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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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Norman E. Bowne and John T. Ball

Abstract

A unique set of data was collected during a large-scale field study of diffusion about an urban complex at Fort Wayne, Ind. Observations of turbulent wind fluctuations were obtained on two towers, one in a typical rural setting, the other in the heart of the downtown area of this medium size city. These turbulence data allow us to compare the state of the atmospheric boundary layer (to a height of 60 m) in its normal rural condition with its state after modification by the city.

Predictions of the effect of the city have been verified and quantized. The increased roughness of the city had a drag effect that changed the shape of the wind speed profile. The city “heat island” decreased the stability of atmosphere most noticeably in the lowest layers and much less with increasing height. This effect was more noticeable when the rural atmosphere was slightly stable.

Turbulence was more intense in the rougher and less stable urban environment, but the ratios of normalized intensifies in dynamic similarity coordinates were quite comparable, indicating the universality of the similarity methods. The shapes of the spectral curves were different in the urban and rural regions and peaks in the plots were shifted toward higher frequencies in the urban atmosphere. Secondary high-frequency peaks were found only in high-wind-speed cakes; lower wind speeds resulted merely in a flattening and shifting of the spectra.

The most interesting observation was the persistence of the turbulence, in both intensity and wavelength. While the city caused the turbulence intensity to increase at low levels, and shifted the wavelength where the maximum intensity was found, the order between trials established in the rural area was maintained in the city. Therefore, the prediction of turbulence intensity and related parameters in the city from wind observations in nearby rural areas appears feasible.

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John T. Ball and Keith W. Veigas

Abstract

The incorporation of diagnostic data obtained from surface observations into a humidity analysis at the 850-, 700- and 500-mb levels is tested using European surface and upper-air data for 22 observation times in February 1962. Sparse data conditions are simulated by withholding a portion of both surface and upper-air data. The rms errors and contingency table per cent correct scores indicate that an improved analysis is obtained in data sparse areas by weighting the diagnostic data relative to the radiosonde data. The most appropriate weighting is a function of the reliability of the diagnosis and the data density. The effects of using different initial guess fields, varying the application of the successive approximation technique (SAT) to these fields, and applying smoothing were also considered.

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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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Duane S. Cooley, John T. Ball, and Abraham M. Pavlowitz

Abstract

A method is developed for the semi-objective analysis of layered cloud amounts and of heights of cloud bases and tops. The analyses are based on satellite television and infrared data, surface synoptic observations and radiosonde observations. After video data are manually extracted, input data are processed and analyzed on a digital computer with the final analysis including an adjustment by a meteorologist who considers the High Resolution Infrared Radiometer (HRIR) data and other available data. The consideration depends in part on a comparison between computed values of effective radiating temperature at grid points and analyzed values of temperature from Nimbus 2 HRIR observations. Where possible, verification is made both of the machine-adjusted layered cloud amounts and of the final manually adjusted values using Navy and Air Force aircraft weather reconnaissance reports. The limited verification sample indicates some value gained by the adjustments, particularly for high and middle cloud amounts.

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John T. Ball, Stephen J. Thoren, and Marshall A. Atwater

Abstract

The spatial and temporal characteristics of cloud coverage during the 21-day GATE Phase 111 period were studied using SMS-1 infrared hourly digitized data and standard hourly meteorological surface observations taken on 18 ships positioned within the A/B and C scale regions. Results were obtained for the entire sample and for portions stratified according to enhanced (E) or depressed (D) convective activity. Separate areal analyses based on ship data and on satellite data were obtained for total coverage, low-cloud amount including cumulonimbus, middle-cloud amount and high-cloud amount. Both the satellite and the ship data indicate that the average coverage for the GATE A/B array area during Phase 111 is close to 80%. The zone of maximum cloudiness exhibits a basic east-west orientation and is centered near 8°N.

Analysis of hourly variations yields a nighttime maximum of total coverage near 0300 GMT on D days while on E days the maximum occurs in late afternoon or early evening. Analysis of low clouds showed a double maxima at 0400 and 1500 GMT. The early afternoon maximum predominates on E days while only the nighttime maximum is present on D days. On days of significant activity, the high cloud maximum occurs in the late afternoon (1900 GMT), ∼4 h later than the low-cloud maximum. The results of this study emphasize that generalizations about the diurnal variations of clouds, convective activity and precipitation over tropical oceans must be carefully evaluated in terms of regional location and prevalent degree of convective activity.

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George H. Milly, John T. Ball, and David B. Spiegler

Abstract

An examination is made of the hypothesis that the inconclusive or marginal effects of many cloud seeding operations are due, at least in part, to an inhomogeneous distribution of freezing nuclei resulting in great ranges of concentration and extensive areas of overseeding and underseeding over the target region. Distributions of silver iodide nucleus concentrations arising from ground based generators were computed using a Gaussian plume diffusion model. Meteorological conditions and the number, locations, and yield of ground based generators were varied in a series of numerical experiments which bracketed conditions typical of many cloud seeding operations. The results indicated that effective seeding concentrations of nuclei can be achieved over a significant portion of the target area only by carefully considering initial atmospheric conditions as they affect nucleus diffusion and activity, and by accordingly designing and deploying the system of silver iodide generators.

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John T. Ball, Marshall A. Atwater, and Stephen J. Thoren

Abstract

A radiation model was used to simulate daily incoming solar radiation at four ships during Phase III of GATE. The accuracy of the simulations from several different cloud analyses based on ship or satellite data was estimated by comparison with measured values. The cloud-data source, objective analysis characteristics and analysis grid interval affected the accuracy of the radiation computations.

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Donald A. Chisholm, John T. Ball, Keith W. Veigas, and Paul V. Luty

Abstract

Northern Hemisphere surface-synoptic and radiosonde data from November 1961 through February 1962 are used to develop diagnostic relationships between surface-observed variables at a single station and the dewpoint spread at the 850-, 700-, 500- and 400-mb levels above that station. The approach consists of two steps: 1) the isolation within a decision-tree framework of those cases for which individual surface-observed variables yield highly reliable estimates of upper-level humidity, and 2) the application of a statistical technique (Regression Estimation of Event Probabilities) to the remaining cases to derive equations yielding probabilities of occurrence of specified categories of dewpoint spread. This approach yields useful diagnostic information with a quantitative measure of reliability.

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