Gregory S. Poulos

CASES-99: A Comprehensive Investigation of the Stable Nocturnal Boundary Layer

Abstract The Cooperative Atmosphere-Surface Exchange Study—1999 (CASES-99) refers to a field experiment carried out in southeast Kansas during October 1999 and the subsequent program of investigation. Comprehensive data, primarily taken during the nighttime but typically including the evening and morning transition, supports data analyses, theoretical studies, and state-of-the-art numerical modeling in a concerted effort by participants to investigate four areas of scientific interest. The choice of these scientific topics is motivated by both the need to delineate physical processes that characterize the stable boundary layer, which are as yet not clearly understood, and the specific scientific goals of the investigators. Each of the scientific goals should be largely achievable with the measurements taken, as is shown with preliminary analysis within the scope of three of the four scientific goals. Underlying this effort is the fundamental motivation to eliminate deficiencies in surface layer and turbul...

Land–Atmosphere Interaction Research, Early Results, and Opportunities in the Walnut River Watershed in Southeast Kansas: CASES and ABLE

Abstract This paper describes the development of the Cooperative Atmosphere Surface Exchange Study (CASES), its synergism with the development of the Atmosphere Boundary Layer Experiments (ABLE) and related efforts, CASES field programs, some early results, and future plans and opportunities. CASES is a grassroots multidisciplinary effort to study the interaction of the lower atmosphere with the land surface, the subsurface, and vegetation over timescales ranging from nearly instantaneous to years. CASES scientists developed a consensus that observations should be taken in a watershed between 50 and 100 km across; practical considerations led to an approach combining long—term data collection with episodic intensive field campaigns addressing specific objectives that should always include improvement of the design of the long—term instrumentation. In 1997, long—term measurements were initiated in the Walnut River Watershed east of Wichita, Kansas. Argonne National Laboratory started setting up the ABLE ar...

An Evaluation of Bulk Ri-Based Surface Layer Flux Formulas for Stable and Very Stable Conditions with Intermittent Turbulence

Abstract High-rate near-surface overnight atmospheric data taken during the Cooperative Atmosphere–Surface Exchange Study-1999 (CASES-99) is used to quantify the representativeness of surface layer formulations under statically stable conditions. Combined with weak wind shear, such conditions generate large dynamic stability (Ri > 1.0), intermittency, and nonstationarity, which violate the underlying assumptions of surface layer theory. Still, such parameterizations are applied in atmospheric numerical models from large-eddy to global circulation. To investigate two formulas, their parameterized sensible heat flux and friction velocity (u∗) values are compared, when driven by CASES-99 measurements, to CASES-99 measurements of the same from various heights. Significant inaccuracies in the magnitude and sign of flux are found with 1) a frequent, large underprediction of heat flux for Rib > ∼1.0, 2) an overprediction of negative sensible heat flux and u∗ for ∼0.2 < Rib < ∼0.8, 3) a systematic underprediction...

The Interaction of Katabatic Flow and Mountain Waves. Part I: Observations and Idealized Simulations

Abstract The mutual interaction of katabatic flow in the nocturnal boundary layer (NBL) and topographically forced gravity waves is investigated. Due to the nonlinear nature of these phenomena, analysis focuses on information obtained from the 1993 Atmospheric Studies in Complex Terrain field program held at the mountain–canyon–plains interface near Eldorado Canyon, Colorado, and idealized simulations. Perturbations to katabatic flow by mountain waves, relative to their more steady form in quiescent conditions, are found to be caused by dynamic pressure effects. Based on a local Froude number climatology, case study analysis, and the simulations, the dynamic pressure effect is theorized to occur as gravity wave pressure perturbations are transmitted through the atmospheric column to the surface and, through altered horizontal pressure gradient forcing, to the surface-based katabatic flows. It is proposed that these perturbations are a routine feature in the atmospheric record and represent a significant p...

An Observational and Prognostic Numerical Investigation of Complex Terrain Dispersion

Abstract The Atmospheric Studies in Complex Terrain Program conducted a field experiment at the interface of the Rocky Mountains and the Great Plains in the winter of 1991. Extensive meteorological observations were taken in northeastern Colorado near Rocky Flats to characterize overnight conditions in the region. Simultaneously, a tracer dispersion experiment using over 130 samplers to track plume development was conducted by Rocky Flats facility personnel. These two datasets provided an opportunity to investigate the accuracy and applicability of a fully prognostic, primitive equation, mesoscale model to the simulation of complex terrain dispersion. Meteorological conditions in the Rocky Flats region are forecast for selected case nights using the Regional Atmospheric Modeling System initialized with sounding data taken during the experiment. The forecast winds and temperature are used in a Lagrangian particle dispersion model to predict tracer plume transport. The results of both models are compared to...

A numerical analysis of Los Angeles basin pollution transport to the Grand Canyon under stably stratified Southwest flow conditions

Abstract This paper presents a numerical investigation of air pollutant transport from the Los Angeles Basin to Grand Canyon National Park (GCNP) under the stably stratified wintertime, synoptic high-pressure conditions that existed during 10–13 February 1987. The Colorado State University Regional Atmospheric Modeling System (CSU-RAMS) is used to develop fields of different atmospheric variables for 54 simulated hours. These fields are applied in a Lagrangian particle dispersion model (LPDM) to simulate the advection of pollutant particles for this period. It is found that under the generally southwest flow conditions presented, particles released from the Los Angeles Basin will impact the Grand Canyon but only in small amounts. By comparing a flat to complex terrain simulation, the importance of the terrain features between Los Angeles and GCNP to the dispersion of Los Angeles Basin pollutants is made obvious. Mountain barriers and undulating land reduce what could otherwise be a very serious pollutant impact on GCNP. Based on these results the conclusion is made that despite southwest average flow aloft during the Winter Haze Intensive Tracer EXperiment (WHITEX) period of 10–13 February 1987, Los Angeles Basin pollution did not contribute significantly to the observed visibility reduction in the Grand Canyon during that time period.

Standardized test to evaluate numerical weather prediction algorithms

In order to assist in comparing the computational techniques used in different models, the authors propose a standardized set of one-dimensional numerical experiments that could be completed for each model. The results of these experiments, with a simplified form of the computational representation for advection, diffusion, pressure gradient term, Coriolis term, and filter used in the models, should be reported in the peer-reviewed literature. Specific recommendations are described in this paper.

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

Influence of Mesoscale Circulations on Long-Range Transport in the Grand Canyon Area

Using a climatological analysis scheme results are presented relating the relative contribution of long-range versus local pollution to visibility degradation in Grand Canyon National Park. Among the results, we demonstrate that the poorest visibility in the winter is generally associated with synoptic stagnation and/or transport of pollution from the northeast.