Designing the UK domain: where the edges go
The first decision in any WRF setup is where the model sees air. Here is how the Convek UK domain is laid out and why the boundaries sit where they do.
Before you pick a PBL scheme, before you argue about microphysics, before any of the physics that actually matters for soaring, you pick a domain. The domain is the rectangle of air WRF integrates over. Everything inside the edges is simulated. Everything outside is borrowed from the global model driving you, nudged in at the boundaries every three hours.
The geometry matters more than most tutorials let on. WRF is accurate in the middle of your domain and progressively less so near the edges, where the boundary conditions from GFS are imprinted onto the solution. A rough rule is that the outer 8 to 10 grid cells on each side are contaminated by boundary adjustment. At 4 km spacing that is 30 to 40 km of unusable rim. Any soaring area sitting inside that rim is going to forecast badly, and there is nothing you can do in the physics to save it.
So the first design question is: where are the pilots, and how much buffer do we need around them? For the UK, the furthest-flown areas in summer are the Scottish border ridges in the north, the Long Mynd and Mid Wales in the west, the South Downs and Kent in the south, and East Anglia. The domain has to comfortably cover all of them and then add the rim on top.
The actual UK domain runs from roughly 47.5 N to 60 N and 11 W to 5 E. That puts Scottish flying well inside the domain, buys a clean buffer of open Atlantic off the west coast where the prevailing weather comes from, and clears the North Sea before the eastern edge. No populated soaring area sits in the contaminated rim.
We run a single domain at 4 km rather than a nested parent-and-nest setup. Nests are useful when you want high resolution over a small area and do not care about the rest, but the UK is small enough to run the whole thing at 4 km for roughly the same compute as a two-way nest would cost, and you get consistent physics everywhere. No nested-boundary weirdness, no ratio-of-three grid jumps, no fiddly feedback options. 1 km nests will come later over specific sites, but the base layer stays flat.
The westerly buffer over the Atlantic is deliberate. UK weather arrives from the west, so giving WRF 300 km of open ocean to develop the incoming airmass on before it hits land means the boundary layer over soaring country is a proper WRF solution rather than a freshly-interpolated GFS field. This is particularly important for moisture: cloudbase forecasts degrade badly if the airmass has not had time to equilibrate inside the domain.
One thing that is not obvious from the map: Ireland sits inside the domain. We are not publishing Ireland coverage yet (that needs a separate validation pass and a different set of soundings), but including Irish terrain in the simulation is what keeps the airmass arriving over Wales physically sensible. If you cut the domain at the Irish Sea, the boundary layer over Snowdonia develops from a GFS field that has skipped Ireland entirely. That is the kind of invisible mistake that shows up three months later as a systematic cloudbase bias.
The map itself will get its own post once the coverage page gets the interactive version. Next post: vertical levels, and why the bottom 3 km of atmosphere gets most of the detail.