Evaluation of wind fields used in Grand Canyon visibility transport commission analyses

The Grand Canyon Visibility Transport Commission (GCVTC) was established by the U.S. Congress to assess the potential impacts of projected growth on a tmospheric visibility at Grand Canyon National Park and to make recommendat ions to the U.S. Environmental Protection Agency on what measures could be taken to avoid such adverse impacts. A critical input to the assessment too l used by the commission was three-dimensional model-derived wind fields us ed to transport the emissions. This paper describes the evaluation of the w ind fields used at various stages in the assessment. Wind fields evaluated included those obtained from the Colorado State University Regional Atmosph eric Modeling System (RAMS), the National Meteorological Center's Nested Gr id Model (NGM), and the National Oceanic and Atmospheric Administration's A tmospheric Transport and Dispersion (ATAD) trajectory model. The model-deri ved wind fields were evaluated at multiple vertical levels at several locat ions in the southwestern United States by determining differences between m odel predicted winds and winds that were measured using radiosonde and rada r wind profiler data. Model-derived winds were also evaluated by determinin g the percent of time that they were within acceptable differences from mea sured winds. All models had difficulties, generally meeting the acceptable criteria for less than 50% of the predictions. The RAMS model had a persistent bias towa rd southwesterly winds at the expense of other directions, especially faili ng to represent channeling by north-south mountain ranges in the lower leve ls. The NGM model exhibited a substantial bias in the summer months by exte nding northwesterly winds in the eastern Pacific Ocean well inland, in cont rast to the observed southwesterlies at inland locations. The simpler ATAD trajectory model performed somewhat better than the other models, probably because of its use of more upper air sites. The results of the evaluation i ndicated that these wind fields could not be used to reliably predict sourc e-receptor impacts on a particular day; thus, seasonally averaged impacts w ere used in the GCVTC assessment.