Unusual radar image on 11 November 2013

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Unusual radar image on 11 November 2013

Postby Alan Sholl » 2013 Nov 14, 15:48

Did anyone notice on the Met Office rainfall radar on Monday 11 November that there was a narrow line of precipitation (approx. 5-10 miles wide) running from the Forest of Dean north through Hereford and Ludlow to Market Drayton in north Shropshire? The line was stationary and persisted for about 9 hours (1400-2300h) sometimes continuous, sometimes more fragmented, with mostly 0.01-1.00 mm/h indicated but sometimes, locally 2-8 mm/h. At my own station in Tenbury Wells which was close to or under the line I did not observe any measureable rain during this period - it was dull and overcast but with only an occasional spit of rain. We were in a warm sector throughout this period with light SW winds and, according to the radar, other patchy light rain was drifting across the country SW-NE as you would expect in these conditions. Perhaps not coincidentally, the analysis chart for 1200h on 11 November shows a curious cold front or trough crossing the warm front and the warm sector and lying SW-NE across the Midlands. I don't recall seeing either of these features previously. Any explanations?
Many thanks.
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Re: Unusual radar image on 11 November 2013

Postby Richard Hunt » 2013 Nov 14, 17:21

Could it be down to anomalous propagation?.

Just a guess.
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Re: Unusual radar image on 11 November 2013

Postby Martin Rowley » 2013 Nov 14, 21:21

... I can't comment on the Met Office radarnet imagery: I use that available via MeteoX.com and via their archive option, the rainfall moves. As noted elsewhere, if it was a static, narrow line, it's possible it was anomalous.

However, more generally, bringing up the 12hr rainfall amounts for 11/1800Z on a map shows that many stations under the path of that area of rain had between 4 and 7 mm of rain. To illustrate the spatial variability of rainfall (which you noted) down here in Dorset, Hurn recorded 4 mm of rain, whilst here in West Moors, just 5.5 km/about 3.5 mi to the NW, we managed just 1 mm of rain. It was an interesting morning, with persistent rain/drizzle and visibility < 5km, but an abrupt change to intermittent rain and much improved visibility occurred around 13Z.

The 'curious' front is an upper cold front: when the 'spikes' or 'bobbles' are not filled in, then the analyst has used some upper-air feature to decide on the alignment of the front - perhaps not obeying the classical 'Norwegian' model, but meteorology has developed tremendously in the last 30 years or so and if you are a frequent user of these charts you'll often see such upper features. The ability to use these symbols has been around for many years - I can remember a WMO 1960s handbook detailing the chart conventions for use by analysts and upper fronts (amongst others) were allowed for.

On the 850 temperature fields for the day (11th), it appears that the Chief Forecaster has used some such level to trace the axis of the upper warm plume - which is active in producing the observed rainfall. I note many will use an upper occlusion - but I think this Chief has correctly identified a zone where differential advection simulates the action of a cold frontal zone aloft - probably releasing potential (or convective) instability which accounts for the sporadic higher intensity - and for those areas that get relatively little rainfall.
Martin Rowley
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Re: Unusual radar image on 11 November 2013

Postby Alan Sholl » 2013 Nov 15, 15:22

Thank you both for your helpful and interesting replies. I've looked at the MeteoX.com charts and, to me, there is a static band of precipitation in a line just to the east of the Welsh Border. Admittedly it is much more fragmented than it was on the Met Office charts, in fact, it seems to dissipate and reform periodically over several hours, but always in the same line. It is most noticeable at 1545, 1830, 2000, 2030 and 2145 GMT.
Alan Sholl
 
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Re: Unusual radar image on 11 November 2013

Postby Martin Rowley » 2013 Nov 15, 18:41

Alan Sholl wrote:I've looked at the MeteoX.com charts and, to me, there is a static band of precipitation in a line just to the east of the Welsh Border. Admittedly it is much more fragmented than it was on the Met Office charts, in fact, it seems to dissipate and reform periodically over several hours, but always in the same line. It is most noticeable at 1545, 1830, 2000, 2030 and 2145 GMT.


... yes, my mistake and apologies: I was mesmerised by the PPN driven by the upper feature. I can see what you mean now and to my eye that's anomalous.

I did wonder if it might be associated with standing wave motion in the warm sector, but to persist for several hours *without any spatial displacement at all* is odd - you'd expect it to move about a little bit as the flow pattern changed.

I'd go with problems with the software used to clean up the imagery before presenting to the network - or one (or more) radar units being absent periodically. Predannack is notorious in this respect as it hasn't got a 'buddy-check' radar close by and often we see semi-permanent lines of apparent PPN across Mounts Bay. In this case, I suspect a 'spike' from Cobbacombe is occasionally over-riding the composite image.

There's a useful pdf of the Met Office web site here ... http://www.metoffice.gov.uk/media/pdf/j ... No._15.pdf

Unfortunately, to categorically decide we'd need access to the 2km radar archive at least (we see 5km) and ideally 1 km, though in this case the 1km wouldn't have covered the area required.

Martin.
Martin Rowley
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