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Bruce B. Hicks

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Bruce B. Hicks

Abstract

In complex terrain, horizontal advection and filtration through a canopy can add substantially to the vertical diffusion component assumed to be the dominant transfer mechanism in conventional deposition velocity formulations. To illustrate this, three separate kinds of terrain complexity are addressed here: 1) a horizontal landscape with patches of forest, 2) a uniformly vegetated gentle hill, and 3) a mountainous area. In flat areas with plots of trees, the elevation of the standard area-weighted dry deposition velocity will likely depend on the product hn 1/2, where h is the tree height and n is the number of plots per unit area. For the second case, it is proposed that the standard “flat earth” deposition velocity might need to be increased by a factor like [1 + Ra/(Rb + Rc)]1/2. For mountainous ecosystems, where no precise estimate of local dry deposition appears attainable, the actual dry deposition rate is probably bounded by the extremes associated with 1) the flat earth assumption involving aerodynamic, quasi-boundary layer, and canopy resistances as in conventional formulations, and 2) an alternative assumption that the aerodynamic resistance is zero. Such issues are of particular importance in the context of atmospheric loadings to sensitive ecosystems, where the concepts of critical loads and deposition forecasting are now of increasing relevance. They are probably of less importance if the emphasis is on air quality alone, because air quality responds slowly to changes in deposition rates. The issues addressed here are mainly appropriate in the context of air surface exchange that is not controlled by surface resistance (e.g., for deposition of easily captured chemicals such as nitric acid vapor, and perhaps for atmospheric momentum) and for chemicals that have no local sources. It is argued that dry deposition rates derived from classical applications of deposition velocities are often underestimates.

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Bruce B. Hicks

The meteorological situation of coastal regions is strongly influenced by the shoreline and by topographic relief. In this instance, we have learned how to take water and land influences into account when predicting changes in meteorology. But we have now stepped beyond the standard meteorological view of coasts affecting air through their complexity, to a new awareness that deposition from the air affects the coastal environment. It is along the coasts that the terrestrial, aquatic, and atmospheric media come into most intimate contact, and where any one of them can affect any other. The importance of the interaction is becoming even more apparent as the population of coastal areas continues to grow, and as demands for energy, food, and recreation are growing even faster. The interactions among the terrestrial, aquatic, and atmospheric media are central in considerations of what must be done to protect an increasingly stressed coastal environment from what could soon be irreversible damage. To generate the understanding necessary to underpin regulations and emissions control strategies, accurate models of pollutant behavior in all of the media must be constructed, and these must then be integrated to protect against the imposition of ineffective controls. The challenges for the atmospheric sciences are daunting. Not only must atmospheric deposition become a focus of attention, but mesoscale models must be constructed to provide the spatial information that existing coarse monitoring networks are incapable of providing alone.

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Bruce B. Hicks

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Destruction of the thin subsurface thermal boundary layer at an air-water interface can be accomplished by relatively low rates of aeration and can result in substantially improved thermal performance when water temperatures are high. The heating and saturating of rising air bubbles can also provide a significant improvement in overall thermal performance when water temperatures and aeration rates are sufficiently great. At 80°C, improvements of ∼20% appear possible with average aeration rates <1 mm s−1.

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Marvin L. Wesely and Bruce B. Hicks

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Temperature and humidity fluctuations at frequencies within the inertial subrange are found experimentally to be partially correlated in the surface boundary layer over warm wet surfaces. The spectral correlation coefficient, deduced from variances and covariances computed by analog electronics, is near unity in the flux-carrying eddies and decreases with increasing frequency, approximately as n½. As a result, optical refractive index fluctuations may have the false appearance of being strongly anisotropic in the inertial subrange.

