The DLSM version 4.2 has two options for estimating the wind variance on the sub-grid scale of the model. The first method computes the wind variance on the grid scale (3 x 3 x 3) and then scaling the variance to smaller scales by the Von Karman relationship. This “reasonable” approach has been used for many years but has not been fully verified by real data. Although the meso-scale models generate a value for the dissipation rate, we do not have confidence in the numbers. This is especially true for heights above the PBL.  The second method represents the uncertainties by scaling them to 20 % of the mean model wind speed. Comparisons of the uncertainties with the NMC rawinsonde profiles suggest that the simulated variances using the 20 % rule are not unreasonable. The user can also choose not to include turbulence.

Last Updated: 02/07/2007