Comments on “a synoptic climatological analysis of air quality in the Grand Canyon National Park”

This note is written to comment on the paper by Davis and Gay (1993, henceforth referred to as DG), in which DG have ignored surface synoptic data sets. The inclusion of these data would permit a more accurate characterization of air quality influences on Grand Canyon National Park. They cannot provide a more accurate definition of air quality by adding synoptic classes based on upper air analyses alone. Their categories are defined exclusively in terms of analyses provided by 21 rawinsonde stations located throughout western U.S.A. and Mexico. They ignore, however, the more numerous surface observation stations which are the only data that can be used to define the pressure field over this region at the surface. During the winter, in particular, the movement of air below the synoptic inversion, which can often be of a different direction than the 500 mb winds, is not considered at all in DG. Atmospheric flow in this layer below the inversion exerts a significant influence on pollutant transport from surfacebased sources. Using rawinsonde data alone, Doty and Perkey (1993) have shown the failure of the operational rawinsonde network to characterize transport paths even in the flatter eastern Unil:ed States, as a result of the sparse spatial representation and limited (12 h) time sampling of the rawinsonde network. This conclusion is also supported by work presented in Kahl and Samson (1986, 1988) and Walmsley and Mailhct (1983)in which the spatial and temporal resolution of the Standard National Weather Service twice daily rawinsonde observations over the eastern United States were found to lack the necessary resolution in determining transport paths during the Cross Appalachian Tracer Experiment (CAPTEX) under relatively calm synoptic conditions. It is important to also note that the spatial resolution of the rawinsonde network in the east is more dense than in the western United States. Therefore, the conclusions of these authors relative to the limitations of rawinsonde analyses would be expected to be even more valid for the western United States where the resolution of the network is coarser and complex terrain effects are also more significant. This i,,; a serious omission which limits the value of the DG study, and results in erroneous conclusions which are illustrated in the next sections. DG also misinterpreted the use of the synoptic classification in the WHITEX study (Maim et al., 1989). The climatological analysis used in WHITEX is a proven, valuable approach to assess synoptic weather categories as reported in Yarnal (1993) and Barry and Perry (1973). Using the surface pressure analyses and frontal positions, the six synoptic classes discussed in Pielke et al. (1987) are further decomposed into geostrophic wind direction and speed classes, Whether or not the Grand Canyon receives pollution from a source such as the Navajo Generating Station (NGS) depends on the flow direction below a synoptic inversion assuming that the effective stack height is below that level. The effective stack height at NGS is generally between 450 and 1050 m above ground level in the wintertime (Whiteman et al., 1991). Whether there is an accumulation of pollution near this industrial site and then a transport towards the southwest depends on both the antecedent surface weather patterns, and flow speed and direction below any synoptic and/or local generated thermodynamic inversion. To demonstrate the seriousness of their oversight, we categorize the 16 periods of highest sulfur levels at Hopi Point, as reported in Maim et al. (1989), according to their synoptic 500 mb maps and corresponding class definitions. The dates of these events are given in Table 1. In order to illustrate the often present directional difference between the surface and 500 mb flow at a point near the Navajo Generating Station a synoptic classification processor was utilized. European Center for Medium Range Weather Forecasting (ECMWF) 2.5 ° latitude/longitude synoptic-scale data were used to compute the wind speed and direction at the surface and 500 mb at 12 GMT on dates when the high pollution levels were recorded (45 in all; see Table 1). The geostrophic surface level wind speed and direction were calculated from the mean sea level pressure gradient. The 500 mb flow information was taken directly from the data set. The synoptic processor also was employed to perform an objective classification based on the synoptic classes discussed earlier. Shown in Table 1 are the dates of high pollution events followed by the classes as objectively and subjectively determined. The last three columns show the geostrophic wind direction at the surface, for the subjective and objective analysis, respectively, and the analyzed model winds at 500 mb. The results indicate that there was agreement in synoptic classification (Columns 2 and 3) for 80% of the days, demonstrating the objective skill of the processor in describing the subjectively determined synoptic categories. Disagreements between the two are associated with transition zones between categories, and the occasional anomalous pattern. In order to validate the wind directions evaluated by the objective processor, a comparison to the subjectively diagnosed wind directions can be made from Table 1. Neglecting the calm days (11 total), the direction agreed on 15 out of the 34 case days. While the direction used is allowed to vary 45 ° in either direction (e.g. easterly geostrophic wind is considered in agreement with a northeasterly or a southeasterly geostrophic surface wind), a total of 32 out of the 34 days are in agreement. The only day with disagreement (14 February, 1987) involves a case where the surface front is located very