Richard Wagener

Direct shortwave forcing of climate by the anthropogenic sulfate aerosol: Sensitivity to particle size, composition, and relative humidity

Recent estimates of global or hemispheric average forcing of climate by anthropogenic sulfate aerosol caused by scattering of shortwave radiation (“direct” effect) are uncertain by somewhat more than a factor of 2. The principal sources of this uncertainty are atmospheric chemistry properties (yield, residence time), and microphysical properties (scattering efficiency, upscatter fraction, and the dependence of these properties on particle size, composition, and relative humidity, (RH)). This paper examines the sensitivity of forcing to these microphysical properties to identify and improve understanding of the properties required to reduce the uncertainty in the forcing. The relations between aerosol loading and forcing developed here are suitable for comparing modeled and measured aerosol forcing at specific locations and for use in climate models, provided aerosol composition and microphysical properties are known, calculated, or assumed. Results are presented showing the dependence of scattering efficiency, upscatter fraction, and normalized forcing (W m−2/g(SO24−) m−2 or W g(SO24−)−1) on dry particle size (expressed as mole(sulfate) per particle), composition ((NH4)2SO4, NH4HSO4, H2SO4), solar zenith angle, latitude, and season. Forcing is strongly dependent on dry particle size and RH but is relatively insensitive to composition. The normalized forcing can be integrated over a known or assumed size distribution to evaluate the sulfate aerosol forcing. Global and annual average values of the normalized forcing are evaluated as a function of particle size and RH. Depending on values of these variables, normalized forcing may be less than, intermediate to, or greater than the range of previous estimates of sulfate aerosol forcing.

Intercomparison of models representing direct shortwave radiative forcing by sulfate aerosols

The importance of aerosols as agents of climate change has recently been highlighted. However, the magnitude of aerosol forcing by scattering of shortwave radiation (direct forcing) is still very uncertain even for the relatively well characterized sulfate aerosol. A potential source of uncertainty is in the model representation of aerosol optical properties and aerosol influences on radiative transfer in the atmosphere. Although radiative transfer methods and codes have been compared in the past, these comparisons have not focused on aerosol forcing (change in net radiative flux at the top of the atmosphere). Here we report results of a project involving 12 groups using 15 models to examine radiative forcing by sulfate aerosol for a wide range of values of particle radius, aerosol optical depth, surface albedo, and solar zenith angle. Among the models that were employed were high and low spectral resolution models incorporating a variety of radiative transfer approximations as well as a line-by-line model. The normalized forcings (forcing per sulfate column burden) obtained with the several radiative transfer models were examined, and the discrepancies were characterized. All models simulate forcings of comparable amplitude and exhibit a similar dependence on input parameters. As expected for a non-light-absorbing aerosol, forcings were negative (cooling influence) except at high surface albedo combined with small solar zenith angle. The relative standard deviation of the zenith-angle-averaged normalized broadband forcing for 15 models was 8% for particle radius near the maximum in this forcing (∼0.2 μm) and at low surface albedo. Somewhat greater model-to-model discrepancies were exhibited at specific solar zenith angles. Still greater discrepancies were exhibited at small particle radii, and much greater discrepancies were exhibited at high surface albedos, at which the forcing changes sign; in these situations, however, the normalized forcing is quite small. Discrepancies among the models arise from inaccuracies in Mie calculations, differing treatment of the angular scattering phase function, differing wavelength and angular resolution, and differing treatment of multiple scattering. These results imply the need for standardized radiative transfer methods tailored to the direct aerosol forcing problem. However, the relatively small spread in these results suggests that the uncertainty in forcing arising from the treatment of radiative forcing of a well-characterized aerosol at well-specified surface albedo is smaller than some of the other sources of uncertainty in estimates of direct forcing by anthropogenic sulfate aerosols and anthropogenic aerosols generally.

