The effect of subgrid velocity scale on site-specific/subgrid area and grid-averaged dry deposition velocities

A method for deriving the site-specific and subgrid area wind speed and friction velocity from regional model output and detailed land type information is developed. The “subgrid velocity scale” is introduced to account for generation of turbulent fluxes by subgrid motions. The grid vector averaged wind speed is adjusted by adding the subgrid velocity scale. This is to account for the fact that the spatial average of the local wind speed is usually larger than the absolute value of the vector averaged velocity ( ), especially when there are different land or surface types within the spatial averaging area and when is small. The assumption of uu*=constant is then applied within a model grid area to obtain wind speed and friction velocity for specific sites and subgrid areas. Using this method, the site-specific and subgrid area wind speed and friction velocity can be estimated from grid-averaged model output. In addition, more realistic air pollutant dry deposition velocities for specific locations and subgrid areas can be calculated. Grid-averaged deposition velocity values calculated using this approach tend to be about 30% different (either larger or smaller) for HNO3 and sulphate and about 10% different for SO2 and O3 compared to values calculated by assuming a constant wind speed over the whole model grid area. These differences are found to be even larger at specific sites or over some subgrid areas. This method can be applied to determine a more realistic wind speed, friction velocity and pollutant dry deposition velocity at specific locations using gridded meteorological data.