Brett C. Bush

Surface aerosol radiative forcing at Gosan during the ACE‐Asia campaign

4.8 W m � 2 /t500 for the total solar, near-infrared, and visible spectral regions. We also introduce a new radiative forcing parameter, the fractional forcing efficiency, defined to express the radiative forcing relative to the total energy incident at the top of the atmosphere. The fractional diurnal forcing efficiency at Gosan during ACE-Asia was � 18.0 ± 2.3, � 16.2 ± 2.4, and � 26.7 ± 3.3%/t500 for the same band passes, indicating that a larger percentage of the flux at visible wavelengths is radiatively forced compared to the total and near-infrared portions of the solar spectrum. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 0394 Atmospheric Composition and Structure: Instruments and techniques; KEYWORDS: radiative fluxes, radiative forcing, aerosols

Absorption of solar radiation by the cloudy atmosphere: Further interpretations of collocated aircraft measurements

We have extended the interpretations made in two prior studies of the aircraft shortwave radiation measurements that were obtained as part of the Atmospheric Radiation Measurements (ARM) Enhanced Shortwave Experiment (ARESE). These extended interpretations use the 500 nm (10 nm bandwidth) measurements to minimize sampling errors in the broadband measurements. It is indicated that the clouds present during this experiment absorb more shortwave radiation than predicted by clear skies and thus by theoretical models, that at least some (≤20%) of this enhanced cloud absorption occurs at wavelengths <680 nm, and that the observed cloud absorption does not appear to be an artifact of sampling errors nor of instrument calibration errors.

Spectral aerosol radiative forcing at the surface during the Indian Ocean Experiment (INDOEX)

[1]xa0We describe a radiation experiment performed at the Kaashidhoo Climate Observatory (KCO), Republic of Maldives, during the Indian Ocean Experiment (INDOEX) field campaign in February through March 1999. Both the total solar broadband (0.3 to 3.81 μm) and visible spectral narrowband (seven spectral channels in the region from 0.4 to 0.7 μm) quantities were measured for the total (hemispherical), direct solar, and diffuse (including forward scattering) components of the radiation field. The aerosol optical depth at 500 nm, obtained from one of the narrrowband spectral channels, ranged from approximately 0.2 to 0.7 during the period encompassing the intensive field phase (IFP) of INDOEX. The diurnally averaged atmospheric forcing efficiencies determined for the total solar broadband and visible spectrum are −72.2 ± 5.5 W m−2 and −38.5 ± 4.0 W m−2, respectively. Model simulations driven by in situ measurements and realistic aerosol optical properties infer a single scattering albedo of 0.874 ± 0.028 at 500 nm.

Electron and proton aurora observed spectroscopically in the far ultraviolet

[1]xa0The only way to get a global, instantaneous picture of the energetic particle input over the auroral oval is through spectral imaging. The major driver of auroral emissions in the high-latitude ionosphere is overall electron precipitation. However, for certain locations and times, such as the equatorial edge of the evening auroral oval, proton precipitation can be the major energy source and thus the primary contributor to auroral emissions. Using kinetic transport models to describe the transport of energetic particles in the atmosphere, we analyze UV spectra from the STP78-1 satellite mission during magnetically disturbed conditions (Kp = 6) in the evening sector of the auroral oval. We discuss the contribution of protons and electrons to the auroral emissions. The energy flux of the incident protons is inferred from the H Lyman α emissions, after removing the H geocoronal background induced by solar radiation. Both the mean energy and energy flux of electron precipitation are inferred from non-H emissions (N II 108.5 nm, N2 135.4 nm, and O I 135.6 nm), after removing the contribution of proton precipitation. From the latitudinal distribution of the incident energy flux the location of the electron and proton aurorae is discussed. The estimation of the particle characteristics allows one to infer the Pedersen and Hall electrical conductances induced by particle precipitation. For the studied substorm period, energetic protons contribute significantly to the Pedersen conductance, ∼25–30% overall of the total particle-induced conductances and much more at the equatorward edge of the midnight aurora. Because protons and electrons do not interact in the same way with the atmosphere, our study shows that while analyzing auroral spectra and studying the state of the ionosphere, it is crucial to separate electron and proton components of the precipitation. The method described to disentangle the relative contribution of precipitating electrons and protons may be applicable to the UV data of the upcoming TIMED and DMSP missions.

Measured and calculated clear-sky solar radiative fluxes during the Subsonic Aircraft Contrail and Cloud Effects Special Study (SUCCESS)

Modeled and measured surface insolations are compared with the purpose of evaluating the ability of a radiative transfer model to predict the amount of solar radiation reaching the surface under clear-sky conditions. Model uncertainties are estimated by performing sensitivity studies for variations in aerosol optical depth, aerosol optical properties, water vapor profiles, ozone content, solar irradiance at the top of the atmosphere, and surface albedo. In this fashion, a range of possible calculated values is determined and compared to observations. Experimental errors are evaluated by comparison with independent, simultaneous measurements performed using two World Radiation Reference instrument arrays that were operational for a limited period during SUCCESS. Assuming a mineral aerosol, it is found that there is agreement between calculated and measured fluxes, with differences approximately equal to and within one standard deviation. Such agreement improves further if a layer containing a small amount of carbonaceous aerosol is added. The presence of carbonaceous aerosols is likely because occasional biomass burning activities took place during SUCCESS in the area around the experimental site (the clouds and radiation test bed operated by the Department of Energy in Oklahoma).

