  A comparison is carried out between two operational wave
forecasting/assimilation models for the North Sea, with the emphasis on the
assimilation schemes. One model is the WAM model, in combination with an
optimal interpolation method (OIP). The other model, DASWAM, consists of the
third generation wave model PHIDIAS in combination with an approximate
implementation of the adjoint method.
  In an experiment over the period February 19 - March 30, 1993, the models are
driven by the same wind field (HIRLAM analysis winds), and the same observation
data set is assimilated. This set consists of a) spectra from three
pitch-and-roll buoys and b) Synthetic Aperture Radar (SAR) spectra from the
ERS-1 satellite. Three analysis/forecast runs are performed: one without
assimilation, one with assimilation of buoy measurements only, and one with all
data assimilated. For validation, observations from four buoys, altimeter data
from ERS-1 and Topex-Poseidon, and scatterometer data from ERS-1 are used.
  A detailed analysis of the "Wadden Storm" (February 20-22) shows the very
different nature of the two assimilation schemes: the wave and wind field
corrections of the WAM/OIP scheme are all in the vicinity of the observations,
whereas the DASWAM adjustments are more of a global nature. The impact of some
individual buoy and SAR observations is visualized. A comparison of the
performance of the two schemes is somewhat obscured by the very different
behaviour of the two first-guess runs.
  A statistical analysis over the whole 39-day period gives the following
results. In a comparison with buoy observations it is shown that a positive
impact of wave data assimilation remains until about 12 hours in forecast in
