A dry May but maybe not a dry summer

[Len]

Only 1.1 mm overnight giving a May total of 17.7 mm so far (average 68 mm).

Any perception of summers(JJA) becoming drier however down here in SW Devon does not hold up.

In fact there is a hint of summers becoming wetter since 1985.

Len

Wembury, SW Devon coast

83 m asl

[Julian Mayes]

Hello Len. I think we had this discussion last year but it is nice to see an update of your smr rainfall graph.  

I'm not sure how you can infer a forecast for the coming summer on the basis of a past trend. There have been a few long range forecasts issued in April that suggested a dry month spell, i.e. into June. We shall see but May could be another dry month. (btw you may have seen coverage of the sunniness of this spring - Reading Univ has recorded its sunniest spring on record already). 

As to your graph, the linear trend of course depends on your start-year. I'd allege that the 80s and 90s were the tail end of a period of quite dry summers in the SE half of the UK and you'd have been on the fringes of that. 2007-2012 is shown clearly as a spell of wet summers but most summers in the south and east since then have been 'fine' - not all, I admit - but your graph shows the downturn from 2013 clearly. As you know, this is all in the direction indicated by climate models for summer, though naturally unlikely to continue without further blips upwards. Surely these models show consistency for summer conditions that is sadly absent in the winter projections - wetter and milder winters suggested what the course of westerliness (and hence regional changes) still seems unresolved.  

Cheers   Julian  

[Len]

Julian,

I can't understand why you think I was making a forecast for the summer.

Actually I was provoked by your post and someone else's on UKweatherworld inferring the  SE has to get used to drier summers.

I wanted to point out that at least in the SW since 1985 this is not apparent.

There is a large year to year variability.

There is a difference between continental SE and maritime SW. Models are poor for UK rainfall projection on this scale.

I don't think there is a signal in the SW for drier summers unless you look from 2012.

Generally the rainfall climate projections have large uncertainties. Not surprising when feedbacks are crudely parameterised.

Len

Wembury

[Julian Mayes]

On Saturday, May 23, 2020 at 5:25:38 PM UTC+1, Len wrote:

Julian,

I can't understand why you think I was making a forecast for the summer.  

 your subject-line. 

 

Actually I was provoked by your post and someone else's on UKweatherworld inferring the  SE has to get used to drier summers.

I'm not clear why you find the orthodox position on future climate change provocative. 

 

I wanted to point out that at least in the SW since 1985 this is not apparent.

There is a large year to year variability.

Yes, and with a recent spell of wetter summers (with drier summers either side), not unreasonable to assert that the recent past may not be a good guide to the future. 

The MO future projections (summarised in this report   https://www.metoffice.gov.uk/pub/data/weather/uk/ukcp18/science-reports/UKCP18-Overview-report.pdf  )  do illustrate the wettest and driest plausible extremes in the projected seasonal climate (10th percentile maps showing the wetter end and even these do not show an increase in summer rainfall in summer Britain). 

Graphs on p. 17 of that report  show the deviation of recent wetter summers from the future projection range, but this is for the UK as a whole. If you look st the SE - and possibly even the SW - the wet anomaly was smaller. I' m hardly going to deny the wet anomaly - let's face it, we wrote a joint article in Weather on the Yorkshire floods of 2007! My comments were for the SE.  If the MO website had decent navigation, I'd add a link here to the maps of monthly and seasonal rainfall anomalies back through the C20th - have looked, cannot find it. It shows my point about the SE missing the worst of the wet anomaly, though not all of it. 

Another thing we have to keep an eye on is if rainfall does become more erratic as per model projections, total rainfall may rise (or be maintained) for a particular month even though the number of dry spells may increase and the number of rain / wet days fall. 

Julian     Molesey    0.8mm yesterday.     5mm for the month so far.  Any more?    

 

[Len]

I was only making a couple of points.

1. Climate change in terms of rainfall is showing a difference between SE and SW Engalnd.

Should n't paint southern Britain with one brush.

2. Climate projections on a regional scale can be done because there are models and powerful computers.

But where is the verification?. As you point out, from when are we looking at?

Everyone can see we are having a warming. But rainfall?

It just annoys me that more powerful computers does not mean better forecasts/projections.

Especially on the very short time scales and the long (climate) timescales.

Ensembles or no ensembles.

We need better physics and better modelling of feedbacks.

Len

Wembury

Grass turning brown here.

.

[Julian Mayes]

Thanks Len - completely in agreement now - especially your first point.  And people do say 'north / south' when they really mean nw- se' or sometimes 'w - e'.   And as you say, SW climatically completely different to SE / E Ang. 

Front garden - grass brown-ish. 

Back garden - grass green. I wonder how that could be?   Flower bed watering spilling onto the lawn - and sometimes in the rain gauge.

Julian   

Molesey

[Graham Easterling]

We also need what's actually happening being fed back into the forecasts. Sometimes the forecast can be rather silly just 2 or 3 hours ahead. A bit of nowcasting is required. 

Also, in this part of the world, the ability to model sea breezes. Sometimes you can get a circular pattern over Penwith (the Lands End) peninsula. In Penzance it typical means the early onset of a S-SE sea breeze, a calm period, then a sharp drop in temperature as the N coast sea breeze arrives through the Hayle Saltings / Marazion Marsh gap. The onset of the north coast sea breeze is often very sudden, around 16:00. Probably influenced by the NW facing 'north' coast not getting the full impact of the sun until mid afternoon. These big differences in sea breeze impacts over a couple of miles are quite beyond the models it would seem, and it's all rather important. The southward movement of the north coast sea breeze front in the afternoon can generate some big showers. 

Graham

Penzance

[Len]

I think it's a big ask Graham to expect the models to forecast the sort of local knowledge you have.

Sea breezes are forecast but they become more and more unreliable the more complex the coastline is.

It could not get much more complicated than it is in west Cornwall.

I think the fine mesh model being used has 10km scale.

Not good enough in many cases so the solution is as you say is to have more intervention from a forecaster with local knowledge to give a nowcast.

It's not going to happen. UKMO did away with all local offices in the 90s. Too expensive.

Len

Wembury, where we have had sea breeze since about 0900 BST, quite an early onset.

[Julian Mayes]

Len,  off hand, IIRC, global models (MO and EC) now ~ 9km (EC HRES) 10km (MO global) (EC ensembles 18km, I think).     hi-res down to 4km and even 1.5km from MO - 4km EURO4 model can appear on the MeteoGroup / BBC graphics if you squint (showers give it away).  

To go back to the showers in the SE discussion in another thread, the 4km model was capable to showing the showers forming along the convergence lines that Smartie discussed. 

I'd say the limiting factor now is the willingness of the media to give forecasters adequate time to do justice to the amt of info available. Nowcasting can be achieved through creating a supplier's model output stats whereby local knowledge can  refine the raw model output.   

Julian  

[Len]

Thanks Julian,

I thought I'd read somewhere that 1.5 km is used. Not sure it is used operationally in terms of the hourly online forecast.

In theory it should help in forecasting flash flooding. Although it didn't with the Coverack flood of 18 July 2017.

According to Graham, it is not able to help with the complex sea breeze front developments and interactions in west Cornwall.

Len

Wembury, SW Devon