Precipitopography

As I write this, the UK and Ireland are being lightly scoured by winter storm Brendan, which is producing some lovely patterns on the radar rainfall maps. Generally speaking, we get our weather from the southwest, and the prominent western coastlines receive the brunt of the rainfall. When you look at one of these maps, it seems as though you can even make out the coastline, marked by the place where the rain begins (it’s a lot more obvious on an animated map – the fronts and clouds move, but the coast doesn’t).

So, today’s question is: can we reconstruct Great Britain’s coastline using only rainfall data?

With the help of Bash and wget, I am now the proud owner of 288 snapshots of the UK and Ireland, sampling every five minutes from 00:00 to 23:55 on the 13th January 2019. During this time, the front visible above swept across the entirety of Great Britain, generally from west to east.

The first step to turning these into useful data is dealing with the awful colourbar in use on netweather.tv. Luckily, the images aren’t antialiased, so it’s as simple as comparing the pixel values to those in the colourmap.

Now, we’ll integrate over our 24-hour period – this just amounts to stacking all the images and summing the values in each pixel. The result should be a map of (a pretty good approximation of) the total rainfall received at each point in the UK and Ireland:

That’s not too bad! It’s a long shot from accurately capturing the coastline, but there’s a lot of detailed topography on view here. Incidentally, this effect is called orographic rainfall, and is caused by the terrain lifting incoming air, which forces water to condense. The area to leeward is termed the rain shadow, and gets less rain because the atmosphere has lost water content, and while descending the hills, the air warms, which increases its water-holding capacity.

But first, let’s talk artifacts. The most prominent is the mysterious blocky island over St. Geroge’s Channel between Ireland and Wales. This isn’t really visible in any of the individual frames, so I guess it must correspond to a region where whatever algorithm they use overestimates the rate of rainfall; it does seem to be in an area that’s quite far away from the radar stations.

You can roughly guess where the stations are by looking for the rays emanating from them – I assume these are caused by shadows and reflections from terrain and objects near the detectors. One neat thing is that you can see how the range of each sensor is about 150 km; this is too far for line-of-sight, but the radio waves are able to diffract over the horizon. I’m able to spot seven stations in the UK and Ireland (marked with black dots on the right-hand map), near Limerick, Belfast, Exeter, Preston, Glasgow, the Isle of Lewis, and a field in Hertfordshire. It feels like there should be more coverage in the east, but it’s hard to tell if there is any there, since it’s so dry.

On that note, there’s a surprisingly huge variation in the amount of rainfall here, after two pixels in Ireland which claimed over half a metre each (which I do not believe), a few spots in the Scottish Highlands claimed over 100 mm of rainfall, while the east of England only got 3 (I guess it felt like more with the wind).

Finally, there seems to be a very prominent (and literal) rain shadow off the northeast coast of Great Britain; for a distance of about 50 km, the radar records no rainfall at all. I’m not sure if this is real, but would be surprised if it were – how can a cloud know if it’s over land or sea? I was wondering if cliffs on the coast could be barriers to the radar signal, but I can’t come up with any reason why they should behave differently to cliffs on land. A mystery to be sure… I might retry this on another day, and see if the shadow’s still there.

Anyway, here’s a nice animated map – watch that front sweep across from 13:00 onwards.