by Jochen Kerkmann and Gordon Bridge (EUMETSAT)Jump to images
During the 3-day period from 6 to 8 January 2005 parts of Northwest Scotland and Northern Wales received well over 100 mm of precipitation, with an observatory on the mountain called Capel Curig (in Snowdonia) reporting 225 mm of rain (see report on severe storm from D. Jameson, PDF, 9 KB).
Over 100 flood warnings were issued by the UK Environment Agency as the relentless rain brought widespread flooding to these areas, with Carlisle, in Cumbria, particularly badly affected. Carlisle was effectively cut off by flood waters and ‘awash’ for some time on the 8th with flood waters nearing the second floor of some houses, leaving many residents stranded and even some having to be airlifted off farmhouses. The town of Keswick was also badly hit by flooding with its residents having to be evacuated by mountain rescue teams and inflatable boats. Many other areas in the north of England experienced similar flooding.
The sequence of 6-hourly colour composite images presented below clearly show the rapid development of the storm over time. An account of the associated atmospheric dynamics can be found in a detailed discussion of the synoptic background and development of the storm by PJ Blight (see synoptic situation and development, PDF, 371 KB).
These images are Brightness Temperature Difference (BTD) images showing the Meteosat-8 IR9.7 channel brightness values minus the IR10.8 channel brightness values. The cloud structure of the frontal zone is easily seen in this combination, but of particular interest is the intrusion of dry stratospheric air behind the cloud head, as it develops, and as shown in the initial stage and advanced stage I images below. By the time of the advanced stage II image the dry stratospheric air is well tucked in to the south and east of the cloud head (see IR9.7 - IR10.8 image, GIF, 283 KB). This is likely the time of most active development of the storm system.
As known from Meteosat First Generation, the WV6.2 channel is essential for the early identification of severe storm development. At 22:00 UTC on 7 January 2005, the WV6.2 image clearly shows a dark stripe between the baroclinic leaf (or cloud head) and the cloudiness from the warm conveyer belt. This dark stripe, or dry instrusion, is caused by the advection of dry sinking air originating from the lower levels of the stratosphere along the cyclonic side of the jet stream (see WV6.2 image, GIF, 277 KB). It is already visible at 15:00 UTC, although to a lesser extent.
Another new possibility of early detection of rapid cyclogenesis (besides the above described BTD between the IR9.7 and IR10.8 channels) is given by the BTD between the WV6.2 and WV7.3 channels. The interpretation of this BTD is not straightforward and can be quite ambiguous (see figure, source: C. Georgiev and Weldon & Holmes, GIF, 45 KB). However, for cloudy pixels this BTD is strongly related to the height of the cloud, i.e. high clouds exhibit small (negative) WV6.2 - WV7.3 BTD and low clouds exhibit larger (negative) differences. Very high clouds that reach the lower stratosphere may even present small positive values (between 0 and +5K). For cloud-free areas, this BTD depends on the vertical distribution of humidity and temperature in the troposphere. A moist layer at about 400-500 hPa will produce large (negative) differences (around -25 K), whereas dry air at upper and middle troposphere above moist air at lower levels will produce smaller (negative) differences (between -10 and -15 K). However, it should be pointed out that such a difference could also result from a moist layer at high levels above dry air at low and middle levels (thus the ambiguity).
At 03:00 UTC there are already signs of descending stratospheric air with high values of Potential Vorticity (PV) on the western side of the upper trough over the West Atlantic, which is just beginning to interact with the long frontal band lying all across the Atlantic (see WV6.2 - WV7.3 image, GIF, 325 KB). This high PV air is indicated by a broad whitish band in the WV6.2 - WV7.3 BTD image. This band perfectly corrisponds to a dark stripe in the WV6.2 image. Although not in this case, sometimes this features also appears as a faint "white stripe" in the IR9.7 image (Ozone channel). It may be that the "limb whitening" when looking obliquely through the atmosphere (high satellite viewing angle) hides the signal coming from the descending stratospheric air.
Also shown below is a RGB composite that combines the best three MSG features for the early detection of rapid cyclogenesis: the WV6.2 channel, the WV6.2 - WV7.3 BTD and the IR9.7 - IR10.8 BTD. All three features show the height of the clouds, but they are also strongly related to airmass characteristics in cloud-free areas. The WV6.2 channel shows the Upper Tropospheric Humidity (UTH), the WV6.2-WV7.3 difference shows the vertical distribution of humidity and the IR9.7-IR10.8 difference is related to the height of the tropopause. Combined together in a RGB, the result is an image where high clouds appear in white colour, mid-level clouds in light ochre colour and cloud-frees 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 the dry descending stratospheric air is marked by a reddish colour (see RGB composite, JPG, 596 KB).
Finally, for completeness, also one of the recommended night-time RGB Composite is shown in the 6-hourly sequence below. While it is not useful for the detection of dry intrusions, it nicely shows the thickness of the clouds: thin (high-level) clouds appear in a dark blue colour; thick clouds in a orange/red colour.
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