How to interpolate region on vectorplot?

Question

Continue from my previous question 'create vectorplot from velocity dataset'. I still have 2 question how to make the figure look like below:

1) How to make the region interpolation? I have tried used interpolate = TRUE but didn't work.
2) How to define the arrow symbol with the same length (It's mean the arrow only show the velocity direction)

Here my data uv.nc and syntax I have written:

library (raster)
flname <- 'uv.nc'
u <- raster(flname, varname = 'U')
v <- raster(flname, varname = 'V')
uv <- stack(u,v)
s <- sqrt(u^2 + v^2)

library(rasterVis)
jet <- colorRampPalette(c('#00007F', 'blue', '#007FFF', 'cyan','#7FFF7F', 'yellow', '#FF7F00', 'red', '#7F0000'))
range = seq(0, 0.5, 0.05)
vectorplot(uv, isField = 'dXY', interpolate = TRUE, col.regions = jet, region=s, length=0.05)

Answer 1

(First question) The interpolate argument needs the panel.levelplot.raster function to be called by levelplot (used internally by vectorplot to render the background). However, this does not work directly with the current version of rasterVis. You can try this trick:

levelplot(s, 
          panel = panel.levelplot.raster, 
          interpolate = TRUE,
          margin = FALSE) + 
vectorplot(uv, isField = 'dXY', region = FALSE)

(Second question) The length of the arrows is determined by your data, because you are using dXY = TRUE. Thus, you should use modify your data to get vectors with the same magnitude.

uv0 <- uv / s
vectorplot(uv0, isField = 'dXY', region = s)