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Claude E. Duchon

Abstract

The steady-state radiation temperature correction for a spherical temperature sensor, defined as the difference between the observed sensor temperature and the true air temperature, is computed for clear nighttime conditions in terms of five meteorological and physical parameters: sphere diameter, wind speed, observed sensor temperature, total incident radiation, and the absorptivities and emissivity of the sphere. The range of values encompassed by these parameters are, respectively, 0.001 to 0.1 cm, 0 to 400 cm set−1, 270 to 300K, 0.009 to 0.020 cal sec−1 cm−2, and 0 to 1.

The infrared radiation incident on a sphere in the boundary layer of the atmosphere is related matic- matically to the radiation observed by a flat plate type of radiometer. The derived relationship is based on previous measurements of the spatial distribution of the intensity of infrared sky radiation.

Numerically, the radiation temperature correction is determined by using a sequence of three graphs in-corporating the various parameters. The correction, to be added to the observed sensor temperature, varies from less than 0.01K to 3K.

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Claude E. Duchon

Abstract

A method is presented for obtaining empirical confidence limits for variance spectra. It can be applied whenever the distribution of spectral variances is unknown but the stochastic model of the data is known or proposed.

For computer-generated white noise the results show that the normalized raw spectral variances do not vary as the theoretical model, χ2 10/10, predicts due to the appearance of expected negative line variances. Thus, the lower a priori and a posteriori confidence limits are less than their theoretical counterparts. The empirical confidence limits for normalized hanned spectral variances agree with those from the theoretical model, χ2 27/27.

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Claude E. Duchon

Abstract

A Fourier method of filtering digital data called Lanczos filtering is described. Its principal feature is the use of “sigma factors” which significantly reduce the amplitude of the Gibbs oscillation. A pair of graphs is developed that can be used to determine filter response quality given the number of weights and the value of the cutoff frequency, the only two inputs required by the method. Examples of response functions in one and two dimensions are given and comparisons are made with response functions from other filters. The simplicity of calculating the weights and the adequate response make Lanczos filtering an attractive filtering method.

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Claude E. Duchon

Abstract

A series of nomograms is presented showing the longwave radiation temperature correction for various thermocouple wire diameters, different values of emitted and absorbed radiation, and for wind speeds from zero to 10 m sec−1. The temperature correction is computed from the steady-state balance between radiative and convective heat transfers and is to be added to the measured temperature. The radiation field is based upon previous measurements made under clear nighttime skies and the free and forced convection correlation formulae are adopted from wind tunnel experiments.

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Claude E. Duchon

Observations at the San Juan, Puerto Rico international airport show that the annual mean temperature has increased by about 2.1°C (3.8°F) from 1956 to 1983. The chief contributors to the increase are an increase in daily minimum temperature from 1956 to about 1970 and an increase in daily maximum temperature from about 1970 to 1983. In addition, there is evidence for veering of the wind direction so that overland trajectories are more frequent. No comparable temperature change has been measured at a cooperative station 8 km to the southwest in the city of San Juan nor at other stations in Puerto Rico, thus ruling out a synoptic-scale phenomenon. It is hypothesized that as the airport expanded in size and the vegetation diminished, a mini heat island evolved. The heat island combined with the development of adjacent residential and commercial areas and a changing wind direction account for the observed annual mean temperature increase.

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Claude E. Duchon

Abstract

A method is developed to predict corn yield during the growing season using a plant process model (CERES-Maize), current weather data and climatological data. The procedure is to place the current year's daily weather (temperature and precipitation) into the model up to the time the yield prediction is to be made and sequences of historical data (one sequence per year) after that time until the end of the growing season to produce yield estimates. The mean of the distribution of yield estimates is taken as the prediction. The variance associated with a prediction is relatively constant until the time of tassel initiation and then decreases toward zero as the season progresses. As a consequence, perfect weather forecasts reach their peak value between the beginning of car growth and the beginning of grain fill.

The change in the predicted yield in response to weather as the growing season progresses is discussed for 1983 and 1976 at Peoria, Illinois. Results are given of an attempt to incorporate 30-day Climate Analytic Center outlooks into the predictive scheme.

