S. K. Pope

Atmospheric absorption during the Atmospheric Radiation Measurement (ARM) Enhanced Shortwave Experiment (ARESE)

The objectives of the Atmospheric Radiation Measurement (ARM) Enhanced Shortwave Experiment (ARESE) are to directly measure clear and cloudy sky shortwave atmospheric absorption and to quantify any absorption found in excess of model predictions. We undertake detailed model comparisons to near-infrared and total solar flux time series observed by surface and airborne radiometric instruments during the ARESE campaign. Model clear-sky absorption biases generally fall within the range of uncertainty generated by sample size, and assumptions of aerosol properties and surface albedo. Direct measurements by stacked aircraft on the overcast day of October 30, 1995, confirm the detection of enhanced cloud shortwave absorption during ARESE. The detection is substantiated by, and consistent with, three independent measures of cloudy sky absorption estimated in previous studies: cloud forcing ratio, insolation forcing ratio, and albedo/transmission slope. A significant portion of the enhanced absorption occurs at visible wavelengths. Collocated measurements of liquid water path (LWP) suggest the magnitude of the enhanced absorption increases with LWP.

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.

Atmospheric Radiation Measurements Enhanced Shortwave Experiment (ARESE) : Experimental and data details

Atmospheric Radiation Measurements Enhanced Shortwave Experiment (ARESE) was conducted to study the magnitude and spectral characteristics of the absorption of solar radiation by the clear and cloudy atmosphere. Three aircraft platforms, a Grob Egrett, a NASA ER-2, and a Twin Otter, were used during ARESE in conjunction with the Atmospheric Radiation Measurements (ARM) central and extended facilities in north central Oklahoma. The aircraft were coordinated to simultaneously measure solar irradiances in the total spectral broadband (0.224-3.91 μm), near infrared broadband (0.678-3.3 μm), and in seven narrow band-pass (∼10 nm width) channels centered at 0.500, 0.862, 1.064, 1.249, 1.501, 1.651, and 1.750 μm. Instrumental calibration issues are discussed in some detail, in particular radiometric power, angular, and spectral responses. The data discussed in this paper are available at the ARM ARESE data archive via anonymous FTP to ftp.arm.gov.

Absorption of solar radiation by the atmosphere as determined using satellite, aircraft, and surface data during the Atmospheric Radiation Measurement Enhanced Shortwave Experiment (ARESE)

Data sets acquired during the Atmospheric Radiation Measurement Enhanced Shortwave Experiment (ARESE) using simultaneous measurements from five independent platforms (GOES 8 geostationary satellite, ER-2, Egrett and Twin Otter aircraft, and surface) are analyzed and compared. A consistent data set can be built for selected days during ARESE on the basis of the observations from these platforms. The GOES 8 albedos agree with the ER 2, Egrett, and Twin Otter measured instantaneous albedos within 0.013±0.016, 0.018±0.032, and 0.006±0.011, respectively. It is found that for heavy overcast conditions the aircraft measurements yield an absorptance of 0.32±0.03 for the layer between the aircraft (0.5–13 km), while the GOES 8 albedo versus surface transmittance analysis gives an absorptance of 0.33±0.04 for the total atmosphere (surface to top). The absorptance of solar radiation estimated by model calculations for overcast conditions varies between 0.16 and 0.24, depending on the model used and on cloud and aerosol implementation. These results are in general agreement with recent findings for cloudy skies, but here a data set that brings together independent simultaneous observations (satellite, surface, and aircraft) is used. Previous ARESE results are reexamined in light of the new findings, and it is concluded that the overcast absorptance in the 0.224–0.68 μm spectral region ranges between 0.04±0.06 and 0.08±0.06, depending on the particular case analyzed. No evidence of excess clear-sky absorption beyond model and experimental errors is found.

Cloud radiative forcing at the top of the atmosphere during FIRE ACE derived from AVHRR data

Cloud radiative forcing at the top of the atmosphere is derived from narrowband visible and infrared radiances from NOAA-12 and NOAA-14 advanced very high resolution radiometer (AVHRR) data taken over the Arctic Ocean during the First ISCCP Regional Experiment Arctic Cloud Experiment (FIRE ACE) during spring and summer 1998. Shortwave and longwave fluxes at the top of the atmosphere (TOA) were computed using narrowband-to-broadband conversion formulae based on coincident Earth Radiation Budget Experiment (ERBE) broadband and AVHRR narrowband radiances. The NOAA-12/NOAA-14 broadband data were validated using model calculations and coincident broadband flux radiometer data from the Surface Heat Budget of the Arctic Ocean experiment and from aircraft data. The AVHRR TOA albedos agreed with the surface- and aircraft-based albedos to within one standard deviation of ±0.029 on an instantaneous basis. Mean differences ranged from −0.012 to 0.023 depending on the radiometer and platform. AVHRR-derived longwave fluxes differed from the model calculations using aircraft- and surface-based fluxes by −0.2 to −0.3 W m−2, on average, when the atmospheric profiles were adjusted to force agreement between the observed and the calculated downwelling fluxes. The standard deviations of the differences were less than 2%. Mean total TOA albedo for the domain between 72°N and 80°N and between 150°W and 180°W changed from 0.695 in May to 0.510 during July, while the longwave flux increased from 217 to 228 W m−2. Net radiation increased from −89 to −2 W m−2 for the same period. Net cloud forcing varied from −15 W m−2 in May to −31 W m−2 during July, while longwave cloud forcing was nearly constant at ∼8 W m−2. Shortwave cloud forcing dominated the cloud effect, ranging from −22 W m−2 during May to −40 W m−2 in July. The mean albedos and fluxes are consistent with previous measurements from the ERBE, except during May when the albedo and longwave flux were greater than the maximum ERBE values. The cloud-forcing results, while similar to some earlier estimates, are the most accurate values hitherto obtained for regions in the Arctic. When no significant melting was present, the clear-sky longwave flux showed a diurnal variation similar to that over land under clear skies. These data should be valuable for understanding the Arctic energy budget and for constraining models of atmosphere and ocean processes in the Arctic.

Determination of Clear-Sky Radiative Flux Profiles, Heating Rates, and Optical Depths Using Unmanned Aerospace Vehicles as a Platform

Abstract In this paper the authors report results obtained using an unmanned aerospace vehicle (UAV) as an experimental platform for atmospheric radiative transfer research. These are the first ever climate measurements made from a UAV and represent a major step forward in realizing the unique potential of long-endurance, high-altitude UAVs to contribute to climate and environmental studies. Furthermore, the radiative flux divergences determined during these experiments are some of the highest quality measurements of this kind obtained from any type of aircraft and constitute an important test of radiative transfer models.

Observations and models of irradiance profiles, column transmittance, and column reflectance during the Atmospheric Radiation Measurements Enhanced Shortwave Experiment

In the fall of 1995 the Atmospheric Radiation Measurement Enhanced Shortwave Experiment (ARESE) was conducted to address fundamental questions about the amount of absorption of solar radiation occurring in clear and cloudy skies. Irradiances measured from instruments flown on stacked aircraft as part of this experiment are compared to modeled irradiances for several representative days. As a basic check of the data and the model, the downwelling irradiances at 13 km are examined and found to be in good agreement, with both model and observations showing day-to-day variations as well as shorter timescale variations due to changing atmospheric conditions. Evidence of cloud-induced enhanced absorption is seen from an analysis of column reflectance and absorptance as determined from simultaneous measurements made from the stacked aircraft. When compared to model results, profiles of irradiance versus altitude as measured by one aircraft indicate cloud-induced enhanced absorption as well.