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John E. Frederick

Abstract

The backscatter ultraviolet spectrometer on the Atmosphere Explorer-E satellite obtained ozone data for the upper stratosphere and lower mesosphere from an equatorial orbit near the time of the March–April 1976 stratospheric warming. A simple inversion technique allows the extraction of information on the absolute ozone (O3) abundance and its longitudinal variation from the measured radiances. Between altitudes of 35 and 60 km tropical O3 in constant pressure layers exhibits increased spatial variations on length scales of 20° to 30° in longitude after the onset of the polar warming, indicating enhanced wave activity. One week after the polar vortex breakdown upper stratospheric O3 begins a relaxation toward the unperturbed state, while that in the lower mesosphere continues to show maximum variations near ±10% of the longitudinally averaged value. Although the degree of horizontal structure increases during the warming, the absolute O3 amounts averaged over longitude between 35 and 60 km in altitude remain nearly constant in time. The observed behavior is consistent with that expected from enhanced vertical transport of O3 in the tropics with accompanying adiabatic temperature changes which alter the chemical loss rate.

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John E. Frederick

Abstract

No abstract available.

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John E. Frederick
and
Hilary E. Snell

Abstract

Measurements obtained from several Robertson-Berger (RB) meters over the course of one year define the role of cloud cover in moderating biologically effective ultraviolet radiation at the Earth's surface. In an annual mean sense, clouds reduce the erythemal irradiance to levels from 62% to 78% of the values that would exist if skies over the measurement sites remained clear and free of pollutants. The RB meter results combined with a simple model of radiative transfer allow one to estimate the response of erythemal irradiance to variations in fractional cloud cover and cloud optical thickness. If local fractional cloud cover during June and July varied by ± 10% of its monthly mean value, erythemal irradiance at the different sites would undergo charm ranging from 1.2% to 6.4% with the opposite sign. Changes in cloud optical thickness of ±10% generally have a smaller impact on surface irradiance than do changes in fractional cloud cover. Variations in erythemal irradiance predicted in these scenarios are, in some cases, similar to those associated with derived trends in total column ozone at middle latitudes between 1969 and 1986.

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Dan Lubin
and
John E. Frederick

Abstract

The National Science Foundation scanning spectroradiometer at Palmer Station, Antarctica (64°46′S, 64°04′W) provides hourly ground-based measurements of solar ultraviolet (UV) irradiance at the, earth's surface. These measurements define the UV radiation environment of the region and, in conjunction with a daily record of sky conditions and radiative transfer modeling, permit a quantitative understanding of the role of cloud cover in regulating UV radiation levels at the Antarctic surface, including the period of the springtime ozone depletion. The transmission properties of cloud types over the Antarctic Peninsula are quantified by taking the ratio of UV-A irradiances measured under them to UV-A irradiances calculated for clear skies and the same solar zenith angle, and the results are then generalized to the UV-B. Under the averse overcast sky in the region, UV irradiance at all wavelengths is slightly greater than half of the value for clear skies. Under the thickest overcast layers, UV irradiance at all wavelengths is roughly 20% what it would be if the sky were clear. In a seasonally averaged sense cloudiness has no effect on the percentage enhancement in UV-B surface irradiance that results from the springtime ozone depletion. However, when considering time scales of hours to several days, an increase in cloud cover can be discussed in terms of its ability to attenuate the solar irradiance; in some cases giving a surface UV-B level comparable to that found under an unperturbed ozone column and clear skies. Depending on the amount of ozone depletion and the type of cloud cover, there will always be a wavelength below which surface radiation levels are excessive during spring.

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John E. Frederick
and
H. Donnan Steele

Abstract

This work examines the use of standard meteorological information to describe the attenuation of sunlight associated with cloudy skies. Datasets consisting of broadband ultraviolet and total spectrally integrated solar irradiance are available for Chicago over the period April–October 1993. A series of regression models seeks to explain variability in the measured irradiances using fractional cloudiness, cloud-ceiling altitude, and visibility as independent variables. Fractional cloud cover alone can explain 39%–51% of the variance in the radiation data. Regression models that combine all three independent variables can increase the explained variance to 61%–77%, depending on local time and spectral region. Comparison of broadband ultraviolet irradiances with simultaneous total solar irradiances indicates that cloudy skies provide less attenuation in the former dataset.

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Mark Allen
and
John E. Frederick

Abstract

Simple polynomial representations of the altitude and zenith angle dependence of effective photodissociation cross sections for molecular oxygen and nitric oxide in the Schumann-Runge band region are presented. Longward of ∼202 nm, the atmosphere is optically thin and the effective cross sections are well correlated with local temperature. Atmospheric transmission values and O2 and NO photodissociation rates calculated using the parameterized effective cross sections are in good agreement with the results of the high spectral resolution computations of Frederick and Hudson. The effective cross section approach allows the use of different solar spectra and avoids the assumption of previous work that ray paths at different solar zenith angles, but with the same O2 absorbing column, have the same opacity and dissociation rates. The errors resulting from this assumption can exceed 20% at optical depths greater than 2.

