The Effects of Calculating Air Mass Trajectory ofear-Surface Wind Field Analysis Methods:ALMET, Variational Analysis, MM5

This study used variational analysis approach, CALMET and MM5 meteorological models, using the wind field data of simulations to discuss the relevance and differences of calculating air mass trajectories in order to understand the transmission of air pollutants in atmosphere. By using the variational analysis of 1st generation scheme (Chang, 1983) to build three-dimensional wind field in Taiwan. Because the effects of the complex terrain area. To add the wind direction shift module to correct the initial wind field by objective analysis according to terrain slope. The results reveal that the horizontal wind field of variational analysis modified by wind direction shift module represents sufficient influence of complex terrain area near the ground level. And the vertical wind field can show the characteristics of local circulations such as sea-land breeze and valley wind. It’s can be found that two-dimensional (2D) back trajectory of level-averaged wind field can reveal the variation of wind field and affiliate the source to pollutants more accurate than 2D back trajectory of rectangular coordinate wind field and three-dimensional (3D) back trajectory in Chiayi area. The transmission distance and retention time of track points in pollution area are superior to others. It’s will absorb more precursor and promote the pollution incidents to occur. The model simulations of wind fields of variational analysis are closer to the observations of EPA and CWB stations than CALMET model. And the simulated performances of station wind speed of variational analysis are slightly superior to CALMET model. The 2D and 3D back trajectories of variational analysis and the outputs of MM5 model will appear the phenomena of flowing around a sharp ridge slope even the circulated flow due to the effects of valley circulations and the blocking effects of terrain in Chushan area in central Taiwan. But the mechanisms of CALMET model aren’t obvious and the trajectory will over fly the Central Mountains. And the simulation speed of MM5 is higher than variational analysis and CALMET model. To synthesize the research results, suggesting that using the data of level-averaged wind field after the wind direction shift with 2D back trajectory analysis will upgrade the accuracy of conjecturing the transmission route of air mass in atmosphere.