Absorption of solar radiation by the clear and cloudy atmosphere during the Atmospheric Radiation Measurement Enhanced Shortwave Experiments (ARESE) I and II: Observations and models

[1]xa0As a follow-on to the Atmospheric Radiation Measurement (ARM) Enhanced Shortwave Experiment (ARESE) I, which provided atmospheric shortwave measurements from collocated aircraft, ARESE II performed similar measurements with a single aircraft flying at an altitude of 7 km over an instrumented surface site. ARESE I and ARESE II absorptance measurements are found to agree with each other and, when converted to top of the atmosphere (TOA) instantaneous column absorption, are also consistent with GOES 8 and Scanner for Radiation Budget (ScaRaB) satellite observations. Measurements are compared to calculations performed with five different radiative transfer models. It is found that the calculated absorption differs systematically from the observations in cloudy conditions, with models underpredicting the absorption. In particular, all the models tested here underpredict the measured instantaneous cloudy column absorption by amounts ranging from 17 to 61 W m−2, depending on the models and cases studied. The various models, using identical input, differ among themselves; for example, for the same cloudy case, absorptance estimates range from 0.22 to 0.27 for the atmospheric column from the surface to the TOA. It is also found that model-calculated absorptances appear not as well correlated to cloud optical depth variations as the measured absorptances appear to be. Measured and calculated clear-sky absorptances agree well within the uncertainties. Cloudy-sky absorptances in the visible spectral region (300 to 700 nm) reach values of as much as 0.02 during ARESE II while the equivalent measurements for ARESE I range around 0.06. This visible absorptance may be related to aerosols and their day-to-day and seasonal variability.

Broadband shortwave calibration results from the Atmospheric Radiation Measurement Enhanced Shortwave Experiment II

[1]xa0The second ARM (Atmospheric Radiation Measurement) Enhanced Shortwave Experiment (ARESE II) used a single aircraft flying above the north central Oklahoma Southern Great Plains ARM central facility to measure the atmospheric absorption in the column of air between the surface and the Twin Otter altitude ceiling around 7 km on both clear and overcast days. For this experiment, three types of broadband radiometers were used to measure upwelling and downwelling shortwave flux on the aircraft at 7 km. This provided redundancy that was lacking in the first ARESE. Further, all instruments used on the ground and on the aircraft were calibrated on the ground against the same radiation standard. Preflight and postflight comparisons of the broadband instruments used on the aircraft during the flight series with the standard, and comparisons of the ground instruments with the standard during the flights, suggest agreement much better than the target uncertainty of 20 W/m2 at the 95% confidence level. Comparisons of the standard and the aircraft instruments for low-altitude passes on clear days indicate more uncertainty as expected for a nonstationary platform. The estimated uncertainty in the measured column absorption based on the difference in measured net irradiances at the surface and near 7 km at the 95% confidence level is 20 W/m2.

COMPARISON OF ARESE CLEAR SKY SURFACE RADIATION MEASUREMENTS

Abstract A variety of broadband and spectral surface irradiance measurements from three instrument platforms show significant agreement with one another during three clear sky days (11, 15 and 18 October 1995) during the ARM Enhanced Shortwave Experiment (ARESE). This agreement is only possible when carefully considering absolute and angular calibration issues and applying the recommended correction procedures to the raw signals. By removing systematic measurement errors associated with the various instruments, it is possible to achieve consistencies in the various broadband data sets good to about 1–2% accuracy. It is through these careful comparisons that the measurement techniques currently used in determining downwelling surface flux radiances reflect the highest degree of accuracy and certainty.

SURFACE RADIATION MEASUREMENTS DURING THE ARESE CAMPAIGN

Abstract We describe a comprehensive set of surface radiative flux measurements made during the Fall 1995 ARM Enhanced Shortwave Experiment (ARESE) at a variety of locations in northern Oklahoma. Accurate measurements of solar spectral broadband (0.224–3.91xa0μm and 0.68–3.3xa0μm) and narrow spectral bandpass (10xa0nm) downwelling irradiances were made over atmospheric conditions ranging from “pristine” clear to heavy overcast. Precise instrument calibrations are used during analyses in deriving absolute downwelling fluxes accurate to about 1% for the broadband and 3% for the spectral instruments. Measured instrument angular responses are incorporated in these analyses to minimize deviations from ideal cosine responses. All calibrated irradiance measurements are available through the ARM ARESE archive via anonymous FTP at ftp.arm.gov and the World Wide Web at www.arm.gov.

Continental aerosol properties inferred from measurements of direct and diffuse solar irradiance

[1]xa0Aerosol particles affect the global energy budget and Earths climate through the absorption and scattering of both solar and terrestrial radiation. Here we present a study of column-averaged aerosol optical properties derived from clear-sky measurements of the direct and diffuse components of the solar downwelling spectral and broadband irradiance during a 10-day observation period at the Atmospheric Radiation Measurement (ARM) facility in central Oklahoma in the fall of 2001. Using a radiative transfer model and Mie calculations of the optical properties of standard continental aerosol components and assuming a constant aerosol composition with altitude, we determine the phenomenological aerosol model that best fits both the direct and diffuse spectral irradiances. Internal and external homogeneous aerosol mixtures were considered, along with single-aerosol and coated sphere models. We find that the irradiance data is well fit statistically by at least one of the models for every clear-sky time considered during the observations period, with no excess gaseous absorption required to fit the data. By far the best fitting model consisted of ultrafine (∼5–30 nm radius) coated spheres with absorbing black carbon cores and nonabsorbing shells of water or a similar substance. The mean value of the 500-nm aerosol single-scattering albedos for the entire 10-day period was 0.76. These results indicate that absorbing carbonaceous aerosols can have a significant radiative impact even in rural settings.