The second COMPARE exercise: A model intercomparison using a case of a typical mesoscale orographic flow, the PYREX IOP3

Fifteen models have been evaluated for their ability to simulate the variou s phenomena of a mesoscale orographic how sampled during the Pyrenees exper iment (PYREX). A pure forecast exercise has been conducted and model perfor mances were assessed using numerous observations. Two additional experiment s were also performed in order to discriminate between small-scale errors a nd large-scale induced errors, and to discuss an optimal specification of m odel terrain height and roughness for use with envelope orography and effec tive roughness length parametrizations. The comparison results reveal systematic errors for all the models: the loc al winds are too weak, the mountain-wave amplitude is too large and the lee vortices are poorly represented. Since forcing by analyses did not correct the errors, they can be therefore mainly attributed to the model represent ation of orography. The blocking created by the model topography at low lev el is under-represented and the model topography does not sufficiently slow the how. A positive consequence of the effective roughness length parametrization is to reduce the mountain-wave amplitude. Negligible benefit occurs from the use of an envelope orography parametrization. Although it favours the appea rance of the lee vortices, the latter appear too early, the local winds rem ain too weak, and the mountain-wave amplitude is enhanced. The comparison o f the computed pressure drag with the observed one is reasonably good for m ost of the models but the pressure drag is found to be more correlated to t he lee vorticity than to the mountain wave.