by ZAMG
|
|
Appearance in METEOSAT 8 imagery:
28 July 2005/10.00 UTC - Meteosat 8 IR 10.8 enhanced image
|
28 July 2005/10.00 UTC - Meteosat 8 WV 6.2 image
|
|
|
|
|
28 July 2005/10.00 UTC - Meteosat 8 HRVIS image
|
In the HRVIS image loop multiple small convective developments over the Northwestern part of Austria can be seen during the initial stage. As has been described above, the WV boundaries are preferred areas for initiation of convection.
An artificial composite of the infrared and water vapour channels of Meteosat 8 is called the Airmass RGB. The Airmass RGB consists of the WV6.2 - WV7.3 Brightness Temperature Difference (BTD, on red), the IR9.7 - IR10.8 BTD (on green) and the WV6.2 channel (on blue). A particular feature of this RGB is that dry descending stratospheric air is marked by a reddish colour. Details on the representation of other colours can be found here .
In the image below left the Airmass RGB for 28 July 2005 is given. The image shows clearly the WV red stripe extending over the border of Italy and Slovenia and then turning anticyclonically over Austria into Slovakia. On the border of this red stripe a convective cell develops in Lower Austria.
28 July 2005/15.00 UTC - Meteosat 8 Airmass RGB image (WV6.2-WV7.3, IR9.7-IR10.8 and WV6.2i)
|
28 July 2005/15.00 UTC - Meteosat 8 Severe Convection RGB image (WV6.2-WV7.3, NIR3.9-IR10.8 and NIR1.6-VIS0.6)
|
|
|
A second RGB usefull to detect convection is the Severe Convection RGB. An example of this RGB is shown in the image above right. The RGB which is build up of the composite of the BTD of WV6.2-WV7.3 on red, the BTD of IR3.9-IR10.8 on green and the BTD of NIR1.6-VIS0.6 on blue. In the image two cells are recognised: a yellow northern cell and a more orange southern cell. The colours of these cells explain the physical content. Whereas the northern cell in yellow is mainly consisting of very small iceparticles, the southern cell has further developed and already consists of larger ice particles. The presence of the BTD of the two water vapour channels on the red beam is clearly visible as the previously WV dark stripe appears in purple in this RGB. More information about the different colours observed in this Severe convection RGB can be found here .
The above image loop, for 19 June case, shows two regions with development of deep convection: first over the UK and second over the Alps appearing at noon over Southern France, Switzerland and northern Italy. Again the development starts within or next to the WV dark stripe. MCS development follows soon.
19 June 2005/15.00 UTC - Meteosat 8 Airmass RGB image (WV6.2-WV7.3, IR9.7-IR10.8 and WV6.2i)
|
19 June 2005/15.00 UTC - Meteosat 8 Severe Convection RGB image (WV6.2-WV7.3, NIR3.9-IR10.8 and NIR1.6-VIS0.6)
|
|
|
In the Severe Convection RGB, above right, both convective systems in France and England appear yellow indicating that they consist of small iceparticle and further development may be expected. The areas with low tropopause are recognised by its purple colouring.
The satellite loop above shows a dark stripe over the Iberian peninsula. It can be seen that the cells grow from the WV boundary into the drier zone (WV Dark Stripe). By 12.00 UTC convective activity is taking place over Catalunya in Eastern Spain. The WV image loop above shows a large number of developing convective cells over Austria, the Czech Republic and Poland. The cells develop on WV boundaries.