SENSITIVITY OF REGIONAL OXIDANT MODEL PREDICTIONS TO PROGNOSTIC AND DIAGNOSTIC METEOROLOGICAL FIELDS

Objective analysis and diagnostic methods are used to provide hourly m eteorological fields to many air quality simulation models. The viabil ity of using predictions from the Pennsylvania State University-Nation al Center for Atmospheric Research Mesoscale Model version 4 (MM4) tog ether with four-dimensional data assimilation technique to provide met eorological information to the U.S. EPA Regional Oxidant Model(ROM) wa s studied. Two numerical simulations were performed for eight days usi ng the ROM for a domain covering the eastern United States. In the fir st case, diagnostically analyzed data were used to provide meteorologi cal conditions, while in the second case the MM4's prognostic data wer e used, Comparisons of processed diagnostic and prognostic meteorologi cal data indicated differences in dynamical, thermodynamical, and othe r derived variables. Uncertainties and forecast errors present in the predicted vertical temperature profiles led to estimation of lower mix ed-layer heights (similar to 30%-50%) and a smaller diurnal range of a tmospheric temperatures (similar to 2K) compared with those obtained f rom the diagnostic data. Comparison of area-averaged horizontal winds for four subdomains indicated minor differences (similar to 1-2 m s(-1 )). These differences systematically affected the estimation of other derived meteorological parameters, such as friction velocity and sensi ble heat flux. Processed emission data also showed some differences (s imilar to 1-5 ppb h(-1)) that resulted from the differing characterist ics of the diagnostic and prognostic meteorological data. Comparison o f predicted concentrations of primary (emitted) chemical species such as NOx and reactive organic gases in the two numerical simulations ind icated higher values(1-5 and 1-25 ppb, respectively) when the prognost ic meteorological data were used. This result was consistent with the lower estimated values of the ROM's layer 1 and layer 2 heights (plane tary boundary layer) with the prognostic meteorology. However, compari son of predicted ozone concentrations did not indicate similar feature s. Area averages of predicted concentrations of ozone for four subdoma ins indicated both increases and decreases (+15 to-10 ppb) over the ar ea averages predicted by the ROM using diagnostic meteorological data. These results indicate that the prediction of trace gas concentration s and the nonlinearity in the model's chemistry are sensitive to the t ype of meteorological input used.