An evaluation of meteorological data prediction over Washington, DC: Comparison of DCNet observations and NAM model outputs

This study presents an example of how outputs of operational and readily-available mesoscale numerical models can be adapted to initialize dispersion calculations within the urban surface roughness layer. Three years of urban meteorological observations from central Washington, DC, are compared against forecast outputs of the North American Mesoscale (NAM) model. NAM wind speed predictions underestimate the observations in light winds and overestimate the measure-ments in high winds. Average wind directions are consistent. However, an adjustment of the predicted direction of the plume by -20 degrees is needed. The uncertainty associated with this adjustment is large in light NAM wind speed with no evident variation by season. The values of the standard deviation of the wind direction, sigma theta derived from NAM model outputs underestimate the observations by a small amount (about -1.5 to -2.5 degrees). The results presented here indicate that mesoscale numerical model outputs can provide information adequate for dispersion cal-culations. However, levels of uncertainty associated with implementation of the suggested pro-cedures increase with decreasing wind speed, causing considerable uncertainty in the implementation of adjustments as low wind speed conditions are approached. Results and rec-ommendations reported here should not be extended to other numerical models or other cities without further testing.