Airmass RGB Time Sequence: 27 July 2006     

In this chapter the convection is studied using the 'Airmass RGB'. This RGB makes use of brightness temperature differences (BTD). The difference of water vapour channels 6.2 and 7.3 shows the distribution of moisture in the troposphere while BTD of the infrared channels 9.7 and 10.8 are related to tropopause height (total ozone concentration). Further, it incorporates the single WV channel 6.2, which shows the horizontal distribution of Upper Tropospheric Humidity (UTH).

High clouds appear in white colour, mid-level clouds in light ochre colour and cloud-free areas in dark green colour (warm air mass with high tropopause) or blue colour (cold air mass with low tropopause). A particular feature of this RGB is that dry descending stratospheric air is marked by a purple colour.

The image shows over eastern Germany blue areas that are related to dry, however, in that case warm air. Further to the West we see a purple area that is equivalent with relatively high IPV. According to the conceptual model "Enhancement of convection by PV" we recognize some convective cells. Over central Germany we observe the transition from purple to blue. That is equivalent to the western edge of the dark line in the WV image. Here we see again convection that occured according to the conceptional model "Convective cloud features at the leading edge of frontal cloud bands".

Surprising is the blue colour because we find very high precipitable water in high level of the troposphere in contradiction to the interpretation key of air mass RGBs.

Note: You can always click on the in the header to bring up the RGB-key.

The interpretation of the airmass RGB becomes more difficult during the formation of big cloud complexes. Airmass RGBs are best applicable before high extending clouds develop because the dynamical processes in combination with different air masses can be investigated.

In the introduction it was already pointed out that we will also consider some situations over West Europe. We find here very instructive examples

a) over England and the North Sea and
b) over Spain and France.

We see a small purple area over NE-England and southerly some small convective cells. Over Spain we find a further purple area. It is likely that convection will be triggered according to the conceptual model "Enhancement of convection by PV".

The purple area over England moved to the East and approached to the convective cells situated Southeast of it. Over central and northeastern Spain a similar process started yielding some convective cells.

This image reveals a West-East orientated purple stripe extending from Southeast Poland to the Ukraine. That is an instructive example for an advantage of the airmass RGB against WV-images: In addition to information about dynamical processes you see already first convection that is line-orientated. An intensification during the next minutes/hours should be assumed.

The purple zone over England continued approaching to the southeasterly situated convective complex that intensified. Over northeastern Spain and the Pyrenees we see a similar process that caused the intensification of convection in this area.

At the right side of the image (purple line-shaped area East Poland-West Ukraine) some convective cells developed.

We still see the triggering of convection by PV over eastern England (CTT in the meanwhile between -52 and -56°C). Analogue to that we see the intensification of convection over northeastern Spain and southern France.

An interesting detail can be seen near the western parts of the Alps and over Italy. The convective cells produced cold air indicated in form of blueish small areas in the airmass RGB.

The convection over England, Spain and southern France reached its maximum. CTT below -64°C were measured.

The cells over eastern England and Spain exhibit bigger areas. However, the CTT decreased during the last hour.