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Marshall A. Atwater

Abstract

A radiation model is used to examine the possible effects that the presence of pollutants, particularly aerosols and nitrogen dioxide, have on the thermal structure of the urban atmosphere. Discontinuities in the lapse rate, which develop at successive iterations of the radiation model, are removed by use of a one-sided space finite difference at the lowest atmospheric grid level. The surface temperature is specified as a function of time and pollutant concentration is assumed constant with time. Pollutants are shown to be a possible cause for the formation of elevated inversions which occur over cities. Therefore, the radiative effects of pollutants should be included in a realistic simulation of urban meteorology.

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Marshall A. Atwater

Abstract

Three models, based on methods of Brooks and Elsasser, and on transmission functions of Davis and Viezee, are used to compute the infrared surface flux and the flux divergence in the lower atmosphere. The three models are compared and brief derivations are given.

Data from 0'Neill, Nebr., during the summer of 1953 are used in the computation of the incoming and net radiation at the surface and the infrared cooling rates in the lower atmosphere. Observed radiation data at O'Neill were compared with computed results from the three models. The computed cooling rates from the three models are compared with observed temperature changes.

The largest surface flux was calculated by the Brooks model and the smallest by the Elsasser model. The computed net radiation, including the solar radiation, was in better agreement with the observed data than was the total flux. The computed cooling rates agree with those of other authors. The cooling rate computed by the Brooks model is smallest of those computed in the day and nearly the same as the values by the other two models at night. The difference between the day and night results are attributable to the omission of carbon dioxide effects by Brooks. The largest cooling rates are shown by the Elsasser model. The distributions with height of the cooling rates are similar for the three methods.

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Marshall A. Atwater

Abstract

A numerical model of the planetary boundary layer, based on the Eulerian conservation equation, is used for two- and three-dimensional simulations of wind, temperature, specific humidity and pollutant concentration within the boundary layer surrounding urban areas. The variations of the terrain are included in the simulations. Two- and three-dimensional simulations of the thermal structure and pollutant concentrations are highly correlated. The simulated urban heat island and vertical thermal structure compare realistically with observations. The temperature changes caused by radiatively active pollutants are generally smaller than effects induced by other urbanization factors. During daytime, the presence of radiatively active pollutants resulted in increased vertical stability and higher concentrations near the surface when compared with radiatively inactive pollutants.

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Marshall A. Atwater

Abstract

A numerical model based on the Eulerian conservation equation simulated the urban heat island in tropical, desert, mid-latitude and tundra climatic regimes in a two-dimensional mode. Data are based on typical climatological descriptions. Experiments to identify the effects of individual urbanization characteristics showed anthropogenic heating to be important in the mid-latitudes and tundra, and that radiative effects of pollutant aerosols are minimal except in the tundra. Four types of urban heat islands, over a day's duration, both negative and positive, are possible in various climatic regimes and seasons. Simulations showed that a positive heat island forms in the urban arm in the absence of sun, and negative heat islands are formed as the daylight lengthens. The largest urban thermal effects occurred in the tundra regions and relatively small effects occurred in the tropics and deserts.

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Marshall A. Atwater

Abstract

The effect of pollutants, both gaseous and particulate, on the air temperature in the boundary layer is examined by computing their interaction with solar and infrared radiation. Carbon dioxide and large concentrations of ozone are found to be the most important gaseous IR absorbers. Nitrogen dioxide and high concentrations of SO2 and O3 produce important heating by absorption of solar radiation. The effect of particles on the solar and IR radiative heating rates computed for realistic numbers, size distributions and complex refractive indexes is found to approach or exceed the heating rates due to water vapor.

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Marshall A. Atwater and John T. Ball

Abstract

Hourly values of total solar radiation are computed by two methods at 11 stations in all regions of the United States for a two-year period. In the first method, precipitable water is computed from radiosonde observations while in the second method it is estimated from surface dew point. Annual differences were 1% or less at 10 of the 11 stations and less than 2% at all stations for both years. Differences in individual months were also less than 2% at all stations with few exceptions.

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Marshall A. Atwater and John T. Ball

Abstract

Data generated by a physically based solar radiation model, consisting of hourly values of total and direct-beam solar radiation, were computed at nearly 50 stations in the United States for the years 1971 and 1972. The radiation model used in the computations includes the effects of Rayleigh scattering, absorption by water vapor and permanent gases, and absorption and scattering by clouds and aerosols. Random and systematic variations of total solar radiation are presented in the eastern United States. Errors in computed radiation on a daily basis were less than distance-induced random variations for distances ≳100 km. Systematic intraregional differences in monthly solar radiation of between 15 and 30% were computed within distances of 200 km in the eastern United States. The spatial distribution of total solar radiation is significantly influenced by coastal-inland and urban-rural climatic differences with radiation minima computed for some large cities.

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Marshall A. Atwater and Philip S. Brown, Jr.

Abstract

A model for incoming solar radiation is developed for use with readily available meteorological data and is designed for efficiency and accuracy. Purameterizations are used to account for Rayleigh and Mie scattering and for absorption by permanent gases, water vapor and aerosols. Clouds are incorporated in the model by employing Manabe and Strickler's methods, and by including the transmission functions of Haurwitz. Results compare favorably with results of more sophisticated models and with observations taken during BOMEX and IFYGL. The latitudinal effects of various aerosol concentrations in the presence of clouds are examined. For a given aerosol opacity, it is found that the doily summer insolation has larger reductions near the poles than near the equator.

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Marshall A. Atwater and John T. Ball

Abstract

A numerical solar radiation model based on standard meteorological data was revised for clouds using data from the GARP Atlantic Tropical Experiment (GATE). Climatic-mean transmittance functions were revised for low and convective clouds independent of zenith angle. Transmittance during partial cloudiness ψc was assumed to vary from the climatic-mean transmittance during overcast skies &ψmacr; by ψc = &ψmacr;c where c is the cloud amount. Mean errors in computed flux as a function of cloud amount were reduced. Results for four GATE ships are presented.

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Philip S. Brown Jr. and Marshall A. Atwater

Abstract

Various numerical methods applied to Schwarzschild's equation have been found to produce solutions that exhibit pronounced two-grid-interval oscillations. Though such solutions remain bounded as the time integration proceeds, the short-wave oscillations eventually obliterate the desired components of the solution. An analysis is presented that explains this oscillatory behavior, and a new finite-difference scheme is proposed that produces a smooth solution.

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