Fog RGB composite from GOES

By Lorena Guerrero (Servizio Meteorologico Nacional Argentina)

In order to detect fog and low clouds during the night time, EUMETSAT has developed an MSG RGB combination of infrared bands based on channels 4, 9 and 10 (IR3.9, IR10.8 and IR12.0, respectively), where low level clouds are show with green colours. When the cloud top becomes higher it turns to a brown colour, the highest clouds have a red colour (see RGB recipe and examples).

The National Weather Service of Argentina has begun to work on the creation of a similar product using GOES-12 images (see RGB recipe and example). It is based on GOES channels 2, 4 and 6 (at IR3.9, IR10.7 and IR13.3, respectively). The replacement of the MSG IR12.0–IR10.8 Brightness Temperature Difference (BTD), on red colour beam with the IR13.3–IR10.7 BTD in the GOES composite was necessary as GOES-12 does not have an IR12.0 channel.

The resulting colour differences between MSG and GOES composites are minor once the thresholds are adjusted (see images below), but users must be aware that the IR13.3–10.7 BTD represents mainly cloud height information (high clouds have a small BTD, low clouds have a larger (negative) BTD), whereas the IR12.0–IR10.8 BTD is used to represent cloud optical thickness (thick clouds have a small BTD, thin clouds have a larger (negative) BTD). Thus, the biggest difference between the GOES and the MSG RGB product is that all high level clouds look reddish in the GOES RGB product, whereas they appear red (thick ice clouds) to dark blue/black (thin ice clouds) in the MSG RGB product.

Synoptic analysis

During 26 and 27 September 2009, a very cloudy cold front moved over the central and northern parts of Argentina. Additionally, a warm front in the north was slowly moving south until 27 September, when it became a frontal wave due to the influence of a trough west of the Cordillera Andes mountains. The front then reversed direction and moved northwards.

Convective activity took place mostly over northeast Argentina, Uruguay and the south of Brazil. In the north and northwest of Argentina, south of Paraguay and south of Bolivia there are generally stratiform clouds.

The first set of images from both satellites relate to 28 September at 00:00 UTC. Due to the fact that the cold front reaches low latitudes, its velocity slows and its slope decreases, staying as a wide frontal zone over northern Argentina, south Paraguay, south Brazil and Uruguay.

From this RGB combination low level clouds can be distinguished (area A) in a green colour while developing clouds (area B) are coloured pink and red, depending on the height of the cloud top. The northern system can be clearly observed as a cumulus line with shallow development (area C). Over the south of Brazil, convective cells have deeper development that extend to a cirrus layer to the east, which can be identified by an intense red or black colour (area D). Very low (cold) IR3.9 values are present as black in the GOES RGB product (red-yellow speckled in the MSG RGB product). To the south of the system, a well defined post-frontal anticyclone is clearly visible as a cloud-free area (E).

The second pair of images relate to 29 September at 08:00 UTC. The frontal zone has moved a few degrees northward, now covering northern Argentina, Paraguay, southern Brazil and east Bolivia (area F). The part of the trough that is associated with the active frontal system is now over the Atlantic Ocean whilst another part remains on the western side of the Cordillera Andes. It is possible that this latter trough is associated with the easterly flow of the post frontal anticyclone, and which results in the generation of mid-level clouds within the cold air along the eastern slopes of the Andes. This is seen as an increase in cloud cover to the south of this region (area G).

In addition, in the 29 September images a large area of cirrus clouds can be identified (area H). They are formed by the jet stream above the low-level clouds (possibly a high-level wave cloud over the Andes mountains, see also wave clouds on 1 June 2007). The GOES RGB product shows these cirrus clouds in red, while the MSG RGB composite shows black in this area due to the low optical thickness of cirrus clouds (negative IR12.0–IR10.8 BTD). The subsidence of the anticyclone, deeper by this time, can still be seen because of the lack of clouds (area I). Finally, the cold air over the relative warmer sea generates air mass instability, seen as open cells near the lower right corner of the image.

Acknowledgements

The GOES images were generated using the Terascan system for GOES data reception and display. The Meteosat images were obtained from EUMETSAT and processed with the Nubes package.