These images are based upon the randomly selected time step 18UTC on 2008-04-04. The layer is always the one at the lowest altitude because only this is subjected to downscaling.

Fig. 1: Pressure as supplied by the COSMO model: The coarse data.

Downscaled data

Due to the existence of a Schomburg rule for pressure, it can be subjected to full downscaling (fig. 3,5,5.1). Figures 2,4,4.1 deal with results from mere interpolation.

Fig. 2: Pressure after spline interpolation. Fig. 3: Pressure after full downscaling.

Fig. 4: Pressure after spline interpolation and subsequent upscaling back to the coarse grid. Fig. 5: Pressure after full downscaling and subsequent upscaling back to the coarse grid.

Fig. 4.1: Pressure bias after spline interpolation, on the coarse grid. Fig. 5.1: Pressure bias after full downscaling, on the coarse grid. The axis limits have changed.

While common fluctuations within the pressure field hardly ever exceed 10^4 Pa = 100 hPa, the interpolation bias fluctuations are much smaller, on the scale of 10^–3 Pa. This encourages the interpretation that the calculated bias is 0 plus some numerical fluctuations. The bias from full downscaling is slightly larger, but still well below what could be considered significant.