Infrared polar brightening on Jupiter: III. Spectrometry from the Voyager 1 IRIS experiment

Spectra from the Voyager 1 IRIS experiment confirm the existence of enhanced infrared emission near Jupiters north magnetic pole in March 1979. The spectral characteristics of the enhanced emission are consistent with a Planck source function. A temperature-pressure profile is derived for the region near the north magnetic pole, from which quantitative abundance estimates of minor species are made. Some species previously detected on Jupiter, including CH3D, C2H2 and C2H6, have been observed again near the pole. Newly discovered species, not previously observed on Jupiter, include C2H4, C3H4, and C6H6. All of these species except CH3D appear to have enhanced abundances at the north polar region with respect to midlatitudes. Upper limits are determined for C4H2 and C3H8. The quantitative results are compared with model calculations based on ultraviolet results from the IUE satellite. The plausibility of the C6H6 identification in discussed in terms of the literature on C2H2 polymerization. The relation of C6H6 to cuprene is also discussed.

High-resolution, low-temperature photoabsorption cross sections of C2H2, PH3, AsH3, and GeH4, with application to Saturn's atmosphere

Using synchrotron radiation as a continuum background light source, the VUV absorption cross sections of (1) C2H2 and PH3 have been obtained under physical conditions approximating the stratosphere of Saturn and (2) AsH3 and GeH4 have been obtained under room temperature conditions. We have combined these new laboratory measurements with International Ultraviolet Explorer observations of the albedo of Saturn, for which improved data reduction techniques have been used, to produce new models for that atmosphere. When the effects of C2H2 absorption are accounted for, additional absorption by other molecules is required. The best fitting model also includes absorption by H2O, CH4, C2H6, PH3, and GeH4. The abundance of H2O is reduced by a factor of 2 as a direct consequence of the new low-temperature cross section data of C2H2. The model abundance distributions of PH3 and GeH4 on Saturn decrease with increasing altitude, suggesting a possible consequence of UV photolysis.

The Jovian stratosphere in the ultraviolet.

The center-of-disk reflectivity of Jupiter in the wavelength range from 1450 to 3150 angstroms has been computed from 30 low-dispersion IUE spectra taken during solar maximum in 1978-1980. A vertically inhomogeneous radiative transfer program is used to compute model reflectivities of various stratospheric compositions for comparison. Ammonia and acetylene are well determined because they show narrow absorption bands in the ultraviolet. Above 1800 angstroms, these two gases provide a good fit to the data, but not below. At shorter wavelengths the fit would be much improved by a small amount (0.5-1.5 ppb) of propadiene/allene (C3H4). Voyager IRIS spectra show that the IR bands of allene are not strong enough to be detected in such a small amount. Additional absorption around 1600 angstroms can be reproduced best with the presence of cyclopropane (C3H6, <15 ppb), although other absorbers (e.g., hydrocarbon molecules with more than three carbon atoms, oxygen- or nitrogen-containing molecules, or a high-altitude haze) could also explain the spectrum in this region. The data are too noisy to detect possible CO Cameron band absorption near 2000 angstroms.

Seasonal, latitudinal, and secular variations in temperature trend: Evidence for influence of anthropogenic sulfate

Tropospheric aerosols increase the shortwave reflectivity of the Earth-atmosphere system both by scattering light directly, in the absence of clouds, and by enhancing cloud reflectivity. The radiative forcing of climate exerted by anthropogenic sulfate aerosols, derived mainly from SO2 emitted from fossil fuel combustion, is opposite that due to anthropogenic greenhouse gases and is estimated to be of comparable average magnitude in Northern Hemisphere midlatitudes. However, persuasive evidence of climate response to this forcing has thus far been lacking. Here we examine patterns of seasonal and latitudinal variations in temperature anomaly trend for evidence of such a response. Pronounced minima in the rate of temperature increase in summer months in Northern Hemisphere midlatitudes are consistent with the latitudinal distribution of anthropogenic sulfate and changes in the rate of SO2 emissions over the industrial era.