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Claude E. Duchon and Gregory E. Wilk

Abstract

Accurate measurements of net radiation are basic to all studies of the surface energy budget. In preparation for an energy budget experiment significant differences were found between direct and component measurement of net radiation, which prompted this investigation of their cause. The instruments involved were an all-black single-dome Fritschen-type net pyrradiometer, two Eppley model 8–48 pyranometers, and an Eppley model PIR pyrgeometer. Each had recently been calibrated. The accuracy of the component instruments was considered first. Comparisons of about one hour on each of three nights between the pyrgeometer and five empirical formulas showed that the average departure over all formulas from the pyrgeometer average was −1%. Other comparisons between the pyrgeometer and an infrared thermometer viewing the surface yielded similar results. Alternate shading and unshading of the pyrgeometer looking upward during daytime resulted in a formula that was used to correct the downward longwave radiation under clear skies. The correction is dependent on wind speed, in contrast to a recent paper showing negligible dependence, but is in accord with earlier findings. Based on the manufacturer's specifications, the pyranometer calibrations were considered to be within 2% of the World Radiation Reference.

Thus a series of experiments was carried out using what were believed to be reasonably accurate component measurements of net radiation and measurements from the net pyrradiometer. The results showed that the sensitivity of the latter was less in the longwave band than in the shortwave band in agreement with findings of others. Speculating on possible further dependence of sensitivity to the upward and downward streams of radiation, a method was developed to determine the magnitude of the individual net pyrmdiometer components. A reflective double-shell hemispherical cup was affixed to the upward or downward face of the net pyrradiometer such that linear regression could be applied to simultaneous measurements from the net pyrradiometer, pyranometer, pyrgeometer, and the inner cup temperature, assumed to he at air temperature, to estimate the individual components. Although a substantial difference in shartwave sensitivity was computed using this method, the result was not definitive because of the limited number and the narrow range of longwave observations. Nevertheless, the method can be employed in the field to verify uniform sensitivity of a net pyrradiometer's sensing surfaces to shortwave and longwave radiation. The method may have particular application to Fritschen-type net pyrradiometers of recently improved design after extended field use.

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James S. Goerss and Claude E. Duchon

Abstract

The effect of ship heating on dry-bulb temperature measurements made during GATE is investigated. It is found that measurements taken on a bow boom are less affected than those taken on the ship's bridge. It is also seen that the ship heating effect must be accounted for before meaningful estimates of the error content of dry-bulb temperature measurement systems can be determined.

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R. Craig Goff and Claude E. Duchon

Abstract

Temperature spectra at three levels in the planetary boundary layer covering a range of periods from about 5 hr to 35 days are presented. Apart from the diurnal and semidiurnal oscillations, there are two identifiable period lengths of interest. One extends from about 3 to 7 days, the other from 7 to 20 days. Comparisons are made between these spectra and spectra from other investigators.

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Claude E. Duchon and Kenneth G. Hamm

Abstract

Time series of daily broadband surface albedo for 1998 and 1999 have been analyzed from six locations in the network of 22 Atmospheric Radiation Measurement Program Solar–Infrared Radiation Stations distributed from central Kansas to central Oklahoma. Two of the stations are in Kansas, and four are in Oklahoma; together they reasonably encompass the variation in geography in the southern Great Plains. Daily precipitation totals locally measured or obtained from nearby Oklahoma Mesonet stations and time series of biweekly maximum normalized difference vegetation index obtained from NOAA’s Advanced Very High Resolution Radiometer were used to determine linkages between surface albedo and amount of precipitation and degree of green vegetation. As part of this determination, daily albedo was categorized according to sky condition, that is, clear, partly cloudy, or overcast, with appropriate boundaries for each category. The more notable results are the following: 1) 2-yr mean annual albedos varied by more than 20% among the six sites, the lowest albedo being 0.18 and the highest albedo being 0.22; 2) the numerical difference was about 4 times the maximum interannual mean difference among the six stations, indicating the importance of geographic location; 3) for sites with a large amount of bare soil, a systematic decrease in albedo in response to rainfall events and a systematic increase in albedo as the soil dried were observed; 4) at the one site with total vegetation cover, that is, no bare soil, albedo response to precipitation events was suppressed; 5) no relation was found between mean annual albedo and annual precipitation; 6) whether days were classified as clear or partly cloudy had little influence on daily albedo, but overcast days typically reduced albedo, sometimes substantially; and 7) the main contributor to low albedos on overcast days with rain was the wet surface; the contribution by the overcast sky was secondary.

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