Line convection developing over SW Peninsula

[Len]

Over land some line convection has developed in last hour.

Len

Wembury

[Graham Easterling]

Sorry Len, didn't see your post until after I posted much the same thing. I hadn't refreshed the screen for a while.

I think Penzance has done a lot better than Wembury today, based on the radar.

Graham

Penzance

[Jack Harrison]

Wind arrows via X/C weather showed convergence possibly caused by sea breezes from opposite coasts.

Jack

[Graham Easterling]

Certainly possible, but think if sea breezes played in part it was very minor. The gradient wind was generally moderate and the temperature over the land was generally cooler than on the coast, and below the current SST (11C offshore - where there was considerable convective activity - some spectacular CB to the south of Penzance). There was also little in the way of sunshine inland, where it was dull & pretty chilly. The best sunshine, and the highest temperatures right on the coast.

e.g 10.3C at Newquay, 10.6C in Penzance etc.

Inland 8.5C at Camborne, <8C at Cardinham etc. Also virtually no sun to warm the ground inland.

I think other factors were at work. Getting out of my area of expertise (if I've got one!) but perhaps uplift over the moors created more instability. Sometimes you get this concentration of rain down the spine of Cornwall during times when the wind's aligned, even when the wind's near gale force and/or the rains frontal.

It would be good to get input from someone like Smartie or Freddie.

Graham

Penzance

[Len]

I thought it might be line convection because it only developed over land where friction would play a major part.

The following is after Browning et al

I know it was not a cold front per se but nevertheless

Len

Wembury

[Freddie]

Shower bands aligned with the peninsula are generally due to frictional or sea breeze convergence. As there weren't conditions conducive to sea breezes then I would suggest frictional as the cause - low level winds deflected slightly to the left overland when compared to over the sea owing to increased friction over and, causing a convergence zone aligned to the coast. You see this in a NNW'ly down the North Sea coast too. Position of the band relative to the coast depends on where the maximum convergence is happening. Upper winds need to be favourable for proper organisation of showers in to stripes/bands.

--
Freddie
Dorrington, Shropshire
115m AMSL

[Graham Easterling]

Shower bands aligned with the peninsula are generally due to frictional or sea breeze convergence. As there weren't conditions conducive to sea breezes then I would suggest frictional as the cause - low level winds deflected slightly to the left overland when compared to over the sea owing to increased friction over and, causing a convergence zone aligned to the coast. 

These 2 events are, in combination, fairly common in Cornwall, and have an interesting impact on the rainfall distribution.

Many actual & average rainfall maps are based on few actual reports and some assumptions, the main one of which seems to be the dominance of the orographic effect. I became increasingly convinced that the this was significantlyoverplayed for Cornwall, the distance from the spine of the County being a dominant feature.

Back in 1999, I got a bucketload of data (as EE would say)  from the EA, a contact was incredibly helpful - I had Excel spreadsheets coming out of my ears. I got more from the likes of the late Philip Eden & Stephen Burt. Within just a few miles of Penzance I got Porthcurno, (drier than me) Drift, (the same) Trengwainton (a bit wetter) etc.  As a result I cam up with this

Just a few actual figures put in as examples.

As I suspected, high ground away from the spine, was a good deal drier than south coast bays. Mount's Bay, Falmouth Bay etc, being wetter than the highest points of the Lizard plateau. Headlands being the driest, and similar to Scilly. Obviously, the orographic effect is superimposed upon this.

The wettest zone is generally just south of the spine. I think this is down to the strength of the north coast sea breeze in the afternoon, as the sun heats the NW facing north coast. (The south coast sea breeze always sets in first.) Whenever you get the line of showers associated with the N coast sea breeze onset, it nearly always starts close to the N coast gradually pushes further south. At Marazion it frequently flows through Marazion Marsh & across the beach, surprising the visitors who suddenly find their windbreaks facing the wrong direction. In Penzance the onshore breeze is commonly suddenly replace by a noticeably cooler NW north coast breeze around 16:00 - the peak temperature recorded in a calm period around 15:00. You can actually end up with a clockwise flow around the Lands End peninsula, useful having all these AWS these days..

Anyone, just thought I'd mention it!

Graham

Penzance