Aerosol Optical Depth over Oceans: High Space- and Time-Resolution Retrieval and Error Budget from Satellite Radiometry

A method to retrieve aerosol vertical optical depth at 0.64 mm from satellite observations of cloud-free scenes over oceans with high spatial resolution (;18) and instantaneous temporal resolution is described and evaluated. The observed radiance is treated as the linear sum of contributions to path radiance by different scattering processes in the atmosphere‐ocean system. This treatment allows examination of errors in the retrieved vertical aerosol optical depth contributed by each process and approximation. Random error in retrieved aerosol optical depth is typically 0.03. The systematic error due to absolute calibration uncertainty in the measured radiance is 0.01. The largest errors and biases are due to radiative transfer approximations (122%) and assumptions regarding aerosol microphysical and optical properties (220%). The latter errors, which are due to the optical properties (e.g., phase function), vary systematically with latitude and season because of the variation of the mean observing geometry. This method is applied to Advanced Very High Resolution Radiometer global area coverage data, and example maps of aerosol optical depth are presented for specific dates in July and October 1986. The aerosol optical depth derived from the satellite data is suitable for examining large aerosol signatures by instantaneous comparison of the amplitude and location of aerosol plumes with model predictions based on meteorological conditions at and preceding the time of observation.

An Overview of ARM Program Climate Research Facility Data Quality Assurance.

We present an overview of key aspects of the Atmospheric Radiation Measurement (ARM) Program Climate Research Facility (ACRF) data quality assurance program. Processes described include instrument deployment and cali- bration; instrument and facility maintenance; data collection and processing infrastructure; data stream inspection and as- sessment; problem reporting, review and resolution; data archival, display and distribution; data stream reprocessing; en- gineering and operations management; and the roles of value-added data processing and targeted field campaigns in speci- fying data quality and characterizing field measurements. The paper also includes a discussion of recent directions in ACRF data quality assurance. A comprehensive, end-to-end data quality assurance program is essential for producing a high-quality data set from measurements made by automated weather and climate networks. The processes developed dur- ing the ARM Program offer a possible framework for use by other instrumentation- and geographically-diverse data col- lection networks and highlight the myriad aspects that go into producing research-quality data.

Strong North/South asymmetry in the Jovian stratosphere

Abstract Large aperture observations of the Jovian north and south polar regions with the International Ultraviolet Explorer (IUE) were spatially calibrated with trailed spectra of standard B stars as the “flat” field light source. The spatial resolution (5 to 6 arcsec) and the extent (20 arcsec) perpendicular to the dispersion direction of IUE spectra are sufficient to extract spectra corresponding to polar latitudes (50 to 90°) and midlatitudess (25 to 50°) from a single observation. The results are (1) Longward of 2000 A the south polar spectrum is a factor of 2.5 lower than the north polar spectrum; (2) shortward of 2000 A strong C 2 H 2 absorption is evident in the north polar and midlatitude spectra, whereas neither continuum nor band absorption features of C 2 H 2 are apparent in the south polar spectrum; and (3) the northern and southern midlatitude spectra are identical within the uncertainties of the data. The very low reflectivity in the south polar region requires that the responsible stratospheric absorber be present at pressures less than 10 mbar. Since no molecular species that can explain the observed reflectivity has been identified, the absorber is assumed to be a small particle haze. To match the observations the haze must exhibit a 200-A-wide absorption maximum near 2000 A. The observed depletion of C 2 H 2 in the south makes a condensed phase of C 2 H 2 derivatives the most plausible candidate for this haze.

The geometric albedos of Uranus and Neptune between 2100 and 3350 Å

Abstract Absolutely calibrated spectra of Uranus, Neptune, and the solar analog stars 16 Cyg A and B between 2100 and 3350 A are reported. The geometric albedos of both planets are close to the curve expected for a semi-infinite Rayleigh-Raman scattering atmosphere between 2100 and 2800 A. Longward of 2800 A the albedos fall below the Rayleigh-Raman values and connect smoothly to the ground-based photometry of J.S. Neff, D.C. Humm, J.T. Bergstralh, A.L. Cochran, W.D. Cochran, E.S. Barker, and R.G. Tull (1984, Icarus 60 , 221–235). Neptune is about 5.5% brighter than Uranus and shows slightly stronger Raman scattering signatures in the MgII lines at 2800 A in accordance with the results of Neff et al. for the CaII H and K lines. This means that the stratospheric haze on Neptune is thinner than on Uranus. The fact that the Neptunian geometric albedo between 2100 and 2800 A is so close to the ideal semi-infinite Rayleigh-Raman scattering atmosphere could be exploited for future absolute calibrations of other Solar System objects in this wavelength region.