays the typical WV distribution as well as a growing cell much better than the coarser global model:
The difference between the moist and dry zones seen in the WV image of 09.00 UTC is well modelled. All stations show a high gradient (from about 60% to 10% relative humidity) between about 800 hPa and 550 hPa.
Grid points within the WV wet zone, like Linz, show a distinct maximum of relative humidity at upper levels around 700 hPa. Drying starts at 700 hPa at the onset of convection.
Aigen is located at a grid point within the transition zone between high and low WV pixel values and next to the observed start of convection in the 09.00 UTC WV image. The vertical distribution of relative humidity shows air with 40% RH up to 750 hPa and a small maximum at 650 hPa. The layer of dry air starts around 600 hPa. Another grid point in the WV dry zone is Graz, with relatively moist air at lower levels up to 800 hPa and the beginning of dry air at about 750 hPa. Relative humidity is well resolved in the ALADIN PSEUDO-TEMPS and there is a good correspondence with the pixel values in the WV imagery.
The high gradient of relative humidity can be seen as the beginning of vertical transport of humidity caused by insolation of short wave radiation and diabatic heating on the ground.
The time of 09.00 UTC is equivalent to 11.00 local time, and insolation would have started about 5 hours earlier.