Evaluations of mesoscale models' simulations of near-surface winds, temperature gradients, and mixing depths

Mesoscale meteorological models are being used to provide inputs of winds, vertical temperature and stability structure, mixing depths, and other para meters to atmospheric transport and dispersion models. An evaluation method ology is suggested and tested with simulations available from four mesoscal e meteorological models (Fifth-Generation Pennsylvania State University-Nat ional Center for Atmospheric Research Mesoscale Model, Regional Atmospheric Modeling System, Coupled Ocean-Atmosphere Mesoscale Prediction System, and Operational Multiscale Environmental Model with Grid Adaptivity). These mo dels have been applied by others to time periods of several days in three a reas of the United States (Northeast, Lake Michigan area, and central Calif ornia) and in Iraq. The authors' analysis indicates that the typical root-m ean-square error (rmse) of hourly averaged surface wind speed is found to b e about 2-3 m s(-1) for a wide range of wind speeds for the models and for the geographic regions studied. The rmse of surface wind direction is about 50 degrees for wind speeds of about 3 or 4 m s(-1). It is suggested that t hese uncertainties in wind speeds and directions are primarily due to rando m turbulent processes that cannot be simulated by the models and to subgrid variations in terrain and land use, and therefore it is unlikely that the errors can be reduced much further. Model simulations of daytime mixing dep ths are shown to be often within 20% of observations. However, the models t end to predict weaker inversions than are observed in interfacial layers ca pping the mixing depth. The models also underestimate the vertical temperat ure gradients in the lowest 100 m during the nighttime, which implies that the simulated boundary layer stability is not as great as that observed, su ggesting that the rate of vertical dispersion may be overestimated. The mod els would be able to simulate better the structure of shallow inversions if their vertical grid sizes were smaller.