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Timothy M. Barzyk
and
John E. Frederick

Abstract

Individual structures within the same local-scale (102–104 m) environment may experience different microscale (<103 m) climates. Urban microclimate variations are often a result of site-specific features, including spatial and material characteristics of surfaces and surrounding structures. A semiempirical surface energy balance model is presented that incorporates radiative and meteorological measurements to statistically parameterize energy fluxes that are not measured directly, including sensible heat transport, storage heat flux through conduction, and evaporation (assumed to be negligible under dry conditions). Two Chicago rooftops were chosen for detailed study. The City Hall site was located in an intensely developed urban area characterized by close-set high-rise buildings. The University rooftop was in a highly developed area characterized by three- to seven-story buildings of stone, concrete, and brick construction. Two identical sets of instruments recorded measurements contemporaneously from these rooftops during summer 2005, and results from the week of 29 July to 5 August are presented here. The model explains 83.7% and 96% of the variance for the City Hall and University sites, respectively. Results apply to a surface area of approximately 1260 m2, at length scales similar to the dimensions of built structures and other urban elements. A site intercomparison revealed variations in surface energy balance components caused by site-specific features and demonstrated the relevance of the model to urban applications.

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John E. Frederick
and
Carynelisa Erlick

Abstract

Measurements of the ground-level solar irradiance from Palmer Station, Antarctica, and Ushuaia, Argentina, reveal a systematic wavelength dependence in the attenuation provided by cloudy skies. As wavelength increases from 350 to 600 nm, the measured cloudy-sky irradiance, expressed as a fraction of the clear-sky value, decreases. Results from Ushuaia for a solar zenith angle of 45° show that a cloudy sky that reduces the spectral irradiance at 500 nm to 50% of that for clear skies is accompanied by irradiances at 350 and 600 nm, which are approximately 59% and 49%, respectively, of the clear sky value. A weaker wavelength dependence appears in the data for Palmer Station. The observed behavior can arise from Rayleigh backscattering of sunlight beneath the cloud, followed by reflection of this upwelling radiation from the cloud base back to the ground. This sequence of events is most effective at short wavelengths and leads to cloudy skies providing less overall attenuation as wavelength decreases.

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John E. Frederick
,
Xufeng Niu
, and
Ernest Hilsenrath

Abstract

A central problem in the detection of long-term trends in upper stratospheric ozone from orbiting remote sensors involves the separation of instrument drifts from true geophysical changes. Periodic flights of a Solar Backscatter Ultraviolet radiometer (SSBUV) on the Space Shuttle will allow the detection of drifts in optically identical sensors (SBUV/2) carried on operational satellites. A detailed simulation of the SSBUV and SBUV/2 datasets defines the accuracy that can be attained by the in-orbit calibration procedure. The repeatability of the SSBUV calibration from one flight to the next is the most critical variable in the analysis. A repeatability near ±1% is essential for detection and correction of drifts in the SBUV/2 radiance measurements. The simulations show that one can infer true geophysical trends in backscattered radiance to an accuracy of approximately ±1.0% per decade when SSBUV flies approximately once per year and provides a precise calibration correction to the SBUV/2 dataset over a full decade.

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John E. Frederick
,
Anne E. Koob
,
Amy D. Alberts
, and
Elizabeth C. Weatherhead

Abstract

This work examines the effects of absorption and scattering in the troposphere on solar ultraviolet radiation reaching the ground. A site was established in the city of Chicago for monitoring broadband ultraviolet irradiance, total sunlight, and the ground-level mixing ratios of ozone and nitrogen dioxide. The radiation sensors were a Robertson-Berger meter and an Eppley pyranometer. Interpretation of the measurements leads to the following conclusions. During the time period studied, the attenuation provided by clouds and haze underwent an annual cycle. The monthly mean ultraviolet irradiance measured by the Robertson-Berger meter ranged from 84.0% of the clear-sky value for June 1991 to 49.1% for January 1992. Average ultraviolet irradiances for June and July of 1992 were 10.6% and 21.7% lower than in corresponding months of 1991, owing to differences in local cloudiness. The attenuation of total sunlight provided by local clouds and haze was the same as their attenuation of ultraviolet radiation. Finally, a statistically significant negative correlation existed between the output of the Robertson-Berger meter and ground-level ozone when the atmosphere was relatively free of clouds and haze. This demonstrates that gaseous air pollution had a detectable effect on ultraviolet radiation reaching the ground.

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