Specification of External Forcing for Regional Model Integrations

Abstract

The effect of vertical and time interpolations of external forcings on the accuracy of regional simulations is examined. Two different treatments of the forcings, one with conventional lateral boundary nudging and the other with spectral nudging, are studied. The main result is that the accuracy of the regional simulation increases very slowly as the number of forcing field levels increase when no spectral nudging is used. Thus, for better simulation, it is desirable to have as many forcing levels as possible. By contrast, spectral nudging improves the regional model simulation when reasonably large numbers of forcing field levels, at least up to nine levels, are given. The accuracy worsens drastically when the number of forcing levels is reduced to less than nine. To improve the simulation, in particular when the forcing field is given at a coarse vertical resolution and at lower time frequency, an incremental interpolation method is introduced. The incremental interpolation in the vertical direction significantly improves the regional simulation at all numbers of forcing field levels. The improvement is largest at very low vertical resolution. Incremental interpolation in time also works excellently, allowing the use of daily output for reasonably accurate downscaling. By using a combination of spectral nudging and incremental interpolation, it is possible to make a reasonably accurate downscaling from the forcing given daily at three?five levels in the vertical direction with low overhead. This considerably reduces the amount of data currently believed to be required to downscale global model integrations.