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Bruce B. Hicks, William R. Pendergrass III, Christoph A. Vogel, and Richard S. Artz

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Data from a network of micrometeorological instruments, mostly mounted 10 m above the roofs of 12 buildings in Washington, D.C., are used to derive average values and spatial differences of the normalized local friction velocity u */u ≡ ()1/2/u (with u being the wind speed reported at the same height as the covariance is measured, w being the vertical wind component, primes indicating deviations, and the overbar indicating averaging). The analysis is extended through consideration of two additional sites in New York City, New York. The ratio u */u is found to depend on wind direction for all locations. Averaged values of u */u appear to be best associated with the standard deviation of local building heights, with little evidence of a dependence on any other of the modern building-morphology indices. Temperature covariance data show a large effect of nearby activities, with the consequences of air-conditioning systems being obvious (especially at night) in some situations. The Washington data show that older buildings, built largely of native limestone, show the greatest effects of air-conditioning systems. The assumption that the nighttime surface boundary layer is stable is likely to be most often incorrect for both Washington and New York City—the sensible heat flux resulting from heating and cooling of building work spaces most often appears to dominate.

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Bruce B. Hicks, William J. Callahan, William R. Pendergrass III, Ronald J. Dobosy, and Elena Novakovskaia

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The utility of aggregating data from near-surface meteorological networks for initiating dispersion models is examined by using data from the “WeatherBug” network that is operated by Earth Networks, Inc. WeatherBug instruments are typically mounted 2–3 m above the eaves of buildings and thus are more representative of the immediate surroundings than of conditions over the broader area. This study focuses on subnetworks of WeatherBug sites that are within circles of varying radius about selected stations of the DCNet program. DCNet is a Washington, D.C., research program of the NOAA Air Resources Laboratory. The aggregation of data within varying-sized circles of 3–10-km radius yields average velocities and velocity-component standard deviations that are largely independent of the number of stations reporting—provided that number exceeds about 10. Given this finding, variances of wind components are aggregated from arrays of WeatherBug stations within a 5-km radius of selected central DCNet locations, with on average 11 WeatherBug stations per array. The total variance of wind components from the surface (WeatherBug) subnetworks is taken to be the sum of two parts: the temporal variance is the average of the conventional wind-component variances at each site and the spatial variance is based on the velocity-component averages of the individual sites. These two variances (and the standard deviations derived from them) are found to be similar. Moreover, the total wind-component variance is comparable to that observed at the DCNet reference stations. The near-surface rooftop wind velocities are about 35% of the magnitudes of the DCNet measurements. Limited additional data indicate that these results can be extended to New York City.

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Bruce B. Hicks, Elena Novakovskaia, Ronald J. Dobosy, William R. Pendergrass III, and William J. Callahan

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Data from six urban areas in a nationwide network of sites within the surface roughness layer are examined. It is found that the average velocity variances in time, derived by averaging the conventional variances from a network of n stations, are nearly equal to the velocity variances in space, derived as the variances among the n average velocities. This similarity is modified during sunlit hours, when convection appears to elevate the former. The data show little dependence of the ratio of these two variances on wind speed. It is concluded that the average state of the surface roughness layer in urban and suburban areas like those considered here tends toward an approximate equality of these two measures of variance, much as has been observed elsewhere for the case of forests.

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Farn Parungo, Joe F. Boatman, Stan W. Wilkison, Herman Sievering, and Bruce B. Hicks

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A statistical analysis using published data on the global distribution of total cloud cover and cloud type amounts over the ocean, reduced from the Comprehensive Ocean–Atmosphere Data Set (COADS), shows a significant positive trend (4.2% increase from the 1930 baseline) in total oceanic cloud amount in the period between 1930 and 1981. The increase of total cloud amount for the Northern Hemisphere (5.8% ) was twice that for the Southern Hemisphere (2.9% ), The more consistent 30-yr ( 1952–1981 ) data show that the change in cloud amount ( 1952 base) was 1.5% for the globe, 2.3% for the Northern Hemisphere, and 1.2% for the Southern Hemisphere. The analysis also shows that the greatest cloud amount increase was for altocumulus and altostratus clouds and that this increase was most pronounced at midlatitudes (30°–50°N). The trend and the pattern of cloud amount variations appear to be in accord with the temporal trend and geographic distribution of S02 emissions. It is hypothesized that sulfate particles converted from S02, may modify cloud droplet spectra, causing affected clouds to be more colloidally stable than unaffected clouds. The longer residence times of affected clouds could cause increases of cloud frequency and cloud amount.

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