wiki:udg/ecoms/RPackage/examples/bias

Version 5 (modified by sixto, 7 years ago) (diff)

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Example 3. Calculating the bias of an ensemble forecast

In this example we will calculate the bias of the multimember forecast loaded in the previous example.

We first load the reference observations for the spatio-temporal domain previously chosen:

ex2.obs <- loadECOMS(dataset = "WFDEI", var = "tasmin", lonLim = c(-15,35), latLim = c(32, 75), season = c(12,1,2), years = 2001:2010)
print(object.size(ex2.obs), units = "Mb") # 60.6 Mb

This is the map of the observed mean minimum surface temperature observed for DJF 2001-2010:

plotMeanField(ex2.obs)

No image "image-20140902-182518.png" attached to udg/ecoms/RPackage/examples/bias

Note that WFDEI provides data for land areas only, and its spatial resolution is finer than CFS (1º vs 0.5º). In order to compare both datasets, it is first necessary to put them in the same grid (i.e., to interpolate). We use bilinear interpolation to this aim, using the downscaleR function interpGridData in combination with the getGrid method, useful to recover the parameters defining the grid of a dataset to pass them to the interpolator:

obs.regridded <- interpGridData(gridData = ex2.obs, new.grid = getGrid(ex2), method = "bilinear")

Note the warnings reminding us that the extent of the input grid is wider than that from CFS. However, in this case we can safely ignore this warnings, since all the land areas we are interest in are within the CFS domain.

Warning messages:
1: In interpGridData(gridData = ex2.obs, new.grid = getGrid(ex2), method = "bilinear") :
  The new longitudes are outside the data extent
2: In interpGridData(gridData = ex2.obs, new.grid = getGrid(ex2), method = "bilinear") :
  The new latitudes are outside the data extent
plotMeanField(obs.regridded)

No image "image-20140902-182614.png" attached to udg/ecoms/RPackage/examples/bias

Now that both model data and observations are in the same grid, we can compute the bias. First, we calculate the mean of WFDEI, which is the reference against which to compute the biases:

ref <- apply(obs.regridded$Data, MARGIN = c(3,2), mean, na.rm = TRUE)

The following lines of code compute the bias of each member w.r.t. the reference and plot them:

# Now we compute the difference agains each of the multimember spatial means:
require(fields) 
n.members <- dim(ex2$Data)[1]
par(mfrow = c(1,2))
for (i in 1:n.members) {
      member <- apply(ex2$Data[i, , , ], MARGIN = c(3,2), mean, na.rm = TRUE)
      bias <- member - ref      
      image.plot(ex2$xyCoords$x, ex2$xyCoords$y, bias, xlab = "lon", ylab = "lat", asp = 1)
      title(paste("Bias member", i))
      world(add = TRUE)
}
par(mfrow = c(1,1)) # To reset the graphical window

No image "image-20140902-182715.png" attached to udg/ecoms/RPackage/examples/bias

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